Why would a rim say "for rim brakes only"?
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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?
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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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?
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?
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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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:
- 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
- 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).
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
https://en.wikipedia.org/wiki/Talk%3...kes_and_spokes
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
https://www.sheldonbrown.com/brandt/brakes.html
*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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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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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.
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.
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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I'm scratching my head as to why this took 3 pages of posts.
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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
Continue debating what others have already proven. I will watch from the sidelines.
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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.
I'm scratching my head as to why this took 3 pages of posts.
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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.
I'm scratching my head as to why this took 3 pages of posts.
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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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:
Disc brakes apply greater
(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.
Last edited by tomtomtom123; 08-19-20 at 04:30 PM.
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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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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.
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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.
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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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.
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.
1mm extra spoke bed thickness on the rim is probably around 30g to 35g extra mass for the above example.
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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?
What am I missing?
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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'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
Last edited by tomtomtom123; 08-19-20 at 04:31 PM.
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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 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 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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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.
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.
I’ve already tried calculating a number. It’s your turn.
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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.
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 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.
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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Plan Epsilon Around Lake Michigan in the era of Covid
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#70
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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?
What am I missing?
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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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.
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.
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.
Last edited by tomtomtom123; 08-18-20 at 09:11 PM.
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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.
I’ve already tried calculating a number. It’s your turn.
(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)
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
Last edited by tomtomtom123; 08-19-20 at 04:34 PM.
#74
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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.
- . 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
- . 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
- 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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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.