Tuesday, October 5, 2010

The Wheel Deal With Trail and Flop

When I build a bicycle, I like to have a reason for doing it other than; That's the way everbody else does it, or that's the way everbody else does it, so I tweak it a bit to be different.

First off you need to define what you want. I hear a lot of talk about handling in reguards to cornering, but how much of your riding is dedicated to corning. I mean if you ride 60 miles on a road bike, what percentage of that ride is spent cornering? Notice I made the distinction between Road Bike, Criterium bike and Mountain Bike. A great deal of "crit" racing and mountain biking is dedicated to cornering, so I'm glad I don't have to concern myself with that . . . yet.

The trail dimension gives us an idea on how the bike is going to handle, Stability vs. Agility. Trail is the distance the front wheel's contact patch trails behind the projection of the head tube's center line to the ground (See The Green Dimension). Trail = ((wheel Radius) * (COS Head Angle Radians)-(Fork Offset)) / ((Sin Head Angle Radians))

For my Raleigh with 72 degree head tube angle, and 62mm rake; it works out to be
Trail=((388)*(.309)-(62)) / (.951)
Trail= 44.63mm.

So what's a good amount? By doing some reading I find bicycle builder's define 56mm as neutral. Anything below (56mm) is quicker agility/lower stability, and anything above it slower agility/higher stability. So by definition my raleigh has low stability despite the 62mm of rake . . . . . errrr how does that work out. I thought rake gives more stability. It's actually backwards when dealing with trail. Less Rake allows the wheel to trail more. So if you have a fork with 45mm of rake and change it to one with 43mm you will increase stability. This is interesting because I also thought this bike wondered more than it should.

So what's with the funky Raleigh geometry? Well first off it's Old School . . . well 70's anyway . . . and it was designed for 27" wheels and not 700c (increased wheel diameter will increase trial).
You can have two bicycles with different head tube angles and rakes that produce the same roughly the same amount if trail (See Chart. I proved a few other manufacture ).
74.0*/40mm = 55.3
73.5*/43mm = 55.3
73.0*/45mm = 56.3
72.0*/50mm = 57.3

So do all these bicycle handle the same? . . . almost. The 72* with slacker angle bicycle is going to have longer fork legs and more rake. So this bike is going to be a little more forgiving on rougher roads. Longer legs, More flex. The 74* bicycle is going to be more sensitive to the texture of the road.
Another reason you might want the head tube more slack is to give you more front wheel/shoe clearance on smaller frames. You don't want to hit your shoe on your tire when cornering.

Also there is something called Wheel Flop. It's a measurment of how far you have to lean the bicycle to make it turn . Wheel Flop is defined as

F=Trail * COS(Head Angle Radians) * SIN Radians (Head Angle Radians)

Then going back to equal trails
72/55mm=51.9mm trail= Wheel Flop of 15.28mm
73.7/45mm=51.9mm trail = Wheel Flop of 14.00mm

What different does 1.28mm make? I'm not sure, and who can ride a bicycle when your handlebars are turned 90 degrees. Where a good read if this hasn't given you a headache already


Before this out of hand, I'll go back to the first fact. You have to decide what you want.

1. I don't want toe over lap
2. I want the bike to be on the stable side of neutral
3. I want the bike to have a balance 55% of my weight on the back wheel, 45% on the front.
4. Short Chainstays, but with today's 10 speed drives, all manufactures recommend chainstays longer than 405mm I believe. any shorter creates a bind in chain cross over. I might go 410mm just to give this some tolerance. So in this case I'm going to have to design the front end working around the back end.
5. I'm typically a long top tube, short stem person. I like to sit between my wheels then on top of my wheels so to speak for long rides
6. I want, what I Like. That mean instead of jamming numbers into equations, I'm going to go with what comfortable for the most part. Those equations are more guidelines than hard figures you have to live by. I say let the bicycle fit the person, and not the person fit to the bike.

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