Fork/crown offsets

So I've been messing with the triple clamp and axle offsets on my moto a lot lately. 2mm fore or aft can have a HUGE effect on the way the front end oversteers or understeers, wants to take high lines/low lines, outside lines/inside lines, and steering input required.

The total amount of offset on a moto axle lug and triple clamp is a bit more than that on a DH bike...I think. So it stands to reason that an even smaller amount of offset change would produce drastic results on a DH front end.

I've noticed the understeer issue on a lot of modern bikes with slacker head angles. Lots of pushing in corners, and a necessary large weight shift to the front end to get it to knife in.

Just out of curiosity, is there any data available for crown offsets and overall offsets for all the major DH forks? 40's, 888's, Boxxers, Dorados, BOS, whatever else. Would be interesting to see what variation there is between each model. Perhaps there is a more ideal offset to match every head angle?

Not a stupid question. On previous bikes, I've swapped out triple clamps (crowns basically) that have a different offset. Usually ones that came with the bike as I bought it because I'm not spending that kind of money on that. Some KTM clamps have the steerer tube held in with a pinch bolt at both ends, and it's made to be 1mm eccentric, so rotating it will change the effective triple clamp offset.

Just tried using this thing called an E-axle on my Yamaha yesterday which is made by Rekluse. Basically it's an eccentric replacement axle that will let you put the hub center at +2mm or -2mm from stock. Pretty clever little piece.

Raising or lowering the fork legs is a different adjustment altogether. That changes mostly rake (and trail ever so slightly), in addition to your front/rear bias. Changing the offset affects the trail measurement. If I remember right it really affects the contact patch of your tire as you lean - someone feel free to correct me on this.

On the newer super slack DH bikes, I'd be inclined to try a bit less offset to correct the push/understeer they sometimes get. -1mm or -2mm probably? Another limiting factor is how much negative offset you can get away with in the caliper, but that could be easily fixed by spacing the caliper back a bit with washers.

This is also why I'm curious what the differences in offset are between fork. An interesting test would be to try a 40, 888, and Boxxer back to back with the same springrate and valving, like an Avy cart, set them at the same a2c height, and see what the cornering differences are.

I would say that the difference you notice in a mm or two of offset change on a moto is very similar to the change you notice between a degree or so of head angle on a DH bike. Not the same sort of change obviously, but the amount of effect it has on handling is in the same ballpark. Same with raising/lowering the forks in the clamps. This seems to have a lot less effect on DH bikes though, but it's still noticeable.

I know the offset on the 888's changed between the 35mm models an the 38mm models. No idea about the Boxxers between generations, or what the measurements are on any of these. Hopefully someone here knows or can measure.

Interestingly enough, at this point I have NO fork to experiment with. I had a bunch of 888's for a while, then a Dorado for the last year, which I was "involuntarily relieved of" along with the rest of my bike, so I don't even have a DH bike at the moment. I went and spent the frame money on a YZ450F instead. So far, a more enjoyable investment that gets more use than a DH bike in the garage.

But yeah, if I still had access to a full CNC shop, just making a single run of a bunch of different crowns with different offsets would be real easy.

It's a good amount of money for the raw aluminum. Then the CNC programming. Then the machine setup time (which is going to be FAR more than the actual run time for one set - this is the driving cost) plus the labor. Then QA. Then some profit for the shop built into it.

Not cheap unless you have either 1) volume or can do most of the legwork and material sourcing yourself or 2) can just do the whole damn thing yourself and have a convenient source of scrap material.

I think they're graphical definition for trail is slightly off (at least from all the explanations that I've seen in the past).

The way I know it is the distance on the ground between the intersect of the steerer angle (with the ground) and a plumb line dropped from the axle. Their diagram shows an angled dimension between the steerer angle and the axle oriented above the ground. Kinda hard to tell where that second line comes from, and the dimension is not isolated from the steering angle. Although they express it as a scalar through their derivations, so the angle doesn't really mean much.

"Rake" is head angle.

"Trail" is a measurement that is derived from head angle, wheel diameter, and offset (lugs plus crowns). What the "trail" measurement affects is the way the contact patch migrates as the wheel is turned/leaned, and consequently, the amount of leverage the contact patch has about the steerer tube axis. It affects the feel of self-centering or wheel flop in the front end.

My original point was that modern DH bike suffer badly from wheel flop due to the extreme head angles. There has never been an update or a real serious critique of offset to go with the head angle changes, and I think that's due. It's the reason a Boxxer equipped bike can turn inside an otherwise equivalent 888/40/Dorado equipped bike.