Frame Geometry vs. Handling Characteristics
#126
Virgo
Thread Starter
Sorry, I don't have time to read this entire thing now. But from engineering standpoint.
The Trail (distance from where line through the steering bearings, to the contact patch (which is directly under front axle) determines the dynamic restoring force.
This is the force trying to straighten the front wheel due to forward motion.
The distance of the front axle in front of the line through the steering bearings is a lever arm, the angle of the steering bearings and that lever arm determine how much the frame is lifted by turning the handlebars. That is the static restoring force. It is proportional to the weight on the front tire too.
To make this short. you must have hit a pothole and bent the front fork. Even a tiny 1/8" bend will change both of these.
Moving the axle back by 1/8" lessens the static restoring force. Now the bike seems less stable at low speeds.
Moving the axle back by 1/8" lengthens the trail and increases the dynamic restoring force. Now the bike is harder to steer at higher speeds
.
Your muscles have been trained to expect a consistent behavior from the bike, and now they are messed up by inconsistent and opposite changes to the restoring force a different speeds.
If it was consistent it would not be so bothersome, but I think it is the relation of the low speed behavior to the high speed behavior that is messing with you.
Even if the front fork does not seem bent, I suspect you want to bend it back so the front axle is farther out front.
The Trail (distance from where line through the steering bearings, to the contact patch (which is directly under front axle) determines the dynamic restoring force.
This is the force trying to straighten the front wheel due to forward motion.
The distance of the front axle in front of the line through the steering bearings is a lever arm, the angle of the steering bearings and that lever arm determine how much the frame is lifted by turning the handlebars. That is the static restoring force. It is proportional to the weight on the front tire too.
To make this short. you must have hit a pothole and bent the front fork. Even a tiny 1/8" bend will change both of these.
Moving the axle back by 1/8" lessens the static restoring force. Now the bike seems less stable at low speeds.
Moving the axle back by 1/8" lengthens the trail and increases the dynamic restoring force. Now the bike is harder to steer at higher speeds
.
Your muscles have been trained to expect a consistent behavior from the bike, and now they are messed up by inconsistent and opposite changes to the restoring force a different speeds.
If it was consistent it would not be so bothersome, but I think it is the relation of the low speed behavior to the high speed behavior that is messing with you.
Even if the front fork does not seem bent, I suspect you want to bend it back so the front axle is farther out front.
#127
Virgo
Thread Starter
If the Raleigh fork is bent, it’s been that way since I got the bike 5 or 6 years ago. If the Fuji fork is bent, it way that way when I got it in July. If both forks are bent, I guess I’m bent too. Anybody have any leads on a 23” 73° parallel frame with a bent fork? Needs room for 28s and fenders. FORK MUST BE BENT, ALL I KNOW ARE BENT FORKS
In other news, lowering saddle height and moving rearward makes Raleigh cockpit a little too long so shorter stem may be needed. Maybe instead of buying a new stem I can bend it back and up to shorten the reach a little. I have a few spares. Should I heat it up real good before I try bending it?
Choo choo!
Since the Raleigh doesn’t have the fender clearance of the Fuji, to get fenders on it I had to use a spare set of hybrid 700c wheels and put some 25mm tires I had lying around on them. The rims are 27mm wide. Looks pretty funky. My 1 mile gravel stretch was “less enjoyable” on 25s at 100psi.
I want an old LeTour frame I think. In that nice orange color they did. I guess it wouldn’t HAVE to have a bent fork, beggars and choosers and all that.
Oh! I could do a budget 650b conversion and use bmx brakes! That’d be cute!
In other news, lowering saddle height and moving rearward makes Raleigh cockpit a little too long so shorter stem may be needed. Maybe instead of buying a new stem I can bend it back and up to shorten the reach a little. I have a few spares. Should I heat it up real good before I try bending it?
Choo choo!
Since the Raleigh doesn’t have the fender clearance of the Fuji, to get fenders on it I had to use a spare set of hybrid 700c wheels and put some 25mm tires I had lying around on them. The rims are 27mm wide. Looks pretty funky. My 1 mile gravel stretch was “less enjoyable” on 25s at 100psi.
I want an old LeTour frame I think. In that nice orange color they did. I guess it wouldn’t HAVE to have a bent fork, beggars and choosers and all that.
Oh! I could do a budget 650b conversion and use bmx brakes! That’d be cute!
#128
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Ah, don't let the bent forks get you down!
Assuming a fork is bent by a head on crash, the result is to reduce fork offset ("rake") and so increase trail. The same crash is likely to bend the frame, increasing the head tube angle.
All these things can be measured. If you have a frame you like, disregard the likelihood that it may be bent, and measure it as is.
But that's a hypothetical. I'd recommend trying a few frames that are guaranteed NOT bent, see how you like them.
Assuming a fork is bent by a head on crash, the result is to reduce fork offset ("rake") and so increase trail. The same crash is likely to bend the frame, increasing the head tube angle.
All these things can be measured. If you have a frame you like, disregard the likelihood that it may be bent, and measure it as is.
But that's a hypothetical. I'd recommend trying a few frames that are guaranteed NOT bent, see how you like them.
#129
Senior Member
It's my experience that pretty much every vintage steel frame is bent. They are fairly large and lightweight tubular structures that are used very hard. By the time they are thirty or forty years old they aren't that straight. If you do something crazy like riding them instead of looking at them they will get progressively less and less straight. Every time I have a frame aligned it rides a great deal better than it did before. Aligned as in on a frame table by a framebuilder. Or you can straighten them to 'good enough', as in good enough that the gears shift, the chain doesn't fly off, and it doesn't pull too hard to the side. Or you can ride them as is and then discuss the fine points of frame geometry.
It's just like tuning a piano. You keep doing it. A good pianist can play an out of tune piano and make great music. For anyone who uses the piano hard the instrument is noticeably further from tuning at the end of the performance than at the beginning. So you call the piano tuner back. Or the instrument slides downhill to junk.
It's just like tuning a piano. You keep doing it. A good pianist can play an out of tune piano and make great music. For anyone who uses the piano hard the instrument is noticeably further from tuning at the end of the performance than at the beginning. So you call the piano tuner back. Or the instrument slides downhill to junk.
#130
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Well, normal riding over any length of time or miles will not change the shape of a good steel (or any other material) frame. Only extraordinarily heavy impacts or other abnormal stresses that exceed the designed yield or fracture strength of the frame will do that.
#131
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I know, I am not sure if this thread is even discussing the original posts.
In Minnesota we have pothole season. If you hit a pothole hard enough to notice something right away, then it would have gotten fixed, usually. The lesser hits accumulate, go unnoticed. What happens is more of a slow realization that something is less than ideal. They could ride the bike for years that way. When they have another bike to compare is when they realize that it could or should be better. Most people start adjusting the adjustable things, searching for a solution. The fork will not appear bent, and even trying to measure it may not show it is bent. But, this is one thing that would make it harder to turn.
Another case is when a smaller wheel is put the front (for the weird look I suppose, it wasn't me that tried it). It really messed up the steering response and was nearly un-ride-able. Nearly the same effect but due to tilting the frame forward.
Putting more of the riders weight on the handlebars:
I would expect this to make the bike more difficult to turn. For one it increases the static restoring force which will make the steering slightly more heavy in the hands. If you just shifted forward in the seat, that heaviness would make it hard to judge the steering input. With more weight forward, the front tire now has to turn that weight too. My expectation would be that the front tire would be more likely to skid, or hop, during the more extreme events. My expectation somewhat depends on the fact they throw a great deal of sand about in the winter here, which a rider may encounter is small or large amounts.
The danger of spilling over the handlebars should be considered too. About 4 weeks ago I came around a slight bend and met 4 riders, one of who was alongside. He tried to stop suddenly to avoid hitting me. He managed to stop the bike in about a 10 feet. He came over the handlebars and did a face plant into the asphalt too. The rest of us stopped in about the same distance without such an incident. I suspect that his weight was a bit too far forward to safely handle the unexpected.
For the general reader,
because you never know how many bikes are out there with subtle bends in the their front forks from hitting things like potholes ::
Anyone who has ever hit a pothole hard, or gone down into one of the slots of a gutter grate, may want to check their steering balance, even if the front fort "looks" ok.
But, It is too hard to measure directly without serious equipment.
Much easier to find a curved line, ride down it and try to stay on the line.
Next, you borrow a friends bike and try the same thing, just to make sure it is not you that is the problem. (For the general public, those who do not have 2 bikes)
The friends bike logically should be harder to steer through this test because it is not as familiar as your own.
But, If the friends bike turns out to be easier to steer this way. then you could end up staring at your bike for long periods of time.
When you cannot stay on the line, especially if it is easy at one speed, but more difficult at slower or faster speeds, you should suspect the fork, (IMHO of course).
This is, of course, unless you have been adjusting the frame into unusual configurations, in which case you would probably want to return it to a normal configuration and
test it again, before bending anything.
Also, the general public should not be bending anything themselves unless they already know how to bend it in tiny amounts in a precise controlled manner (which generally they don't),
and know how to measure how much they have bent it.
Another case is when a smaller wheel is put the front (for the weird look I suppose, it wasn't me that tried it). It really messed up the steering response and was nearly un-ride-able. Nearly the same effect but due to tilting the frame forward.
Putting more of the riders weight on the handlebars:
I would expect this to make the bike more difficult to turn. For one it increases the static restoring force which will make the steering slightly more heavy in the hands. If you just shifted forward in the seat, that heaviness would make it hard to judge the steering input. With more weight forward, the front tire now has to turn that weight too. My expectation would be that the front tire would be more likely to skid, or hop, during the more extreme events. My expectation somewhat depends on the fact they throw a great deal of sand about in the winter here, which a rider may encounter is small or large amounts.
The danger of spilling over the handlebars should be considered too. About 4 weeks ago I came around a slight bend and met 4 riders, one of who was alongside. He tried to stop suddenly to avoid hitting me. He managed to stop the bike in about a 10 feet. He came over the handlebars and did a face plant into the asphalt too. The rest of us stopped in about the same distance without such an incident. I suspect that his weight was a bit too far forward to safely handle the unexpected.
For the general reader,
because you never know how many bikes are out there with subtle bends in the their front forks from hitting things like potholes ::
Anyone who has ever hit a pothole hard, or gone down into one of the slots of a gutter grate, may want to check their steering balance, even if the front fort "looks" ok.
But, It is too hard to measure directly without serious equipment.
Much easier to find a curved line, ride down it and try to stay on the line.
Next, you borrow a friends bike and try the same thing, just to make sure it is not you that is the problem. (For the general public, those who do not have 2 bikes)
The friends bike logically should be harder to steer through this test because it is not as familiar as your own.
But, If the friends bike turns out to be easier to steer this way. then you could end up staring at your bike for long periods of time.
When you cannot stay on the line, especially if it is easy at one speed, but more difficult at slower or faster speeds, you should suspect the fork, (IMHO of course).
This is, of course, unless you have been adjusting the frame into unusual configurations, in which case you would probably want to return it to a normal configuration and
test it again, before bending anything.
Also, the general public should not be bending anything themselves unless they already know how to bend it in tiny amounts in a precise controlled manner (which generally they don't),
and know how to measure how much they have bent it.
#132
aka Tom Reingold
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I've been thinking about this thread a lot, though I'm not sure I have any new insights. I own lots of bikes, and they all feel different, though I believe I have the fit adjustments of most of them dialed in well enough to work well for me.
But I went to Philly this weekend, and my friend loaned me one of his bikes for two 17-mile rides. (We rode from a suburb into the city for the Philly Bike Expo and then back in the evening.) I had trouble discerning what was unique to the design of the bike and separate it from the fit. The fit was very unusual, with the handlebars a lot higher than I'm used to. It took a long time to feel comfortable on it because of the fit, not because of the bike's design.
Then after we got back, he had me try two more of his bikes. One of them also had the handlebars high, but for some reason, it felt terrific. The other one had the handlebars much too low, and not surprisingly, the bike felt awful, but the awfulness was clearly from fit and not the bike.
But I went to Philly this weekend, and my friend loaned me one of his bikes for two 17-mile rides. (We rode from a suburb into the city for the Philly Bike Expo and then back in the evening.) I had trouble discerning what was unique to the design of the bike and separate it from the fit. The fit was very unusual, with the handlebars a lot higher than I'm used to. It took a long time to feel comfortable on it because of the fit, not because of the bike's design.
Then after we got back, he had me try two more of his bikes. One of them also had the handlebars high, but for some reason, it felt terrific. The other one had the handlebars much too low, and not surprisingly, the bike felt awful, but the awfulness was clearly from fit and not the bike.
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Tom Reingold, tom@noglider.com
New York City and High Falls, NY
Blogs: The Experienced Cyclist; noglider's ride blog
“When man invented the bicycle he reached the peak of his attainments.” — Elizabeth West, US author
Please email me rather than PM'ing me. Thanks.
#133
Member
Frame sink
Is anyone familiar with frame sink? This is the effect of rake, trail, head tube angle, and wheelbase, where as the front wheel is turned, the front of the bike lowers a tiny bit. It's significant because it requires effort to turn back to center and raise the bike back up against gravity. Supposedly this affects handling.
#134
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Is anyone familiar with frame sink? This is the effect of rake, trail, head tube angle, and wheelbase, where as the front wheel is turned, the front of the bike lowers a tiny bit. It's significant because it requires effort to turn back to center and raise the bike back up against gravity. Supposedly this affects handling.
#135
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On a nicely balanced motorcycle it feels like you can almost let go in a turn and the motorbike will hold the line, too much squat and it wants to fall into the turn, to much trail and it feels like you’re fighting the bike to get it to lean.
The tendencies are a lot more noticeable at highway speeds than bicycle speeds, but it’s still noticeable.
#136
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Is anyone familiar with frame sink? This is the effect of rake, trail, head tube angle, and wheelbase, where as the front wheel is turned, the front of the bike lowers a tiny bit. It's significant because it requires effort to turn back to center and raise the bike back up against gravity. Supposedly this affects handling.
Trail is a geometric function of rake and head tube angle, as is wheel flop, shown in the online calculator below. The effect of wheelbase on wheel flop is essentially insignificant, but certainly affects handling.
Bicycle Trail Calculator | yojimg.net
#137
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I did this a while back. My 3 main rides. All Italian. Top is 1933, middle is 1959, bottom is 2009. This picture shows the similar trail, over 75 years.
Geometry2 by iabisdb, on Flickr
This compares the geometries, and show the seat/bars relationship is essentially the same, within millimeters, but the BB & wheelbase is all over the place.
Geo_2 by iabisdb, on Flickr
I do have a "new" Italian bike from the mid 20s. I should do a drawing of it to compare.
Geometry2 by iabisdb, on Flickr
This compares the geometries, and show the seat/bars relationship is essentially the same, within millimeters, but the BB & wheelbase is all over the place.
Geo_2 by iabisdb, on Flickr
I do have a "new" Italian bike from the mid 20s. I should do a drawing of it to compare.
Can you please let me know how you did the above drawings?
Thanks a lot
#138
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Yes, checked with a plumb bob and tape measure, tip of saddle is 2 3/4" behind bottom bracket on both bikes. 21" between tip of saddle and center of bars on both bikes. This is accomplished with a 100mm stem on slack angle frame, 90mm stem on 73/73 frame, the top tubes are slightly different lengths.
Also, this wasn't asked but the angles I stated are measured, not nominal.
I spent some time comparing these angles against modern bikes a while ago and I agree with your statement that 72/71-ish seems to be used on CX bikes and 73/73 seems to be used more on road/racing bikes. The stack and reach of the slack angle bike are also shared with CX bikes. I never ride off road, so any advantages to the slack HT angle in that respect are never realized. I also never do any group rides.
Also, this wasn't asked but the angles I stated are measured, not nominal.
I spent some time comparing these angles against modern bikes a while ago and I agree with your statement that 72/71-ish seems to be used on CX bikes and 73/73 seems to be used more on road/racing bikes. The stack and reach of the slack angle bike are also shared with CX bikes. I never ride off road, so any advantages to the slack HT angle in that respect are never realized. I also never do any group rides.
#139
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It's my experience that pretty much every vintage steel frame is bent. They are fairly large and lightweight tubular structures that are used very hard. By the time they are thirty or forty years old they aren't that straight. If you do something crazy like riding them instead of looking at them they will get progressively less and less straight. Every time I have a frame aligned it rides a great deal better than it did before. Aligned as in on a frame table by a framebuilder. Or you can straighten them to 'good enough', as in good enough that the gears shift, the chain doesn't fly off, and it doesn't pull too hard to the side. Or you can ride them as is and then discuss the fine points of frame geometry.
It's just like tuning a piano. You keep doing it. A good pianist can play an out of tune piano and make great music. For anyone who uses the piano hard the instrument is noticeably further from tuning at the end of the performance than at the beginning. So you call the piano tuner back. Or the instrument slides downhill to junk.
It's just like tuning a piano. You keep doing it. A good pianist can play an out of tune piano and make great music. For anyone who uses the piano hard the instrument is noticeably further from tuning at the end of the performance than at the beginning. So you call the piano tuner back. Or the instrument slides downhill to junk.
#140
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The Geometry of Handling
I just happened to notice resurrection of this thread and thought this would be the perfect time and thread to post this Bicycling! 1980 article co-written by a frame builder and a mechanical engineer.
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WTB: American Cycling May - Aug, Oct, Dec 1966.
WTB: Bicycle Guide issues 1984 (any); Jun 1987; Jul, Nov/Dec 1992; Apr 1994; 1996 -1998 (any)
WTB: Bike World issue Jun 1974.
WTB: Slingshot bicycle promotional documents (catalog, pamphlets, etc).
WTB: American Cycling May - Aug, Oct, Dec 1966.
WTB: Bicycle Guide issues 1984 (any); Jun 1987; Jul, Nov/Dec 1992; Apr 1994; 1996 -1998 (any)
WTB: Bike World issue Jun 1974.
#141
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#142
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I am don't think that its really fit in this post but I am sure will be quite few people interested about this Bicycle Geometry 101
Thanks and respect to JohnDThompson!!!
Thanks and respect to JohnDThompson!!!
#143
Senior Member
63, where do you take frames to be aligned and get good results, in your area? I can't think of too many Michigan shops I would trust to do it correctly and whom are still in the game. I had a Trek aligned once by Ron Boi, and the improvement was magical. We used to have Mike Nobilette here in Ann Arbor, but that was >25 years ago. My local good shop went out of business less than a year ago. but they also were not 100%. I don't really have a problem sending or carting frames to Chitown to have a good alignment done.
Ron Boi has been a friend for fifty years. He is barely in business. He did a fork for me just a couple weeks ago. Each time I am less sure what will happen. Talk to him, he loves to talk. If he takes the job it will be good.
Andy Muzi and Tim Bouche at Yellow Jersey in Arlington, Wisconsin do most of my frame work. Have only known Andy 46 years.
You're right, it's magical. Bikes are fun when they are close enough. When truly straight they ride like magic.
#144
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Bikes are vertical, BBs are set at a particular spot. Picture taken of each bike.
Pictures brought into Illustrator, outlines created of each bike. I measured the diameter of each rim and scaled the Illustrator drawings to full size. I cross referenced the seat tube, top tube and wheelbase dimensions to check for accuracy. Everything was within 3mm. Then it is just a simple job of playing with combinations of the outline drawings.
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