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Braking And Geometry


Thor
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I keep reading and hearing that being on the brakes when you turn compresses the forks and tightens the geometry causing the bike to steer more quickly.

 

This hasn't been my experience other than a mild effect on initial turn in. I find that it is more difficult (requires more force) and slower to turn while on the brakes.

 

I understand that it is true that the geometry will tighten, but my sense when riding is that the front tire is loading up and it is more difficult to turn.

 

This reminds me of the argument that toilets water spins in opposite directions when flushed based on which hemisphere you are in. Yes, the Coriolis effect is true, but it isn't strong enough to effect the water in a toilet. But people still believe that it is the cause.

 

Is this another "toilet fallacy?"

 

Do people believe in the misapplication of science in this instance, or is it rather a case where the benefits are so small compared to the potential hazards that it isn't recommended?

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Like it or not, when you are hard on the brakes, the geometry of the bike changes SIGNIFICANTLY. You are dropping the front in the neighborhood of 5" and raising the rear probably 3". That is a massive change on wheelbase which WILL affect turn-in.

 

The 'difficulty' in your turn-in feel, is likely because the front end is so packed down that it simply "feels" goofy.

 

Geometry is geometry, and physics are physics. The bike will steer quicker on a shorter wheelbase and a smaller degree of rake angle as well as less trail. All of which occur when you pack the front down and jack the rear up (i.e., hard on the brakes).

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Like it or not, when you are hard on the brakes, the geometry of the bike changes SIGNIFICANTLY. You are dropping the front in the neighborhood of 5" and raising the rear probably 3". That is a massive change on wheelbase which WILL affect turn-in.

 

The 'difficulty' in your turn-in feel, is likely because the front end is so packed down that it simply "feels" goofy.

 

Geometry is geometry, and physics are physics. The bike will steer quicker on a shorter wheelbase and a smaller degree of rake angle as well as less trail. All of which occur when you pack the front down and jack the rear up (i.e., hard on the brakes).

 

I agree that the geometry changes, but there is also a SIGNIFICANTLY greater force being applied to the front tire when on the brakes.

 

Do the forces applied to the front tire have only a negligable effect on the resistance of the tire to turn?

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I agree that the geometry changes, but there is also a SIGNIFICANTLY greater force being applied to the front tire when on the brakes.

 

Do the forces applied to the front tire have only a negligable effect on the resistance of the tire to turn?

 

In considering it, while I don't know the exact answer and I'm not a physicist (nor did I stay at a holiday inn express last night), I think they are pretty much a wash.

 

Under neutral load (balance front/rear, no braking or accelleration) the bike will be less apt to turn because the geometry of it should be very stable and promoting stability in a straight line.

 

Under heavy braking, the geometry gets altered signficantly, removing that straight line stability and making the bike WANT to turn. However, the counter force of the braking on the front tire may very well affect the traction and feel of the bike while initiating a turn.

 

Put it this way. If you could alter geometry WITHOUT BRAKING, going into a corner whereas the front end dropped down (wheel came up, suspension compressed) and rear end rose (wheel went down, suspension extended), the bike would be VERY twitchy and prone to changing direction (aka - turning).

 

The variable here that plays into the problem is the braking. What impact is that force on the front end? I don't know...

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I agree that the geometry changes, but there is also a SIGNIFICANTLY greater force being applied to the front tire when on the brakes.

 

Do the forces applied to the front tire have only a negligable effect on the resistance of the tire to turn?

 

In considering it, while I don't know the exact answer and I'm not a physicist (nor did I stay at a holiday inn express last night), I think they are pretty much a wash.

 

Under neutral load (balance front/rear, no braking or accelleration) the bike will be less apt to turn because the geometry of it should be very stable and promoting stability in a straight line.

 

Under heavy braking, the geometry gets altered signficantly, removing that straight line stability and making the bike WANT to turn. However, the counter force of the braking on the front tire may very well affect the traction and feel of the bike while initiating a turn.

 

Put it this way. If you could alter geometry WITHOUT BRAKING, going into a corner whereas the front end dropped down (wheel came up, suspension compressed) and rear end rose (wheel went down, suspension extended), the bike would be VERY twitchy and prone to changing direction (aka - turning).

 

The variable here that plays into the problem is the braking. What impact is that force on the front end? I don't know...

 

One thing you have to take into account is that the trail increases by up to 25mm under heavy braking. Trail is measured at the CENTER of the contact patch which becomes way different under hard braking, it moves rearward. 25mm of trail is enormous and is no doubt what contributes to the heavy feel you get at the handlebars under braking.

 

So while the rake and wheelbase go towards a quicker turning machine the shape of the contact and its new location are huge offsetting factors.

 

Keith

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