What's Fork Offset?

I’m building a 29er, looking at getting a SID fork, has the option of buying one with 51mm offset. Any idea what that is, how that differs from standard? Thanks

http://www.bike24.com/1.php?content=8;navigation=1;menu=1000,2,121;product=28009

same as rake isn’t it? it’s how much the axle is offset from the steering axis.

Thats what I figured…..I cant see anything on the RS website that explains this though, how it differs from standard. Anyone know? Am having a frame built, builders want to know geom of the fork I get.

“Standard” 29er offset is 46mm. 51mm is Fisher G2 geometry, didn’t know they were available aftermarket.

Can the builder explain how they can work with the rake/offsets on offer to get the best layout? They should be able to if they know what they’re doing.

I asked a similar question a while ago.
http://singletrackworld.com/forum/topic/what-will-happen-if-i-put-g2-forks-on-a-regular-29er-frame

Be careful using the term “Rake” with a frame builder. It can mean either Fork Offset or Head Angle. Best avoid it really.

trail – offset and head angle
sheldon brown

Doesn’t the combination of head angle and fork offset produce the rake?

I understand rake to be the horizontal distance on the ground between the tyre contact patch and the point where a line drawn from the steering axis hits the ground…

I found the geometry in the RS website eventually, yes looks like 46mm is standard. Will show him both, see what he recommends given what I’m trying to put together. Thanks.

I was too slow

Doesn’t the combination of head angle and fork offset produce the rake?

No, that’s trail.

I was too slow

So was I 🙂

MTG, the term ‘rake’ should only confuse a frame builder if they learned their skills at Orange County Choppers )

No, that’s trail.

Ah yes, remember now 😳

Here’s a guide to what the different things are called.

Calfee[/url] explains all about what they actually do. The more offset a fork has, the less trail – giving faster steering, and a less stable bike at speed, to sum it up briefly.

The more offset a fork has, the less trail – giving faster steering, and a less stable bike at speed, to sum it up briefly.

Interesting.

I have a Jeff Jones fork on my Ti29er and find it very direct, not atl unstable. However it is at a lower a-c of about 460mm as opposed to the On-One 470mm/47mm and Rebas (100mm, sagged to 80mm effective) 490mm/47mm. So does the lower a-c cancel out the shorter trail or amplify it?

The more offset a fork has, the less trail

Presumably for a given head angle..?

So does the lower a-c cancel out the shorter trail or amplify it?

won’t a different a-c effectively change the head angle? if so then it will affect the trail one or the other compared to forks of different a-c but same offsets

Presumably for a given head angle..?

I assume so.

looking at that diagram, steepening the head angle (by lowering the a-c) has the same effect on trail (reducing) as increasing the offset.

So in theory jef jone with longer offset and lower a-c than my o-o carbon forks should ride with less stability. Funny, doesnt feel like that. feels fantastic.

Comparing offset between forks only makes sense if they have the same axle to crown – a slacker head angle will increase trail. A shorter fork will amplify the effects of having a large offset – the bike will want to fold its wheel underneath every time you turn.

Of course, different people might enjoy having a more direct steering feel. YMMV.

Comparing offset between forks only makes sense if they have the same axle to crown – a slacker head angle will increase trail. A shorter fork will amplify the effects of having a large offset – the bike will want to fold its wheel underneath every time you turn.

Anecdotaly I find the opposite, adding longer forks to frames not designed for them seems to produce stable bikes, untill you turn the bars past a certain point, after which the steering flops over.

I vaguely remember Cy posting up some equations that quantify it, but basicly bigger offset forks reduce the trail and make steering feel quicker whilst going fairly straight, but does something else (presumably less floppign about) at tighter angles (i.e. riding switchbacks). The idea being to ballance out the offset and head angle to make the steering as neutral as possible through a wide arc.

I certainly find I can clean switch backs with the JJ forks better than I ever could with the taller forks.

The only time the front wheel has dived underneath me I was trying to mount the bike on a slope in the pub car park after 5 pints of Butty Bach. Bike and I went down like a sack of shit and I now have a scuff on my XTR reservoir cap 😳