zklone

So if I was to change a 37.5cm fork with 48 rake to a 36.5cm one, how much the new one should have rake to make least difference to the original trail? the head tube angle is 74.5




40?

marqueemoon

This might be something good to cross post in Framebuilders.

urbanknight

Isn't this going to change the overall geometry of the bike completely? I couldn't find a tall enough fork for my Bianchi when my stock one cracked, and I ended up with a seat tube too steep to set my saddle far enough back to fit me.

zklone

you're right, still curious.

IK_biker

zklone,

375 mm fork with 48 mm rake results in 43.9 mm trail.
365 mm fork with 43.9 mm desired trail needs 48 mm rake, assuming the same wheel size as above.

You may find this hard to believe, but it's true.

Cheers,
IK_biker

ccdisce

Trail with 375mm fork and 48mm rake is 43.6mm.

Assuming that you are using the same frame, the shorter crown on the fork will tilt the top tube down towards the front by 0.7 degree and steepen the headtube from 74.5 deg to 75.2 degree.

Fork length at 365mm:
With the fork at 48 mm rake the trail goes from 43.6mm to 39. 7mm.

With the fork at 40mm rake, trail is 48mm

With the fork at 43mm rake, trail is 44.9mm

With the fork at 45mm rake, trail is 42.8mm

IK_biker

OK, ccdisce,

Judging by your 75.2 degree figure, apparently you executed the fork crown rotation (drop) around the BB, while the true rotation should occur around the rear axle.

I, too, have some fancy tools in my box. Since zklone did not specify tire size and wheelbase, the assumptions I had to make are a 375 fork on a bike with 980 mm wheelbase and 337 mm tire radius. See attached images.

First image is the fork "as is". 48 mm rake and 43.65 mm trail.
Second image is the 365 mm fork with same angle to headtube (as if the 375 mm fork legs got trimmed down to 365 mm). Rake goes down due to shorter legs, trail is 41.68 mm, wheelbase drops to 976.16 mm.
Third image shows what the OP requested. 365 mm fork on the same bike, trail as requested is 43.65 mm, thus requiring rake of 44.61 mm.

44.61 mm rake is the correct answer for said imaginary bike.

Nessism

Your trail is very low with the stock setup assuming your numbers are correct. Based on that alone, I'd go with the 40mm offset fork.

WheresWaldo

Is this a road bike or is it a cross or tandem. I have seen very few modern road bikes with that little trail but tandems routinely come in that trail range? Just asking.