# Wheel and Fork

#### Phrak

How much rake and trail do need to use in a design?

My design is essencially a bicycle front end. The various forces and changing dihedral angles are making this a real challenge. I don't know where to start.

Some of the forces involved are the normal and lateral force exerted by the wheel contact pad, the gyroscopic action of the wheel, and the resistance to torque that results from the wheel contact pad being twisting on the road surface.

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#### NateTG

Homework Helper
Depends on what you're doing. People write books about that sort of stuff.

Tony Foale's page has some more discussion.

#### Phrak

I'm considering a proof of concept design. For this I need, for the most part, the basic physics and governing equations.

I took a peek at that web site. Unfortunately, what I am looking for seems to be hidden within his software.

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Too hard?

#### Ranger Mike

Gold Member
Phrak, you got way more posts than i ever thought about,,and are far more versed in the mechanics and math..all i can add it that you are asking how much caster to have in the fork..
too much will really build your forearms trying to turn it..not enuff and a peeble on the track will cause the fork to change direction, having the proper amount will cause the front end to stay straight enen when you take your hands off the bars (gyro effect)..all i can recommend is to copy the successful amount of caster on another similar design
regarding rake, longer gives way smoother ride..i had a Harley with the fork neck raked out and 4 inch longer sliders..most smooth ride but tok two counties to turn around..( large turning radius)
hope it helps.you post super good stuff

#### Phrak

Phrak, you got way more posts than i ever thought about,,and are far more versed in the mechanics and math..all i can add it that you are asking how much caster to have in the fork..
too much will really build your forearms trying to turn it..not enuff and a peeble on the track will cause the fork to change direction, having the proper amount will cause the front end to stay straight enen when you take your hands off the bars (gyro effect)..all i can recommend is to copy the successful amount of caster on another similar design
regarding rake, longer gives way smoother ride..i had a Harley with the fork neck raked out and 4 inch longer sliders..most smooth ride but tok two counties to turn around..( large turning radius)
hope it helps.you post super good stuff
Thanks Mike, that does help alot. I need to know what to expect if I change the rake or the fork length. Do you know if anyone changes-out the steering head, and what effect it has?

Speaking of long rake, there's an S turn in an avenue around here. I was watching a guy who may have just made some front-end changes with a long fork. The front wheel starts oscillating, shaking the handlebars. I've never seen that before or since, but that's not something you want.

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#### berkeman

Mentor
Thanks Mike, that does help alot. I need to know what to expect if I change the rake or the fork length. Do you know if anyone changes-out the steering head, and what effect it has?

Speaking of long rake, there's an S turn in an avenue around here. I was watching a guy who may have just made some front-end changes with a long fork. The front wheel starts oscillating, shaking the handlebars. I've never seen that before or since, but that's not something you want.
I've seen a lot of discussion about rake and trail, but more in the context of sportbikes and motocross bikes.

I don't know if you're designing a road bike or a mountain bike, but if it's a MTB with front suspension, the geometry gets even more complicated under braking with the forks compressing.

I did a quick google search on motorcycle rake trail theory, and got some good hits. Here's the hit list in case it helps:

.

#### turbo

Gold Member
I have been involved in street motorcycles (owning, riding, and modifying them) for 40 years. One of the trickier concepts for people doing modifications is how to evaluate steering geometries. There was a fad years back involving people buying raked triple-trees and mating them with long fork tubes for a "chopper look". The problem with that is that the bikes ended up with reduced trail and were unstable. It's possible to modify the frame to have a shallower head angle, which, combined with longer tubes, can give a stretched "chopper look", but you can only push this so far, because it can result in increased trail to the point that maneuverability is severely compromised (performance on curvy roads is reduced). It may be possible to combine a shallower head angle (increased trail) with a raked triple-tree (reduced trail) and come up with a stretched-out front end that performs well, but unless you are made of money (and foolhardy), it is probably not a good idea to jump into this type of experimentation. Darwin Award, anyone?

#### Ranger Mike

Gold Member
Well said - Turbo 1..excellent advice, I have reached the limit of my knowledge ref: cycles
all I can say is that engineers spend huge  and time developing a good reliable product and once you modify it, serious things happen..ifin you monkey with the steering head, more stress due to longer lever, requires beefier bearings, maybe up size the fork thickness and all kinds of handling problems , braking probs, result..if you got to do it, make gradual changes and CHECK everything..especially check for cracks...not nice to top a hill at speed and forks go south..ooops

#### Phrak

I've seen a lot of discussion about rake and trail, but more in the context of sportbikes and motocross bikes.

I don't know if you're designing a road bike or a mountain bike, but if it's a MTB with front suspension, the geometry gets even more complicated under braking with the forks compressing.

I did a quick google search on motorcycle rake trail theory, and got some good hits. Here's the hit list in case it helps:

.
I've been looking over some of the links. The general concensus is that Trail is the most important value. I don't have a lot of redesign room, without completely rebuilding it, So I need a mathematical model.

If I come up with a mathematical model, but it has just one sign error somewhere--- it's good to have a feel for it to check the math.

I'm considering a minimal scoot-around--paved road conditions, very manuverable, maybe a 6 foot turning radius. And able to handle pebbles in the road without throwing the rider.

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#### Phrak

If a bike, with hands off the handlebars, tips in one direction, the front wheel tends to turn into the direction of fall due to the gyroscopic precession of the wheel. The angular inertia of the wheel is a factor in balance.

This restoring action is proportional to the speed of tipping.

The contact point of the front wheel exerts a torsional force on the front fork about the steering axle. The larger the trail, the more force. While moving in a straight line on a smooth and horizontal surface the force is zero.

Then there's offset. Even with the same rake and same trail, the offset of the front wheel axle from the steering axis makes a difference.

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