Bicycle, Scooter and Motorcycle Dynamics

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Alan B   100 GW

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Bicycle, Scooter and Motorcycle Dynamics

Post by Alan B » Jan 20 2012 10:23am

Reading through this article on two wheel vehicle dynamics I found some things that folks here might be interested in. Some of them are counter-intuitive at least to the first order.

Before commenting in this thread please read the article which will be linked at the end of this post. Then perhaps post some useful highlights, questions, or items for discussion.

Some highlights of what you will find there:

Higher center of gravity increases bike stability. This is why scooters are unstable, their cg is too low.

Wheel gyroscopic forces do not help bike stability. Scooters are not unstable because their tires are small. Big wheels don't help with stability except by adding height.

Weight on the rear of a bike should be low for control and stability, but weight in the front should be high for best control and stability. The center of mass is best high and forward to minimize steering input required for balance. This is why recumbents are harder to balance, their center of mass is too low and to far from the steering, also the long wheelbase slows the steering response. This is also why Oatnet and others have had such good success with high front mounted battery packs.

Braking of the front tire can approach 0.5g while braking of the rear tire limits at about 0.1g. A similar argument in reverse applies to front versus rear wheel drive.

Turns begin by steering in the wrong direction.

Lots more here:

http://en.wikipedia.org/wiki/Bicycle_an ... e_dynamics
Last edited by Alan B on Jan 21 2012 1:10pm, edited 3 times in total.

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Farfle   1 MW

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Re: Bicycle, Scooter and Motorcycle Dynamics

Post by Farfle » Jan 20 2012 1:50pm

Interesting page, a good read for the diy frame building crowd for sure. Its interesting how there is an optimum length based on how much traction the tires get. Too long and the bike won't transfer enough weight to the tire that needs it. Too short and either end will lift when the weight transfers.
Last edited by Farfle on Jan 20 2012 1:57pm, edited 1 time in total.
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Re: Bicycle, Scooter and Motorcycle Dynamics

Post by Alan B » Jan 20 2012 1:53pm

Farfle wrote:Interesting page, a good read for the diy frame building crowd for sure.
What nuggets of wisdom did you see there for framebuilders?

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Re: Bicycle, Scooter and Motorcycle Dynamics

Post by Farfle » Jan 20 2012 1:59pm

Oops, edited the original post:
Farfle wrote:Interesting page, a good read for the diy frame building crowd for sure. Its interesting how there is an optimum length based on how much traction the tires get. Too long and the bike won't transfer enough weight to the tire that needs it. Too short and either end will lift when the weight transfers.
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Re: Bicycle, Scooter and Motorcycle Dynamics

Post by Farfle » Jan 20 2012 2:09pm

A perfect example of this is lukes race bike at the grange track, it would peel out, because it was too long and low for the tire it had. I guess that in a perfect world, the bike has to be at 99% of traction to lift a tire, whether that's under acceleration or braking. And that the native weight of the bike front to rear determines how biased it is on it's ability to accelerate/decelerate.
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Re: Bicycle, Scooter and Motorcycle Dynamics

Post by Alan B » Jan 21 2012 9:32am

So Luke's bike could accelerate a bit faster if it was shorter or the cg was a bit farther back due to more weight transfer to the rear and more traction.

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Re: Bicycle, Scooter and Motorcycle Dynamics

Post by dogman dan » Jan 22 2012 8:32am

Good info there, though the math parts go right over my head. I see nuggets for frame builders there in the explanation of why to increase or decrease trail. You'd head one way or the other depending on the intended use of the bike.

The other thing for a frame builder is you get to choose where to carry the battery, instead of compromising based on a frames limitations. I see it explains why the high battery carry that is right behind the handlebars feels good. Some tout carrying down by the crank, and the article explains why that can feel good too. It's also fairly obvious that if you must carry to the rear, carry low as possible.

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Re: Bicycle, Scooter and Motorcycle Dynamics

Post by Drunkskunk » Jan 22 2012 10:44am

Thats a good read. Lot of good information in that.
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Re: Bicycle, Scooter and Motorcycle Dynamics

Post by emotofreak » Jan 23 2012 11:11pm

Alan B wrote:So Luke's bike could accelerate a bit faster if it was shorter or the cg was a bit farther back due to more weight transfer to the rear and more traction.
Or he could just put a stickier tire on...

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