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The catch-all bicycle geometry thread

Posted: 17 Oct 2018, 9:15am
by Samuel D
This is a place to discuss elements of bicycle geometry that you don’t feel warrant their own thread.

I’ll start by mentioning a recent Lennard Zinn podcast on steering geometry.

I didn’t know this, but Zinn did a physics degree and specifically studied steering geometry for his thesis (about an unrideable bicycle). And of course he then worked as a custom frame builder for many years. So he’s probably worth listening to on this topic.

The podcast is over an hour long, so try putting it on while doing some menial task around the house.

I was especially intrigued by the discussion of countersteering in MotoGP motorcycle racing (inside arm pushing in deep leans), and the steep head tube angles that he talks about on his own bicycles (though keep in mind he’s a very tall man).

Re: The catch-all bicycle geometry thread

Posted: 17 Oct 2018, 9:41am
by Brucey
countersteering is a standard way of muscling a motorbike into a turn, and is not confined to MotoGP riders. Folk can be doing the same thing on a bicycle and not even know it. Also shifting bodyweight can have a similar (but less dramatic) effect.

Tall folk have special issues with bike frames; it is a different (and often somewhat wibbly-wobbly) world up there! Anytime I have slung a leg over a very tall frame (with the saddle well down of course) I am struck by how different the bike feels even though the weight distribution and the steering geometry might be 'fairly normal'. Quite what it must be like to be sat six inches higher yet is difficult to imagine, but in (say) unicycles and recumbents it is for sure a world of difference. Some of my chums are tall and have all kinds of problems that shorter folk don't; for example standard cranks may feel very short to them, and (because wheelbases are not scaled with CoG height increase) every time they use the brakes hard they are in more danger of the rear wheel lifting.

cheers

Re: The catch-all bicycle geometry thread

Posted: 5 Nov 2018, 10:33pm
by StephenW
Hello Samuel D

I started listening to the podcast, although I only got through about 20 minutes. I'll listen to more of it when I have time.

Have you come across JBike6? It is a program in Matlab, which calculates the stability of bicycles based on wheelbase, weight distribution, trail, wheel interia etc. If you have access to Matlab, it is worth downloading and having a play with.

Re: The catch-all bicycle geometry thread

Posted: 6 Nov 2018, 8:42am
by Samuel D
StephenW: no MATLAB access, unfortunately.

Brucey wrote:countersteering is a standard way of muscling a motorbike into a turn, and is not confined to MotoGP riders. Folk can be doing the same thing on a bicycle and not even know it. Also shifting bodyweight can have a similar (but less dramatic) effect.

I realise we all countersteer to initiate a turn, but I hadn’t thought about the need to keep pushing the handlebar with the inside arm to hold the lean as Zinn says motorcycle racers do.

After listening to the podcast I noticed I do something similar in sweeping corners on my bicycle, although the force on the handlebar needed to maintain the turn is low.

Brucey wrote:Tall folk have special issues with bike frames; it is a different (and often somewhat wibbly-wobbly) world up there! Anytime I have slung a leg over a very tall frame (with the saddle well down of course) I am struck by how different the bike feels even though the weight distribution and the steering geometry might be 'fairly normal'. Quite what it must be like to be sat six inches higher yet is difficult to imagine, but in (say) unicycles and recumbents it is for sure a world of difference. Some of my chums are tall and have all kinds of problems that shorter folk don't; for example standard cranks may feel very short to them, and (because wheelbases are not scaled with CoG height increase) every time they use the brakes hard they are in more danger of the rear wheel lifting.

On this topic, I’m convinced it’s harder to corner in a low tuck than when sitting upright. It feels less stable and more attention to steering is necessary. Continuing this line of reasoning, perhaps taller people find cornering more stable. Zinn talks about winning races by descending faster than his competitors in the podcast.

Re: The catch-all bicycle geometry thread

Posted: 6 Nov 2018, 9:08am
by meic
but I hadn’t thought about the need to keep pushing the handlebar with the inside arm to hold the lean as Zinn says motorcycle racers do.

Trying to remember quite what I was doing in those days.
You give the bars a tug/push to initiate a "crash" which you catch before you hit the ground.
That is how you throw the bike around rapidly.
You then hold a pressure on the bars to keep the motorbike at a different angle to your body, so that you try and keep your body and head upright and use a gentle touch of countersteer to keep it down rather than by using your weight in a direct line with the motorcycle.
You can ride along a straight line flicking the bike from side to side underneath you, a push of countersteer to make it lean then keeping a gentle pressure to hold it down there. It is a method of shifting your body weight relative to the motorbike's weight.

I wasnt a racer, just a dispatch rider.

Re: The catch-all bicycle geometry thread

Posted: 6 Nov 2018, 9:26am
by Brucey
remember that motorcycle tyres are far from narrow; this means that the contact patches of the tyres deviate from the centreline of the machine considerably once you lean over. This means that some combination of countersteer and/or weight shift is probably needed to maintain a turn, in a way that isn't usually required on a bicycle (with skinny tyres).

BTW there is clearly no agreement about 'the right way' to steer a motorcycle; there are fairly obvious differences in the way that riders hang off Moto GP bikes and this year's champion elect hangs of the bike differently to most of his contemporaries. He also makes the tyres last better than them too. These two things may not be unrelated...

cheers

Re: The catch-all bicycle geometry thread

Posted: 7 Nov 2018, 5:48pm
by scottg
If you want a quick education in counter steering, try riding an racing trike.
A Longstaff or the like, if you try to ride it like a 2 wheeler,
you will find yourself heading straight at the nearest wall.

Re: The catch-all bicycle geometry thread

Posted: 7 Nov 2018, 6:32pm
by fastpedaller
scottg wrote:If you want a quick education in counter steering, try riding an racing trike.
A Longstaff or the like, if you try to ride it like a 2 wheeler,
you will find yourself heading straight at the nearest wall.


Indeed..... BTDT

Re: The catch-all bicycle geometry thread

Posted: 7 Nov 2018, 9:50pm
by mig
Samuel D wrote:On this topic, I’m convinced it’s harder to corner in a low tuck than when sitting upright. It feels less stable and more attention to steering is necessary. Continuing this line of reasoning, perhaps taller people find cornering more stable. Zinn talks about winning races by descending faster than his competitors in the podcast.


because you do it far less frequently and therefore it doesn't feel as much of a normal movement?

Re: The catch-all bicycle geometry thread

Posted: 8 Nov 2018, 4:59pm
by Samuel D
mig wrote:because you do it far less frequently and therefore it doesn't feel as much of a normal movement?

I do it often on descents with sweeping curves that don’t require braking, so I don’t think it’s that. The steering simply needs more attention when my mass is lower. It’s harder to follow the desired line.

This makes intuitive sense. Consider balancing a broom upright on your fingertip. It’s easy. But it’s harder to balance a pencil in the same way, because the corrections must be more frequent, rapid, and precise. In a similar way, taller cyclists must feel more stable than short ones.

Some recumbent bicycles are hard to balance on for the same reason.