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More rake = less trail

DougSloan said:
I never really "get" the fork rake thing, so here's but another question. If I have a 43 mm rake fork, and I replace it with a 50 mm, what happens? Does the bike get more stable and more compliant (all else equal)? Is there a negative? Thanks.
Doug, let me add a little something.

The more trail, the greater the stability. The greater the rake, the less trail.

Think about those choppers you see on the roads. They have very slack head tube angles and therefore have a lot of trail. They are very stable, but the payback is that wheel flop becomes a serious issue.

With a steep headtube (like one of those silly scooters), increased rake decreases the trail. Once trail vanishes altogether, the wheel will want to turn around like a caster on a grocery cart. Very unstable.

If you were to increase rake on your road bike to the point where trail is zero, the wheel will naturally want to pivot all the way around.

Roadbike designers have found that trail of 56-59 mm is optimal for stability and control. Everytime I look at one of those scooters with a hint of trail, I think about how easy it would be for the front wheel to turn in a real hurry. No telling how many kids are getting hurt on those things.

-Plus Vite...
 

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Disagreement here?

Mike T. said:
The wheel on a grocery cart turns around BECAUSE of the trail. Push or pull a cart and the Trail moves the wheel in the opposite direction of the movement of the cart - just like a bike when we push it forward. They both have the tire contact patch behind the steering axis.
I am not sure we are disagreeing. My goal is to make the idea of rake/trail as understandable and as clear as possible.

Let me revise and extend my comments:

Trail, in and of itself, can't do anything. It's just a definition of the physical relationship of a wheel, its steering axis, and direction of travel. Bikes always move forward, so that's a given.

Doug asked what would happen if you increased rake. The obvious answer is this: trail decreases and the result is less stability. When you get to the magic level of zero trail, then a wheel will want to spin into a position in which it has trail, i.e., the caster effect. You could fight and hold it with some kind of steering mechanism, but when left to its own, the wheel will naturally spin 180 degrees into a position where it has trail.

Technically speaking, when you move a grocery cart forward and back, the switching of wheel directions occurs because trail disappears in relation to the direction of motion just as you describe it above.

-PV
 
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