Caster angle for front forks

[eHomer]

I'm modifying my old Dahon Boardwalk folder to an ebike.

It's not quite such a good spec as the later ones, but it is fairly light and rides well with 6 deraillier gears.

I'm trying out my BMS 20" 250w 290rpm front wheel that I bought a while ago. It was intended for my Hase Lepus trike (and may still end up on there), so the 100mm dropout width was no problem.

The Dahon however, has a very narrow front wheel, just 75mm between the dropouts. I didn't want to try springing them that much, so I bough a brand new steel fork from Ebay with 100mm dropouts.

The choice is fairly restricted in that size, so I've ended up with conventional curved forward forks, whereas the original Dahon ones were straight, and in direct line with the steerer tube, not offset.

This has reduced the Caster angle by moving the wheel forward by about an inch. It's still got some positive caster though.

I've been Googling it and found that the recommended is about 80 degrees.

If the bike is now too "lively" on the steering, I was thinking about reversing the fork. The vee brake would then be working behind it instead of in front, but it's well clear of the frame downtube, because of the Dahon single bar design.

Anyone ever tried anything like this ? or tinkered with caster angles ?

 

[Deleted member 4366]

What you're describing is the trail rather than caster. The trail has decreased due to the offset axle. In theory, it'll quicken the steering a it, but I'd be surprised if you notice much difference.

 

[flecc]

And a rear mounted front V brake can be if anything an advantage, since the forks are taking the thrust directly. Some Kalkhoffs like the original Agattu have had this kind of V brake mounting, and a current Pro Connect has a hydraulic rim brake mounted on the rear of the fork.

And of course nearly all rear V brakes are mounted in the same way, thrust force into the frame.

 

[eHomer]

Thanks for the quick replies.

I'm not sure of the terminology but my understanding was that the trail of a pivoting wheel was the distance behind the pivot, and the caster was the angle between the wheel ground contact and the steering axis.

I'd originally thought the reversed forks might look weird and be a bit of a kludge, but it now sounds as though it's got definite advantages.

As you say, the brakes are then on the thrust push side, like at the back.

It would make the bike about 3" shorter too.

I like the idea of more self centering, especially with the small wheels.

mmm, I think I'll give it a go.

 

[Deleted member 4366]

Caster and rake are the same.Trail is the distance between the contact point and the projected line through the steering axis measured perpendicular to the axis. The distance between the axle and the projected steering axis is the offset.

let us know how it goes.

 

[flecc]

mmm, I think I'll give it a go.

Good for you, go for it, but be prepared for the cyclists who approach you to say your forks are fitted the wrong way round.

 

[eHomer]

Yes, I'll certainly report back when I give it a test.

I will try the forks the right way round first, I would prefer it that way as long as it's not too skittish.

 

[eHomer]

Just to wrap this thread, the forks worked fine.

I took it for a test ride, and the reduced angle didn't make the bike skittish at all.