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Old 05-22-10, 07:27 PM   #1
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Question on geometry

I'm not a framebuilder but I figure good framebuilders would know the answer to this question better than anyone else.

I ride a '73 Raleigh Gran Sport, Reynolds 531 DB, with 27" wheels. I recently switched from 28mm to 25mm tires and noticed that the bike felt twitchy. Of course lighter tires would do that perhaps and it may have been my imagination because I'd spent time riding an even older bike with more laid back geometry. But the smaller tires create an effectively smaller wheel and that got me thinking about the effect of switching to 700c wheels.

Trail is the distance between the center of the tire contact patch and the point where the steering axis intersects the ground. The rake of the fork serves the purpose of reducing trail. Now, if you switched to smaller wheels the trail will decrease. For a 72deg head angle, for example, the 4mm smaller radius will reduce trail by 1.3mm. That sounds like a lot, and here's why.

Were there no rake, the trail and radius would change in proportion, so theoretically (I would think) the handling of the bike wouldn't change. But trail has already been reduced by the rake, i.e. the rake is a constant subtracted from the radius times the cotangent of the head angle. Numerically the change in trail is a much greater percentage than the percentage change in radius.

I would assume that a certain amount of trail is desirable because too little would make the bike very quick to change direction, in a word. twitchy. So the switch to 700c wheels would seem to change the bike's handling in that direction. In other words, it's a switch one should do only after careful consideration.

Yay or nay? Am I out to lunch? I know the math, but I'm not sure how it translates to bike handling. Is the effect maybe just so small that it won't make any real difference?

Thanks!
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Old 05-23-10, 01:57 PM   #2
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There's also a phenomenon known as "pneumatic trail". Essentially, a wider tire exerts a more pronounced stabilizing effect on a bike's steering. My guess is that this is what you're feeling. The steering geometry of the GS, as with most bikes of that era, was optimized for the common non-racing tire width of the day: 32 mm or so. Switching to a 700x32c or 28c will probably have LESS impact on handling than switching from 27x1 1/8 (28mm) to 27x1 (25mm).

Free advice, take it for what it's worth...

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ps - Going to a narrower, higher-pressure tire isn't necessary (or always desirable) for better performance. Given two tires OF EQUAL QUALITY AND SIMILAR DESIGN, the wider, lower-pressure tire will actually be faster in most real-world situations (chipseal, rough roads, etc), as well as being more comfortable, longer-lasting and better-cornering.
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Old 05-23-10, 05:27 PM   #3
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There's also a phenomenon known as "pneumatic trail".
Interesting. I've never heard that term or heard of that concept. I wonder what physical factor causes it.

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ps - Going to a narrower, higher-pressure tire isn't necessary (or always desirable) for better performance. Given two tires OF EQUAL QUALITY AND SIMILAR DESIGN, the wider, lower-pressure tire will actually be faster in most real-world situations (chipseal, rough roads, etc), as well as being more comfortable, longer-lasting and better-cornering.
That's certainly not the common opinion even here in BF. For example someone in one thread, possibly in C&V, mentioned how 25mm was the "sweet spot" for tires in my area (due to road conditions, partly). A good friend is an ex-bike racer and ex-bike mechanic and he tells me in no uncertain terms that narrower higher pressure tires is the lower-effort way to go. Of course, comfort isn't part of the race equation. So I'd be delighted if you could expound on your statement.

I just got back from 9 miles around town on new 25mm Pasela TGs pumped up to 115lbs, and I believe they roll more easily than the 28mm of the same tire. But then, what do I know? I never pumped the 28s up that high. And the 25s pumped up like that do feel harder.
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Old 05-23-10, 05:56 PM   #4
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I know the math, but I'm not sure how it translates to bike handling. Is the effect maybe just so small that it won't make any real difference?

Thanks!
You got it, a 1.3mm change in trail is imperceptable. As a matter of fact, I'd say anything below a 10mm change can't be reliably felt/interpreted.
For those of you that doubt, build the forks and you'll see.

Really though, trail is just a part of the handling equation- wheel base/front center changes have a more pronounced effect.
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Old 05-24-10, 02:45 PM   #5
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trail is over thought in many cases
as the be all and end all
74 HTA/40mm
73 HTA/45mm
72 HTA/50mm
all give the same trail
but different geos
and they all do not feel the same in a bike if all the rest of the geo is the same
there is so much other stuff going on
position first
bike design under neath that

this subject is fraught with peril when discussed in forums
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Old 05-24-10, 11:36 PM   #6
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... I just got back from 9 miles around town on new 25mm Pasela TGs pumped up to 115lbs, and I believe they roll more easily than the 28mm of the same tire.
More than likely due to the increased inflation pressure, and not tire width. As Schwalbe states:

"The answer to this question lies in tire deflection. Each tire is flattened a little under load. This creates a flat contact area. At the same tire pressure, a wide and a narrow tire have the same contact area. A wide tire is flattened over its width whereas a narrow tire has a slimmer but longer contact area. The flattened area can be considered as a counterweight to tire rotation. Because of the longer flattened area of the narrow tire, the wheel loses more of its "roundness" and produces more deformation during rotation. However, in the wide tire, the radial length of the flattened area is shorter, making the tire "rounder" and so it rolls better."

http://www.schwalbetires.com/tech_in...resistance#why

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Old 05-26-10, 09:57 AM   #7
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Interesting. I've never heard that term or heard of that concept. I wonder what physical factor causes it.
Short answer: Drag on the contact patch of a rolling tire causes the centroid of pressure to be behind the centroid of the contact patch. This displacement functions as an adder to the geometric trail. That's pneumatic trail. It's more with a softer tire and I think with a wider tire.

I think there's something else that affects twitchiness: harder tires have less contact area, so there is less scrubbing friction impeding the rotation of the fork assembly as forces are applied to the handlebar, and in other ways.
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Old 05-26-10, 11:29 AM   #8
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Short answer: Drag on the contact patch of a rolling tire causes the centroid of pressure to be behind the centroid of the contact patch.
Thank you. (Cool! Someone who uses the word "centroid"! )
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Old 05-26-10, 07:09 PM   #9
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I think there's something else that affects twitchiness: harder tires have less contact area, so there is less scrubbing friction impeding the rotation of the fork assembly as forces are applied to the handlebar, and in other ways.
Yes.

Also, handle bar width (or more precisely, grip width) can be altered to help subdue input sensitivity. Wider to reduce it - narrower to increase it.
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Old 05-27-10, 05:36 AM   #10
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Really though, trail is just a part of the handling equation- wheel base/front center changes have a more pronounced effect.
What does front center do to affect a bike? Mostly just for the change to F/R weighting? Stem length for a certain reach distance?
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Old 05-27-10, 08:37 AM   #11
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What does front center do to affect a bike? Mostly just for the change to F/R weighting? Stem length for a certain reach distance?
I'm not sure what the term front center means.

However stem reach is has a dynamic effect that is not just pure static geometry. A shorter stem brings the weight of the arms closer to the steering axis, even behind it for traditional non-drop bars which curve backwards. A balance adjustment to the steering to correct a lean or wind gust to a particular side throws the arms' weight to the opposite side if their weight is behind the steering axis. This contributes the lean one is trying to correct. If the arms' weight is in front of the steering axis the dynamic effect throws the bike away from the lean, helping the correction instead of contributing to it. Someone learning to ride will find balance much harder even if he/she thinks the upright seating position is more comfortable.
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