Rake, trail, bend: fork questions.
 

So I've got this nice old French bike, a VeloSoleX. DB 531 throughout, and it is the smoothest-riding steel bike I've ever had. It's a keeper.

But... When i got it, the stem was seized. I worked and worked and worked to extract it but the corrosion weld was total. Tragically, in the process of trying to extract it, I tweaked the steer tube. As luck would have it, one of the guys I work with is apprenticing with Paul Wyganowski, and he took it on as a project. He's going to give me a 1" threadless tube and add a second set of eyelets on the dropouts,and I think I'm going to ask him to give me a set of mount points for a Nitto rack.

But the experience kind of got me to thinking: How much do rake, trail and bend each contribute to handling and ride? How much of ride quality comes from the fork at all?

I don't anticipate building my own frame but there's a good chance I might commission a custom frame at some point and I'd like to understand the theory better.

my working theory is that almost all of the ride qualities come from the front end geometry. This is an over-simplification because wheelbase and weight distribution are very important as well, but it seems to me that the way a bike feels to the rider is mostly determined by the way the front wheel reacts to rider input. Thus rake/trail and head tube angle are very important. Bend is in the noise level unless you are trying to impress Jan Heine, in which case you want the smallest radius possible.

I've seen some discussion on how tire size and contact patch interact with rake and trail and how that affected handling vis-a-vis relative weight distribution. I tried to follow it but then I heard something snap in the middle of my head and then my right eye started twitchin'.

It's Oscar run up time, and I was watching an amusing film called Mon Oncle. The main character gets around on gas assisted VeloSolex. Looked pretty cool.

AFAIK KonAaronSnake and I have the only two nonmotorized versions seen on Bikeforums, and I think he sold his. They were apparetnly St-Etienne builds and shipped to VSX dealers as an expansion of the lines. From what I have been able to find, they were shipped as frames only with the dealers installing whatever gruppos were handy. His had a mostly Super record build and mine was Huret with Nervar cranks and gold MAFAC Comp brakes. Nice frames though, as i said DB 531 in one of the lighter gauges (my bare frame, headset and fork weighs in at 3 lb 14 oz) with Bocama long point lugs and decent paint. Build quality is no better than run-of-the mill, Wyganowski at least isn't terribly impressed. The reason I tweaked the steer tube is probably traceable to a dearth of braze in the crown, which undoubtedly contributes to the marvelous, silky ride.

Not bad, and without writing a book.
I always laughed when I read folk say that a sharp radius fork bend is more compliant.

Why laugh? I look at the forces involved and it totally seems to me that a tighter radius coupled with greater trail would be springier. Look at the Bates forks.

If I'm wildly out here, please correct me; or explain how, because I keep thinking there's something I'm missing.

There IS evidence to support that, but since Jan did the test, it MUST be suspect.
[/sarcasm]

SP
Bend, OR

Ah. Gotcha.

a tighter radius should be a little more compliant, but it is not a significant contributor to bike handling or feel. For example, I find it really difficult to tell the difference between a straight bladed fork and a fork with rake. I think it's mostly an aesthetic choice. These secondary effects may add up to significant differences in bike handling, but that's an assertion that I find difficult to buy into.

I think what you mean here is a fork with bend, not a fork with rake. A straight fork and a curved fork, both on the same bike and with the same length and rake, should handle the same. Rake and head angle (which together can be used to calculate trail) will have an effect on handling. It can be major or insignificant, depending who you ask. I think bend and fork blade cross section can have an effect on ride quality, but can't prove it. But not on handling.

Length can affect handling because it affects head angle.

Tire diameter also affects handling because it affects trail.

This much is just physics, and to his credit, Jan's tests do not contradict it.