Fork Offset Question.

[Ken Hicks]

So I'm pretty new to the frame building game but anyway, I have a particular geometry in mind for a road bike for myself. I want to emulate a Cervelo R3 and I would like to run a 3T Funda fork on it. The (old) R3 uses a 3T fork with a 43mm offset. The 3T Funda Team can be had with either a 43mm or 49mm offset but the 3T Funda Pro only comes in a 49mm offset. I would actually like to run the Pro because it is less expensive and - because I'm really anal about things matching - I like the white stripes on the Pro as opposed to the red stripes on the Team.

So the question is what is this 6mm difference going to feel like? I currently ride a Look 586 that has 43mm of offset so I am used to it but I am also no spring chicken and I'm getting less aggressive when it comes to descending so maybe the 49mm offset would offer a little more comfort and slow things down a bit.

Input?

[David Tollefson]

No. More offset makes for less trail, which will quicken things up, steering wise (given the same head tube angle).

[devlin]

Offset, head angle, front center, chain stay length, seat angle, saddle setback, stem length etc. etc. etc. will all determine to the final outcome. Don't go by what the catalogue geometry table says either. Guranteed to not be what the frame actually is. Dial your position then build the bike to suit you.

[Kumo Cycles]

You can always paint a fork

[Ken Hicks]

No. More offset makes for less trail, which will quicken things up, steering wise (given the same head tube angle).

Thanks for answering my question. After looking at a bike geometry diagram I can see what you mean. So maybe you can indulge me on something else. Obviously you don't know the mind of 3T but I wonder why if they are only offering one (49mm) offset on the less expensive fork why they would offer the offset that gives the quicker steering - especially in light of the fact that all three upper end race bikes that I am familiar with (Look 586-which I own, Time RXRS, Cervelo R3) all (in my size) have 73 degree head angles and 43mm of offset? Thoughts?

[David Tollefson]

Thanks for answering my question. After looking at a bike geometry diagram I can see what you mean. So maybe you can indulge me on something else. Obviously you don't know the mind of 3T but I wonder why if they are only offering one (49mm) offset on the less expensive fork why they would offer the offset that gives the quicker steering - especially in light of the fact that all three upper end race bikes that I am familiar with (Look 586-which I own, Time RXRS, Cervelo R3) all (in my size) have 73 degree head angles and 43mm of offset? Thoughts?

My initial answer was in the context of "all other things being equal" -- meaning that if you swapped the 49mm rake fork onto a bike that was otherwise equipped with a 43mm rake fork, you'd get a quicker feel. As was stated by Devlin, all the frame/fork design factors come into play when you're talking about how a bike handles, and no one thing can be discussed away from the others. But... likely that fork comes in different rakes for the OEM market, and only the 49mm rake is offered aftermarket (maybe as a means to get you to buy the more expensive fork?). Many others come in 43, 45, and 50mm. Just depends on what the bike is designed around. The larger rakes would be paired with a slacker head tube angle to yield a similar trail.

Don't know if that really helps you.

[Ken Hicks]

Thanks again. So I understand the "all things being equal" notion and once again looking at the bike geometry diagram I can see how changing to a slacker head angle will move the tire contact point forward and more offset will compensate keeping trail in the desired range. So a 73 degree head angle and 43mm of offset gives 58mm of trail. Once again I refer to the three bike mentioned earlier and they all seem to follow the same school of thought as far a geometry goes (for a size medium or 53-54cm frame). But then looking at specs for the same bikes in other sizes I see that trail increases for the smaller sizes - I'm assuming that this is to help ameliorate toe overlap (somewhat). But it also decreases with larger frames - would this be to keep wheelbase closer to a certain measurement?

This being the case it seems that there is actually more of an "acceptable range" as far as steering geometry goes as opposed to a hard and fast (ideal) number one is shooting for. Correct?

Sorry for so many questions and thanks again for indulging me.

[Jonathan]

You can always paint a fork

Or build it?

[Eric Estlund]

Or build it?

Of buy it, ride it, get a first hand impression of those few MM on ride quality, THEN build the one to spec for the bike.

[Todd Mosley]

If you are just interested in that fork option, it looks like 3T does make the Funda Pro in 43mm rake:
3t funda 43 | eBay

[GrayJay]

Thanks again. So I understand the "all things being equal" notion and once again looking at the bike geometry diagram I can see how changing to a slacker head angle will move the tire contact point forward and more offset will compensate keeping trail in the desired range. So a 73 degree head angle and 43mm of offset gives 58mm of trail. Once again I refer to the three bike mentioned earlier and they all seem to follow the same school of thought as far a geometry goes (for a size medium or 53-54cm frame). But then looking at specs for the same bikes in other sizes I see that trail increases for the smaller sizes - I'm assuming that this is to help ameliorate toe overlap (somewhat). But it also decreases with larger frames - would this be to keep wheelbase closer to a certain measurement?

This being the case it seems that there is actually more of an "acceptable range" as far as steering geometry goes as opposed to a hard and fast (ideal) number one is shooting for. Correct?

Best description of effects of fork trail I have ever seen is by Tom; Spectrum Cycles | Geometry Think of trail more as neutral vs erratic instead of fast vs slow. Non-neutral trail may be beneficial for some conditions but is not as predicatable feeling and non-uniform stability at different speeds. I would estimate that change of trail by less than 5mm is probably impreceptible to most riders.

The change in trail for large/small frames is mostly due to the changes to the head tube angle needed to provide short riders with a short top tube (and secondary benefit of reduced toe overlap) and for larger riders to keep the front-center measurement from growing at larger top-tube lengths which would tend to screw up overall bike handeling. Most large production manufactures are too "lazy" to match fork rake to the HTA across all sizes, just take a one-size-fits-all approach and ignore trying to maintain uniform trail across all frame sizes.