Author Topic: Understanding bicycle frame geometry  (Read 4626 times)

Andre Jute

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Understanding bicycle frame geometry
« on: December 20, 2015, 09:19:08 pm »
We have several thesis-quality threads on this forum on bike geometry and the resulting dynamics, and the Thorn catalogue is the world's leading authority on fitting a bike to the cyclist. Here's a simplified view of how the bike shape feeds into all this, with all the parts labelled on clear sketches, and explained in jargon-free language:

http://cyclingabout.com/understanding-bicycle-frame-geometry/

So good I wished I'd written it myself, than which no higher praise.

(sgnd) Baffled N. O. More

energyman

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Re: Understanding bicycle frame geometry
« Reply #1 on: December 20, 2015, 09:41:47 pm »
Riding into a head wind up a gentle slope today on my RST I appreciated the "bike fit" session some years ago.  Now I (nearly) understand what Lisa was on about !
Oh yes, Happy Christmas (Not Happy Holiday or Happy Winter Holiday) to you all.

Andre Jute

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Re: Understanding bicycle frame geometry
« Reply #2 on: December 20, 2015, 10:10:06 pm »
Riding into a head wind up a gentle slope today on my RST I appreciated the "bike fit" session some years ago.  Now I (nearly) understand what Lisa was on about !
Oh yes, Happy Christmas (Not Happy Holiday or Happy Winter Holiday) to you all.

Man is a clever animal who invents tools to magnify his reach and power. (As I said sniffily when I got thrashed on a fives court and then took the guy who did onto a tennis court and thrashed him in his turn.) The bicycle is a tool that answers to the same ergonomics.

Merry Christmas, especially to the Calvinists. You can be grim for the rest of the year.

John Saxby

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Re: Understanding bicycle frame geometry
« Reply #3 on: December 23, 2015, 02:40:48 am »
Andre,

Reading the cyclingabout blog, I find the article on frame geometry to be well written as always -- economical, accessible, and clear.

But, unless I'm wholly mistaken, I think they've got their basic terms backwards, thus:

1)    Their diagrams are correct: Diagram #1 shows higher trail to be a product of forks with limited curl towards the dropout, so that the vertical drops from close to the start of the curl, producing more trail.  #2 shows something closer to a touring fork (such as my Raven), which is a low-trail set-up.  All OK so far, then.

2)    In the text that follows, the boldface says that "Less trail equates to faster steering," and "More trail equates to slower steering."  Surely this should be reversed?  That is, the touring-fork bikes have lower trail and slower steering.

3)    The same error (as I see it), is replayed throughout the article.

I read this shortly after reading one of the articles in "Bicycle Quarterly" on the virtues of low-trail randonneuring bikes, with their slow, deliberate steering, allowing the rider to steer by lean as well as by bar inputs.

Have I got myself in a tangle? Or did the blogista switch his terms around?

Matt2matt2002

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Re: Understanding bicycle frame geometry
« Reply #4 on: December 23, 2015, 06:14:51 pm »
André!
Fives. That takes me way back to my mis spent youth.
What a fantastic game. I well recall the stinging pains in my palms.
Thanks for the memories.

Matt
Never drink and drive. You may hit a bump  and spill your drink

Andre Jute

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Re: Understanding bicycle frame geometry
« Reply #5 on: December 23, 2015, 06:30:50 pm »
...unless I'm wholly mistaken, I think they've got their basic terms backwards, thus:

1)    Their diagrams are correct: Diagram #1 shows higher trail to be a product of forks with limited curl towards the dropout, so that the vertical drops from close to the start of the curl, producing more trail.  #2 shows something closer to a touring fork (such as my Raven), which is a low-trail set-up.  All OK so far, then.

2)    In the text that follows, the boldface says that "Less trail equates to faster steering," and "More trail equates to slower steering."  Surely this should be reversed?  That is, the touring-fork bikes have lower trail and slower steering.

3)    The same error (as I see it), is replayed throughout the article.

...

Have I got myself in a tangle? Or did the blogista switch his terms around?

I'm sorry to say, John, that the blogger's text is correct, and you're being misled by drawings intended to make a theoretical point i.e. what happens when you change differently raked forks on a bike with the head angle held constant. Follow with me:

Trail is the distance the centre of the front tire contact patch lags behind the steering axis interception of the ground. (This is often stated as the distance at ground level between a vertical line through the axle centre and the steering axis but in fact the contact patch is not necessarily, nor usually, directly under the axle centre. It's a nitpicky point but we're often measuring small fractions here with decidedly non-linear effects, so it is something to be aware of. The axle centre is merely a shortcut. Motorcyclists speak of "mechanical trail" to distinguish this point. There, now you can baffle the Harley-Davidsons...)

This causes a castor effect, as on a shopping trolley but the other way round, in which the bicycle front wheels are set back (technically: "offset") behind the vertical steering axis, so that the force of force of gravity tries to center the wheel in the direction of travel.

How much this self-centring force is, and the force at the handlebars needed to overcome the effect of trail, which is the result of offset, is what gives different bikes different inherent stability and loadbearing and steering characteristics.

A bike with zero trail, i.e. produced steering axis and contact patch centre coinciding exactly, and thus zero self-centring, will be extremely quick-turning but also entirely unpredictable (technically it will have 0% roadholding and require 100% handling, heh-heh), in a word, lethal.

The larger the distance between the produced steering axis at the ground and the contact patch, the stronger the centring force becomes. Another way of looking at it is that the cyclist turning the handlebars is actually lifting the bike, its luggage and his own weight up over the height differential between the contact patch and the produced steering axis intersection with the ground, for which the algorithm will give everyone migraine, except maybe Jim Kukula. It is enough to know that larger trail requires larger disturbing force to drive the bicycle off the straight line, and less to return it to that line, hence a large trail is a requirement for stability.

Consider a bike with a fixed head angle in which you can change only the fork. Offset the hub forward, and you reduce  trail.  Offset the hub backward and you increase trail. This is all that the illustration which misled you shows.

Let's make another theoretical case, a straight fork but you can vary the head angle on the bike. The slacker the head angle, the larger the trail, with all that follows, the more upright the head tube, the smaller the trail, ditto.

Now we come to the crux of the matter. Nippy, even twitchy bikes, usually have LESS fork rake than stable tourers. But when you add in the slack headset angle of touring bikes to their fork rake, they have MORE trail as the combination of headset angle and fork rake.

If you were to take the drawing with the touring-type fork and angle it to an inclination more appropriate to a touring bike, you will see that it produces a larger trail than the drawing of a road bike type of fork on a road-angled head tube. Then the text will make more sense to you.

***

I regret to report that you haven't discovered a great contrary truth of bicycle dynamics, John. (I'd love to know the fellow who does.) But you've certainly made a good case for my original encomium being too enthusiastic if this blogger can mislead a thoughtful fellow like you.

« Last Edit: December 26, 2015, 09:49:13 pm by Andre Jute »

Andre Jute

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Re: Understanding bicycle frame geometry
« Reply #6 on: December 23, 2015, 11:39:40 pm »
It occurred to me as I went down the stairs to dinner that I could have shortcut the entire long explanation above with this albeit negative, observation:

When you're shown a stylized bicycle wheel and wish to discuss the trail, there are only three important details, and none of them is the shape of the fork. They are the fork rake, the radius of the inflated tyre (which isn't mentioned above but also increases trail proportionately) and the head tube angle.

Or, without knowing the shape of the fork, you can take three points from tyre circumference, axle centre offset and head tube angle and by plotting them from axle level mathematically or geometrically determine the trail at ground level. Designers use this process in reverse from desired trail to arrive at the steering axis inclination, which they then devide between head angle and fork rake according to the tubes they have or can get.

Thus, in this illustration from http://cyclingabout.com/understanding-bicycle-frame-geometry/
— the better short wheelbase fast touring bike, the one you want to be on after fourteen hours in the saddle with the hotel still ten miles up an Alpine cliff is the one on the left with the large trail even if the one on the right has the softer "touring fork".

The one on the right would need to have the head angle slackened considerably, so that the trail exceeds that of the lefthand illustration, to be a more stable bike. Then, because of the curve in the fork, it would also have a better ride.

The curvature of the fork may be good as a suspension element but as a trail element it comes into play (as a derived rake function) only in conjunction with head tube angle and tyre circumference.

You can never judge the stability of a bike from the shape of the fork alone.

John Saxby

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Re: Understanding bicycle frame geometry
« Reply #7 on: December 26, 2015, 11:16:12 pm »
Thanks, Andre, that's very helpful. I understand your explanation of "mechanical trail" from my prolonged dalliance with motorcycles -- the "reading" is more straightforward in that domain, because telescopic forks are in line with the angle of the head tube. (I have no idea how things like a Greeves leading-link setup changes the reckoning...)

I'll go back to the original text and review it, but it strikes me that the original presentation of the fork + wheel diagrams needed to factor in the steering-tube angle (and the tire profile & patch, as you say); OR, if presenting only the fork shape, to add an "other things equal".

I'll also see if I can assess my two bikes' respective steering-tube angles and the respective offset of their forks, and let you know what I find. (They also have different wheel and tire sizes, so there may be just too many variables at play to allow an easy reckoning of trail.)  They do, however, have fork profiles comparable to those in the original presentation (the Raven having the more pronounced forward curl). The handling characteristics of the Eclipse are those of a lighter-weight light touring machine with carbon forks (nippy, especially unladen, but reasonably stable with a load), while the Raven is much more deliberate, and with a load, it's rock-solid. (I'd much rather be in the saddle of the Raven after 14 hours, but that's probably on account of a lot of other qualities of the bike, especially its better fit.) There's only about 750 gms difference between the two bikes, unladen, despite the fact that the Eclipse has a titanium frame & carbon forks. (When the Eclipse gets its winter "renewal", it will emerge with a set of steel forks with slightly greater offset -- will be interesting to see if the handling becomes closer to that of the Raven.)

Cheers,

John

Andre Jute

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Re: Understanding bicycle frame geometry
« Reply #8 on: December 27, 2015, 03:31:23 am »
You're baking the pudding as we speak, John. Don't forget to send the riding comparison once you have the new fork on the Eclipse. This will be very interesting.