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Why would a rim say "for rim brakes only"?

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Why would a rim say "for rim brakes only"?

Old 08-18-20, 09:21 AM
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Originally Posted by Skulking
I'd like to circle back to the original question and see if I've understood everything correctly. It seems like the answer is actually that there is no reason that a rim would be for rim brakes only, but because of how the spokes are laced, a wheel may only be suitable for rim brakes. If a wheel is sold fully assembled with the spokes laced in such a way that is only appropriate for rim brakes, then it would make sense for the rim to be labeled as only being suitable for rim brakes, but if it was desired the spokes could be re-laced appropriately for disk brakes because there is nothing specific to the rim which makes it only suitable for rim brakes. Have I correctly understood the gist of this thread?
Wheels generally aren’t laced differently between disc and rim brakes. Yes, you can do a radial lace on a front wheel and a radial lace on the nondrive side of rear hub. Those are mostly fashion...there no real good reason to do it...and radial lacing stresses the hub shell which is why many manufacturers void warranties when a hub is laced radially. Radial lacing probably puts extra stress on the rim as well but front wheels (no disc) are symmetrical and very strong in general. It takes a lot to break a front wheel...either spokes or rim.

But most wheels are going to be laced in a 3 cross pattern for both rim and disc. There is no reason that a rim brake rim couldn’t be used for a disc brake rim.
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Old 08-18-20, 09:53 AM
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Originally Posted by cyccommute
Both transfer the braking force to the fork. The only difference is one of symmetry. All of the force on the disc is transferred to the left fork leg. The load at the spoke bed...remember this is about the rim not the brake...is the same in each case. Rim brake rims aren’t inherently weaker than disc rims.

Your test wouldn't test the rim, it would test spokes. The only way to test the rim would be a longevity test. How many years do you have to dedicate to the test?
We're past discussing rims, there is little dispute there.

You keep on saying the load at the spoke bed (and obviously the spokes and hub spoke holes) are the same, rim or disc brake. But the rim brake clamps at the rim/tire, and disc clamps at the hub. This is the big difference and the reason why you cannot run a radial wheel with a disc brake. Please explain this fact away.
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Old 08-18-20, 09:55 AM
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Originally Posted by FiftySix
Following the thread with interest.

---------------------------------
  • A rim brake works directly on the rim and the attached tyre; a disc brake applies a potentially large torque moment at the hub. The latter has two main disadvantages:
  1. The torque moment must be transmitted to the tyre through the wheel components: flanges, spokes, nipples, and rim spoke bed. An engineered disc brake would reduce weight by not having most of the metal rim components
  2. A front disc brake places a bending moment on the fork between the caliper anchor points and the tip of the dropout. In order to counter this moment and to support the anchor points and weight of the caliper, the fork must be of a certain size (most likely heavier).
https://en.wikipedia.org/wiki/Bicycle_brake#Disc_brakes
The bolded statement above argues that the rim for a disc brake rim can be made lighter but reducing the mass of metal needed...most of which comes through losing the brake track. It does not argue that a rim for rim brakes is weaker.

I think that the critical difference between rim brakes and disc brakes, that the addition was trying to convey, is that with disc brakes the torque of braking occurs between the hub and the rim, while with rim brakes the only time torque occurs between the hub and the rim is at the rear hub during pedaling. It is true that rim brakes can work with radial spoke lacing on the front wheel, while disc brakes require cross-lacing (usually 3x). So while the addition was somewhat inarticulately expressed, it is indeed based in fact. Matt Gies 17:25, 11 May 2006 (UTC)

https://en.wikipedia.org/wiki/Talk%3...kes_and_spokes
Watch a fork when you brake sometime. The fork flexes rearward as you brake and it flexes more as you brake harder. This happens with both types of brakes. The rearward flex wouldn’t happen if there is no torque between the rim and hub or if braking with a rim brake worked the way that people are describing it here. We are translating rotational motion to linear motion. That’s how braking works. We aren’t stopping the contact patch...at least we aren’t stopping only the contact patch. We are stopping the bike that is moving linearly but changing the angular momentum into linear momentum. Apply the brake (of either kind) and the hub attempts to run ahead of the contact patch which results in a twisting of the spokes in the rim. The other spokes are there to prevent that twisting and better translate the angular momentum to linear momentum.

With rim brakes, forks are loaded in bending at the fork crown that similarly supports road shock, while a disc brake places an equal bending torque at the tip of the fork and only on one blade. A fork can only be slender and light because it carries no bending loads at the dropout. With disc brakes, forks would require a substantial increase in cross section (and weight) and the brake would be heavier.

https://www.sheldonbrown.com/brandt/brakes.html
I have no argument with the structure of the fork. The fork has to be stiffer to resist the twisting caused by the disc caliper being mounted behind the fork blade on only one side.* That’s not the original question from crankholio. Remember him? His question was about the rim. Nothing presented yet has really presented an argument for why rim brake rims can’t be used for disc brakes.


*The location of a disc brake on the fork is about the dumbest place it could ever be placed. The force on the rotor causes the wheel to be pushed down during braking. The fork tip is pushed forward while the braking force acts rearward. One blade of the fork is bending forward while the other is bending rearward. No wonder that the fork has to be much beefier.

If the caliper were mounted on the front of the fork, the force would be up and into the fork. Ejection of the wheel wouldn’t be a concern and the fork blades would be working together rather than fighting each other.
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Old 08-18-20, 10:02 AM
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Originally Posted by AlmostTrick
Thank you, 56 and HT. You would think that the simple, well known fact that rim brakes can run radial spokes and discs can't would quickly put this discussion to rest. I'm kinda glad it doesn't though, because reading these explanations is fun!

If the discussion were about the spokes, your are correct. But the discussion isn’t about the spokes. It’s about the rim. Please provide reasoning as to why a rim brake rim couldn’t be used for a disc wheel.
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Old 08-18-20, 10:21 AM
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Originally Posted by AlmostTrick
We're past discussing rims, there is little dispute there.

You keep on saying the load at the spoke bed (and obviously the spokes and hub spoke holes) are the same, rim or disc brake. But the rim brake clamps at the rim/tire, and disc clamps at the hub. This is the big difference and the reason why you cannot run a radial wheel with a disc brake. Please explain this fact away.
With rim brakes, rim is practically pushed towards the rear of the bicycle. Braking does not put much turning torque.
Radially laced spokes are really poor at transmitting the turning torque (either rear drive torque, or disc braking induced torque).
That is why it is not very wise lacing rear wheels, or disc brake wheels radially - especially on both sides of the hub.

My long winded (with pictures) explanation of radial vs cross lacing pattern pros and cons.
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Old 08-18-20, 10:29 AM
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Originally Posted by cyccommute
If the discussion were about the spokes, your are correct. But the discussion isn’t about the spokes. It’s about the rim. Please provide reasoning as to why a rim brake rim couldn’t be used for a disc wheel.
The issue is the seat on the rim for the spoke nipples. If it isn't design to take the addtitional pull of disc braked spokes and the spokes pull out, that's a real rim issue. On a lot of older design, lighter rims, that spoke/rim interface was the weak point. Adding a sticker warning not to make the issue worse is simply common sense.

I'm scratching my head as to why this took 3 pages of posts.
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Old 08-18-20, 12:35 PM
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Originally Posted by cyccommute
The hub is free to roll, yes. The hub isn’t free to turn relative to the spokes. Applying force to the wheel through the brake system, whether disc or rim, put stress on the spokes. The hub shell (which the spokes are attached to) tries to turn relative to the spokes but the hub is locked in place. The amount of force on the spoke bed is the same in both cases. A disc doesn’t generate more force at the spoke bed than a rim brake. Therefore there is no reason for a limitation on using rim brake rims with a disc hub...aesthetics aside
Yes, but the point is that there is MORE torsional torque exerted on the spokes with a hub type brake. Sorry, but that has been something that has been accepted by people for years, in fact, Erik Buell used large rotors, mounted at the rim of his motorcycles so he could use a lighter weight wheel. The larger disc also gave better heat management, but it helped reduce the stress to the wheel, because of... torsion placed on the wheel due to braking forces. Mounting the brake disc to the rim, rather than at the hub lowered the torsion applied to the wheel. The rim mounted brake disc had many other benefits as well, but overall, it allowed Buell to reduce unsprung weight, one disc instead of two heavy discs, one caliper, and, in his own words, "We used a totally new front wheel concept based on the idea that without braking loads being fed through the hub and back out through the spokes, then there would be no torsional loads through those spokes. That enabled us to take a significant amount of weight out of the wheel." And, it worked, very well, because, physics.

Continue debating what others have already proven. I will watch from the sidelines.
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Old 08-18-20, 12:35 PM
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Originally Posted by 79pmooney
The issue is the seat on the rim for the spoke nipples. If it isn't design to take the addtitional pull of disc braked spokes and the spokes pull out, that's a real rim issue. On a lot of older design, lighter rims, that spoke/rim interface was the weak point. Adding a sticker warning not to make the issue worse is simply common sense.

I'm scratching my head as to why this took 3 pages of posts.
Because...cyccommute...
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Old 08-18-20, 03:30 PM
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Originally Posted by 79pmooney
The issue is the seat on the rim for the spoke nipples. If it isn't design to take the addtitional pull of disc braked spokes and the spokes pull out, that's a real rim issue. On a lot of older design, lighter rims, that spoke/rim interface was the weak point. Adding a sticker warning not to make the issue worse is simply common sense.

I'm scratching my head as to why this took 3 pages of posts.
The problem is that no one has shown that spoke pull out is a problem when using a rim brake rim with a disc brake. Rim brake rims have been used for a very long time with disc brakes without problems. If “a lot of older designs” were an issue, it would have been addressed long before now. I’ve never seen a sticker or catalog listing where the user is advised not to use the rim for disc brakes. It just doesn’t come up. Velocity doesn’t address it. Nor does Mavic. When I search for “for rim brakes only”, it doesn’t return any rim manufacturers that list a rim for rim brakes only. The closest is a link a Cycling UK forum about Alex CX28 rims having a sticker like cycleholio asked about. But Alex doesn’t list that fact in their catalog.

The upshot of that Cycling UK forum is that the sticker is a CYA as I’ve said above...long ago.
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Old 08-18-20, 04:16 PM
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Someone please lock the thread to stop the spread of misinformation.

I believe everyone should stop trying to convince the people who don't believe that spokes and spoke holes experience greater forces with disc brakes than rim brakes. It's pointless to keep on trying. It's like trying to convince someone who believes the earth is flat, that in fact it's round.

Just to summarize the difference in forces in case anyone wants to know:

Radial load when decelerating while braking:
Disc brakes apply greater radial load to the spokes and spoke holes (braking force) than rim brakes that are behind the axle. Imagine a see-saw. If you place a mass on one end of the see-saw (the brake force) closer to the center, it needs to be heavier to balance the weight on the other end (road friction). Therefore the center of the see-saw (the axle) will experience greater loads (which transfers to the spokes and spoke holes). Disc brakes are much closer to the center of the wheel than rim brakes. However, if you use a disc rotor that is the same diameter as the rim, then the brake force on that large disc would be equal to the rim brake and therefore the radial load on the spokes (braking force) would be the same. But this is not the case, because disc rotors are usually much smaller in diameter than the rim.

(Radial load when decelerating while braking:
Rim brakes apply radial loads to the axle which transmits through the spokes)


Torque load when decelerating while braking:
Rim brakes do not apply torque load to the spokes, because nothing is turning the hub flange (where one end of the spokes are attached) against the rotation of rim (where the other end of the spokes are attached), since the hub runs on bearings, which makes it a pin/roller system (except for the small amount of friction from the bearings).

Disc brakes apply torque load to all the trailing spokes (mostly on the disc side, much less on the drive side), because the disc brake is turning the hub against the rotation of the rim. This causes a "twist" or torque. You cannot run all radial lacing on rims for disc brakes because the spokes would then be 90 degrees to the axle and would not be able to resist the rotational torque load of the disc brakes trying to turn the hub against the rotation of the rim.

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Old 08-18-20, 04:19 PM
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Originally Posted by phughes
Yes, but the point is that there is MORE torsional torque exerted on the spokes with a hub type brake.
Okay, let’s try a different tack. How much “MORE”? 50% more? 100% more? 10 times more? How much?

Let’s, for the sake of argument, go with the value provided by Jobst of about 5%. Velocity says on their rims to use up to 130 kgf. A 5% increase in tension at the spoke bed would result in an absolutely whopping increase of 3 kgf. Wow! That is just going to shatter the rim Velocity recommends 1010 N (110 kgf) on the low end which translates to 1130N (115 kgf). Again, not exactly shattering.

And, to cover the bases, let’s say the tension increase by 50%. The 110kgf would increase to 165kgf which might cause problems with the rim but disc rims would need to far beefier than they currently are to withstand that kind of (unbelievable) increase. Mavic sells the Open Pro in both disc and rim versions. The weights are the same as are the profiles.
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Old 08-18-20, 04:45 PM
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Originally Posted by cyccommute
Mavic sells the Open Pro in both disc and rim versions. The weights are the same as are the profiles.
The reason why both rim brake and disc brake version of a rim model may have the same extrusion profile is for economics. Why spend several thousands of dollars for a second set of extrusion dies when you can use a single set of extrusion dies for both rim and disc? The point is that disc brakes will apply greater forces to the spoke holes (including an additional torque load, which rim brakes do not have, which makes radial lacing no good for disc brakes). By how much extra force? You figure it out, or ask the manufacturer who has to have calculated this.

And actually, for rim models that have the same extrusion profile for both rim brake and disc brake versions, the disc brake version will actually be a little more heavier than the rim brake version, because the rim brake version would have some of the sidewall material milled away. If you assume 640mm diameter with a 16mm sidewall, with 0.2mm material removed on each side, that's [(320mm^2 x π) - (312mm^2 x π)] x 0.2mm x (0.0027g/mm3 density of aluminum) x (2 sides) = 17 grams.

1mm extra spoke bed thickness on the rim is probably around 30g to 35g extra mass for the above example.

Last edited by tomtomtom123; 08-18-20 at 05:02 PM.
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Old 08-18-20, 05:27 PM
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Originally Posted by tomtomtom123
Someone please lock the thread to stop the spread of misinformation.
You do know that you aren’t forced to read this thread. I haven’t received any messages saying that I have to read it. I assume you haven’t either.

I believe everyone should stop trying to convince the people who don't believe that spokes and spoke holes experience greater forces with disc brakes than rim brakes. It's pointless to keep on trying. It's like trying to convince someone who believes the earth is flat, that in fact it's round.
Fine. You want to say that the spokes and spoke holes experience greater force then how much greater is the force? Give me a number. I’ve already laid out the amount that I think it is. What’s your estimate? And how much more force would the spokes have to undergo to cause the rim to fail? Is an increase in force on the spoke from 3kgf going to break the rim? If that were the case, rim manufacturers would give more detailed specifications on each rim type. Velocity’s suggestion of 110 to 130 kgf on spoke tension is for every rim they make. They don’t say to use 110 on rim brake models and 130 on disc brake models. They give a simple blanket value for every rim.

Frankly, I find the suggested values for spoke tension to be mostly useless because they aren’t more specific.

Finally and again, I can’t find any rims that are listed on websites as being “for rim brake only”. I can’t find that string of words through a Google search that doesn’t just lead back to here for the most part. If it were as important as some have tried to make it out to be here, I’d expect to see a warning everywhere.
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Old 08-18-20, 05:51 PM
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Originally Posted by tomtomtom123
The reason why both rim brake and disc brake version of a rim model may have the same extrusion profile is for economics. Why spend several thousands of dollars for a second set of extrusion dies when you can use a single set of extrusion dies for both rim and disc? The point is that disc brakes will apply greater forces to the spoke holes (including an additional torque load, which rim brakes do not have, which makes radial lacing no good for disc brakes). By how much extra force? You figure it out, or ask the manufacturer who has to have calculated this.
Look, the “radial lacing” thing is a red herring. No, no one is going to use radial lacing with disc brakes. That has nothing...not a damned thing!...to do with the load on the rim and how the rim reacts to that load.

I have “figured it out” based on the estimate by Brandt and the “supposed” increase is tiny. Do you really think that 1 to 3 kgf is significant? The variance due to tension of the wheel is greater than that. The range of suggested tensions is far greater than that.

And, yes, I understand why Mavic would use the same die. But the point is that they use the same die. The (supposed) change in loading doesn’t seem to concern them.

And actually, for rim models that have the same extrusion profile for both rim brake and disc brake versions, the disc brake version will actually be a little more heavier than the rim brake version, because the rim brake version would have some of the sidewall material milled away. If you assume 640mm diameter with a 16mm sidewall, with 0.2mm material removed on each side, that's [(320mm^2 x π) - (312mm^2 x π)] x 0.2mm x (0.0027g/mm3 density of aluminum) x (2 sides) = 17 grams.
You are going the wrong way. The disc version is a lighter than disc version because it generally doesn’t have the brake track. The brake track isn’t needed so why not remove it and sell the rim as lighter. An old example is the Mavic XC717. The disc version weighed 395g. The rim version weighed 420g. These rims had a significantly different rim profile due to the removal of the brake track in the disc version.

1mm extra spoke bed thickness on the rim is probably around 30g to 35g extra mass for the above example.
Got an example where the spoke bed is a millimeter thicker in the disc version than the rim version?
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Old 08-18-20, 05:51 PM
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Originally Posted by crankholio
I understand "for disc brakes only" or "compatible with rim brakes", but I'm not understanding what property a rim would have that would make it incompatible with disc brakes. Seems like if you've got a hub that supports disc brakes, there shouldn't be an issue.

What am I missing?
Spokes.

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Old 08-18-20, 05:52 PM
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Originally Posted by cyccommute
Fine. You want to say that the spokes and spoke holes experience greater force then how much greater is the force? Give me a number. I’ve already laid out the amount that I think it is. What’s your estimate? And how much more force would the spokes have to undergo to cause the rim to fail? Is an increase in force on the spoke from 3kgf going to break the rim? If that were the case, rim manufacturers would give more detailed specifications on each rim type. Velocity’s suggestion of 110 to 130 kgf on spoke tension is for every rim they make. They don’t say to use 110 on rim brake models and 130 on disc brake models. They give a simple blanket value for every rim.
I think you're missing the point. In your previous posts, you claimed that the forces on the spokes and spoke holes will be identical for both rim brake and disc brake models. You also said that rim brakes would transfer torque to the spokes. Both of these claims are wrong. That is what I've been trying to explain.

I've already given calculations in the previous posts with the simplified see-saw diagram, in which proportions can be easily calculated. I estimated at least 4 times (edit: 8 times, could be off, see latest post with calculations) more additional extra load on the trailing spokes behind the axle for disc brakes compared to rim brakes when braking while decelerating. How much extra load on top of the pretension of the spokes? I don't know, and I'm not going to try to calculate an actual number. You can try it if you want to know. However, if you brake to decelerate at the same rate as you accelerate while pedaling, then the extra load while braking with a disc would be similar to the extra load when pedaling. But I assume that normally the rate of deceleration can be much greater than acceleration, especially if you brake the front wheel hard enough to lock it and skid (or just before that point).

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Old 08-18-20, 06:04 PM
  #67  
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Originally Posted by cyccommute
And, yes, I understand why Mavic would use the same die. But the point is that they use the same die. The (supposed) change in loading doesn’t seem to concern them.
You are going the wrong way. The disc version is a lighter than disc version because it generally doesn’t have the brake track. The brake track isn’t needed so why not remove it and sell the rim as lighter. An old example is the Mavic XC717. The disc version weighed 395g. The rim version weighed 420g. These rims had a significantly different rim profile due to the removal of the brake track in the disc version.
Got an example where the spoke bed is a millimeter thicker in the disc version than the rim version?
You've completely missed the point. You claimed that some model of rim had the same profile for both the rim brake and disc brake version. This implies using the same extrusion die. If both versions of rims use the same extrusion die, no (financially sane) company would waste time and money to mill away the sidewall for the disc brake version to reduce it's weight and then polish the surface.

You claim that a model of rim has a lighter disc brake version than the rim brake version because the sidewall of the disc brake version is thinner. In that case, no financially sane company would use the same extrusion die for both versions of the rim, because it would be cheaper (and less time consuming) to use 2 sets of dies with different profiles than to use the same die and have to mill away material for the disc brake version. Therefore, a model of rim that is lighter with the disc brake version than the rim brake version would most likely use different extrusion profiles for each version (not the same profile).

The calculation for weight of 1mm thickness of spoke bed was to explain why a company may forego the idea of using 2 different sets of extrusion dies, instead of spending money on an extra set of extrusion dies for a thinner profile for the minimal amount of weight savings, and simply use the same thicker and stronger extrusion profile for both rim brake and disc brake versions,

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Old 08-18-20, 06:11 PM
  #68  
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Originally Posted by tomtomtom123
I think you're missing the point. In your previous posts, you claimed that the forces on the spokes and spoke holes will be identical for both rim brake and disc brake models. You also said that rim brakes would transfer torque to the spokes. Both of these claims are wrong. That is what I've been trying to explain.

I've already given calculations in the previous posts with the simplified see-saw diagram, in which proportions can be easily calculated. I estimated at least 4 times more additional extra load on the trailing spokes behind the axle for disc brakes compared to rim brakes when braking while decelerating. How much extra load on top of the pretension of the spokes? I don't know, and I'm not going to try to calculate an actual number. You can try it if you want to know.
Sorry but I find 4 times additional load to be extremely high. That would stress spokes enough to the breaking point and would rip spokes out of a spoke bed with even several millimeter of aluminum. 4 times the 130kgf would be in excess of 520 kgf. On an area basis, that would convert to almost 8000 psi. The rim would need a spoke bed of around 8mm of steel to withstand that force. Assuming 1/3 strength for aluminum, the spoke bead would need to be 24mm thick or about an inch. No rim is that thick.

I’ve already tried calculating a number. It’s your turn.
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Old 08-18-20, 06:23 PM
  #69  
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Originally Posted by tomtomtom123
You've completely missed the point. You claimed that some model of rim had the same profile for both the rim brake and disc brake version. This implies using the same extrusion die. If both versions of rims use the same extrusion die, no company would waste time and money to mill away the sidewall for the disc brake version to reduce it's weight and then polish the surface.
No, I haven’t missed any point. Yes, they use the same extrusion die for both rims. One isn’t milled but milling isn’t going to remove much material because the wall of the rim is already thin and removing more material results in less useful life. The amount of material remove is on the order of fractions of a millimeter which translates in to fractions of a gram.

You claim that a model of rim has a lighter disc brake version than the rim brake version because the sidewall of the disc brake version is thinner. In that case, no financially sane company would use the same extrusion die for both versions of the rim, because it would be cheaper (and less time consuming) to use 2 sets of dies with different profiles than to use the same die and have to mill away material for the disc brake version. Therefore, a model of rim that is lighter with the disc brake version than the rim brake version would most likely use different extrusion profiles for each version (not the same profile).
The profile of the XC717 disc version and the rim version differ only in the height of the side wall. The profile is the same eccept that the rim version has a higher wall for the brake trackl and is thus heavier.

The calculation for weight of 1mm thickness of spoke bed was to explain why a company may forego the idea of using 2 different sets of extrusion dies to save money, and instead use the same thicker and stronger extrusion profile for both rim brake and disc brake versions, instead of spending money on an extra set of extrusion dies for a thinner profile for the minimal amount of weight savings.
And now you are missing the point. Using the same extrusion profile for both the disc and rim version says that they aren’t concerned about any supposed need for a stronger rim in the disc version. In fact, they could have saved themselves some stocking costs by just selling one version.

As for the minimal weight savings, that’s what I’ve been saying all along. The difference in strength is going to be marginal between a disc and a rim version because the difference in forces (if they even exist) are minimal. There is no need to mark a rim as “for rim brake use only” other than for legal reasons...and even those are rather ridiculous.
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Old 08-18-20, 06:36 PM
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Originally Posted by crankholio
I understand "for disc brakes only" or "compatible with rim brakes", but I'm not understanding what property a rim would have that would make it incompatible with disc brakes. Seems like if you've got a hub that supports disc brakes, there shouldn't be an issue.

What am I missing?
For a second time... spokes.

If you have a low spoke rim, the manufacturer doesn’t want you, or a shop, to lace it to a disc hub.

You might need 28h or 32h depending on the manufacturer’s analysis.

I’m guessing that since disc wheels started out on mountain bikes it wasn’t really an issue since no one is going to do 3ft drops on 16 spoke wheels. Now that they are on road bikes, it is a different animal as there will be those who will want to shed weight and use a low spoke rim.

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Old 08-18-20, 07:01 PM
  #71  
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Originally Posted by cyccommute
And now you are missing the point. Using the same extrusion profile for both the disc and rim version says that they aren’t concerned about any supposed need for a stronger rim in the disc version. In fact, they could have saved themselves some stocking costs by just selling one version.

As for the minimal weight savings, that’s what I’ve been saying all along. The difference in strength is going to be marginal between a disc and a rim version because the difference in forces (if they even exist) are minimal. There is no need to mark a rim as “for rim brake use only” other than for legal reasons...and even those are rather ridiculous.
It's the other way around. If they decided to use a single extrusion die, then they would not design a weaker profile for rim brakes and then use the same version for disc brake. Instead they would design a stronger profile for disc brake and then use the same profile for rim brakes.

No, they would not sell one exact same version of rim for both rim brake and disc brake versions. This is because milling the sidewall for the brake track costs time and money. They would not sell a milled sidewall for disc brakes, because there is extra machine time, tooling expenses (which wear out and have to be replaced), and payment for a worker to place the rim into the milling machine and flip it onto the other side. Instead they would sell one version with a milled wall for rim brakes, and a non-processed version for the disc brake (cheaper too). That is why, if they did use a single extrusion die for both versions, the rim brake version would be lighter because material would be removed from the sidewall, while the disc brake version would not have any additional material removed (other than the spoke and valve holes). If the disc brake version was actually lighter because of thinner sidewalls than the rim brake version, then they would have used 2 different extrusion dies with different profiles.

Also 1mm extra thickness to the spoke hole bed in the rim for only 35g of extra mass is actually a big increase in strength against cracking of the rim at the holes. My cheap $15 rim is only 2mm thick. 1mm extra would increase the depth of the hole by 50%. I don't know how this would affect the increase of strength for the the nipple to bear on, but maybe if we assume that the load radiates out at a 45 degree angle from one side of the hole to the other side, that's an additional square root of 2. So that extra 1mm of thickness may increase the distance that the load travels through the material by 70%. Whether or not the extra strength is needed (or how much is needed), I don't know, because I don't know exactly how much greater is the total load on the spokes with a disc brake.
​​
A rim might also utilize eyelets in the spoke holes to better distribute the load around the holes instead of increasing the thickness of the bed.

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Old 08-18-20, 07:03 PM
  #72  
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Originally Posted by cyccommute
Sorry but I find 4 times additional load to be extremely high. That would stress spokes enough to the breaking point and would rip spokes out of a spoke bed with even several millimeter of aluminum. 4 times the 130kgf would be in excess of 520 kgf. On an area basis, that would convert to almost 8000 psi. The rim would need a spoke bed of around 8mm of steel to withstand that force. Assuming 1/3 strength for aluminum, the spoke bead would need to be 24mm thick or about an inch. No rim is that thick.

I’ve already tried calculating a number. It’s your turn.
Not 4 times the total load. 4 times (edit: 8 times, could be off, see latest post with calculations) the additional load from the braking (on top of the pretension).
(edit: after thinking more, disc brakes don't create radial loads on the axle, so no radial loads get transmitted to the spokes)

I've tried simplifying the see-saw example even further.
(edit: ignore the blue arrow for radial load on disc brake spoke, in the diagram, but it is still relevant for showing the relationship between the different braking force)


The (additional) radial load from decelerating while braking on the spokes behind the axle would be 2.5 times greater from the disc brake than the rim brake, assuming that the disc is 1/4 the diameter of the rim (and we ignore the thickness of the tire).

There is additional torque load from decelerating while braking with a disc brake, on the trailing spokes on the disc side (and a little bit on the drive side). If you look at the previous examples, I calculated that if you add this torque load to the radial load, then the trailing spokes on the disc side behind the axle would experience 4.3 times (edit: 8 times, could be off, see latest post with calculations) greater (additional) load from decelerating while braking with a disc brake than (radial load from) a rim brake, but this is making a lot of assumptions, like the number of spokes, and the proportion between the diameter of the disc to the diameter of the effective spoke hole circle (depends on the exit angle of the spoke which is why a greater number of spoke crosses is important. radial lacing cannot take torque on the spokes).

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Old 08-18-20, 07:24 PM
  #73  
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Originally Posted by 70sSanO
Spokes.

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Old 08-18-20, 07:36 PM
  #74  
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Originally Posted by cyccommute
There is no rim design that I can think of that would be incompatible with a hub mounted disc brake. The forces on the rim are small. The spokes do most of the work. There are legitimate reasons for not using a disc rim with a rim brake...the sidewalls have been reduced and there is no brake track...but the opposite isn’t true of rim brake rims. Rims for rim brakes tend to be heavier because the brake track adds to the weight but, if anything, that would add strength to the rim, not take it away.
let's see point by point what happens while braking and show me when am i getting it wrong :
  • . braking in general uses the fork to slow down the wheel
  • . braking power goes from the fork through the wheel to the ground
    • . if its a disc brake, braking forces get from the hub, through the spokes to the rim and create pulling forces on the spoke bed
      • the path taken by the forces is longer
      • braking power creates spoke tension
      • momentum also creates tensinon
    • . if it's a rim brake, braking forces create tangential compression in the rim (as quoted by Bike Gremlin from jobst brandt's book )
      • the path taken by the forces is shorter
      • braking power creates compression in the rim
      • momentum creates tension in spokes
  • therefore, spoke tension is smaller in rim brakes.
  • disc brakes require a stronger spoke bed.

can you explain at what point am i missing the wholistic picture of the wheel ?
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Old 08-18-20, 07:48 PM
  #75  
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I'm enjoying this discussion of mechanics and engineering, but let's be honest: no way in hell is a rim brake rim not able to handle the forces of a disc brake system. It's erroneous to label it that way. It's a mistake.
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