I've obtained a frame which came without a fork. I know of the fork which normally comes with this frame and its axle-crown and rake measurements (365 and 38mm, respectively). I would like to look for a cheaper option rather than buying this particular fork new. What I'm wondering is, say if I found a fork that would work (i.e. correct steerer tube length and diameter) but which has a 28mm rake what would be the necessary axle-crown length so my top tube would remain level? Is there some sort of formula out there for this? Seems like it should be some fairly basic trigonometry.

The axle to crown distance will define the top tube's position and also have an effect on head and seat tube angles. Rake will have little effect on keeping your top tube level but will have a big effect on "trail" which will control how the bike steers.

I do not know of a formula per se, but with all other things being equal, including fork length, a 10mm change in rake will either raise or lower the front end of your bike about 3mm.. ie. A 365 mm fork with an offset of 28 will raise your bars about 3mm.....a 365 fork with an offset of 48 will lower them about 3mm. This is not significant, or anything to worry about. Your top tube angle would change only .2 deg...something that would not be noticeable. However, as HillRider notes, trail will be affected significantly. For example with 700c x 23 tires and a 73 deg head tube angle, switching from a fork with 38mm of offset to one with 28mm will move the trail from 62mm to 73mm, which will definitely affect handling.

Dave

FYI...a formula for calculating trail can be found here: https://en.wikipedia.org/wiki/Bicycle...cycle_geometry

A 10mm change in the fork length will change the HTA by at least .5 degree and make a signiicant difference in the trail.

Trail is R/tanH - (offset/sinH). The first half of the equation is the trail without any fork offset and the second half is the reduction in trail due to the offset. An offset of only 38mm is on the small side to start with (makes for slow steering). You wouldn't generally want a lot less. You might want more to get a little quicker steering.

https://www.sheldonbrown.com/rinard/forklengths.htm

Yes, this is correct for a 10mm change in fork length with a similar offset. However, I understood that the OP was asking what affect 10mm of offset would have for a given fork length. ie... how would two forks that are the same length, but with different offsets, affect the height of the front of the bike...or how long of a fork would he need to maintain the height of the front end of the bike if the rake changes.

Before & after drawings attached for a 365mm fork and 38 vs 28mm of rake.

+1 There seem to be quite a lot of forks out there with 40mm offset, you might check one of them out. As DaveSSS says, 38mm will be slow steering.

Draw it out on a big piece of paper?

A large difference in offset would be unusual, but you're right that for a given fork length, the head tube would be lowered by 3mm for each 10mm of additional offset. Their really is a formula - multiply the offset difference times the cotangent of the HTA. To keep the head tube in the same position, you'd need 3mm more length. In the OP's case, using less rake would require 3mm less length.

Keeping a similar steering trail is another story. Reducing the offset by 10mm increases the trail by a similar amount. Rarely a good idea.


I'd ask the OP if he's sure about the 28mm offset - never seen one that small - most are 38-52. That's also the reason I've never thought about a fork length change due to offset. Most people keep the changes down to 5mm or less and the length change is insignificant. What's more important is the brand of fork and the fact that some brands don't even bother to publish the axle to crown race length, as if it was standard.

A 3 mm difference in rake offset is significant, I've done that. Not a huge difference in handling but immediately noticeable.