Fork question

[cwjm38]

I've dug out an old mongoose tank and have been eyeing as a likely conversion frame. Other than some (many) surface rust spots, the frame and suspension components are in good shape.

One problem: The fork is a double-crown, triple-clamp type. I've taken a look at the conversion instructions, and I see that the fork must be flopped around. Theoretically, I could flip it all around, but I'm afraid that my steering range will be quite compromised.

So, if I don't flip this fork around, will it become a non-starter?

Thanks,

Chris

 

[Doug Burton]

Hi Chris,

The fork must be rotated 160 degrees to allow room for the cassette/freewheel when the adapter brackets are attached to mount the rear wheel on the front fork. This allows the rake and trail of the steering geometry to be returned to what it was on the original bike.

Not turning the fork around will result in negative trail in the steering, which will make the bike very unstable at anything above walking speed.

Also the "triple tree" type fork attachment is likely to interfere with your knees, because your legs will straddle the frame head tube when your bike is complete.

Best,

Doug

 

[cwjm38]

Hello Doug, thanks for the reply.

I don't think the width of the triple clamp style is much of a concern for my knees. I'm cut from a larger cloth than most (and do a pretty good job blocking the light from a doorway) and will likely need to find a set or two of pedal extensions to get job done. But it will be good check this to make certain.

I'll have to figure out the impact on the steering range with a flipped fork. I've never taken notice as to how far I spin the handle bars during my commute, or what the worst-case scenario would be. Is there a rule of thumb for this? 120 degress (60 left and 60 right)? I've got no sense of this, but perhaps it won't be an issue.

Also, I'll go ahead and calculate the loss of positive trail with a non-flipped fork. Do you (or anyone else out there) have a measurement for the added rake? Are the old and new drop outs parallel? This will help in my calculation.

I suppose the last calculation may be the most practical: figure out the price of a new fork assembly.

Thanks again,
Chris

 

[cwjm38]

After pictures, drawings, and calculations....

Flipping this fork over would give me only 62 degrees (31 left and 31 right) of turning range. So I'd say thats out.

Not flipping the fork looks like it takes the trail from 3.05 in to -0.5 in. To get this number, I estimated from a picture of the new drop outs that the fork rake would be increased by 3 in and the head of the bike would be lowered about an inch.

A few referenced formulas I had found (which may or may not be appropriate) suggested that the ideal trail for my estimated characteristics may be between 1.5 to 2 inches. I was not optimistic, but I had to put those years of geometry to use to prove it to myself.

Darn. It's a bit like having a losing lotto ticket. You didn't expect to win, and conventional wisdom says that you shouldn't, but you hope that the beat up bike in the basement might finally be really useful.

Now, on to a more suitable donor configuration. The search continues.....

 

[John Tolhurst]

Yep
that's it. Onto a search for a very ordinary fork. Expensive forks control pogo. You won't have that problem. Alloy ones might save you a little weight.