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I'm considering turning my DR650 (2016) into a road race track day bike. Having already put 17" tires on front and rear for supermoto/canyon duty, the trail has become so short that it is not very hard to tuck the front, and it is just on the stable side of being prone to tank slappers. I kinda expected this, and I know this is road race content in a dirty area of the site, but I was hoping to pick the brains of the engineering/experimenter sort.

What downside can I expect if I were to fab up some different trees with less offset to bring the trail back into a more acceptable range? Anybody experiment with this?

I can continue lowering the rear to get things back in line, but I don't want to lose cornering clearance. I've seen H-D road kings with negative offset.

Any thoughts?

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I don't think there'll be much downside if it's currently not hard to tuck and just short of tank slappers.  You'll gain stability, and what you lose in steering quickness you'll (maybe?) gain in fall over (lean in, turn in).  Fall over is increased somewhat with more offset, but increased more so by more trail.  You'll also get lighter steering by having the weight of the forks and wheel closer to the steering axis.   I think all of that is upside in your case, hopefully not high side ;)

I would start by measuring/calculating your current trail and increase it slightly.  If you have fab skills, you could make eccentric bushings for the triple clamps for a few different offsets.  Or you could copy something like the Rekluse E axle that would change the trail without changing the offset, but you wouldn't get the lighter steering that you would with offset.

Edited by bg10459

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21 hours ago, bg10459 said:

Or you could copy something like the Rekluse E axle that would change the trail without changing the offset, but you wouldn't get the lighter steering that you would with offset.

In fact, the effect of changing the offset is identical regardless of whether it changes at the triple clamp or at the axle lug, because it changes exactly the same thing about the front end geometry. 

There is more than trail involved, too.  Steering head angle is at least as important, if not more so.  MX bikes set up at around 27 degrees, typically, and are prone to pushing because of it.  Sport Touring bikes like the FJR come in at 26, while the R1 has a head angle of 24.  Flat trackers are usually built around 22-24 degrees, and Speedway rides are often less than 20.  The trend line is that the steeper the head, the more precisely the chassis will follow the front wheel but often at the expense of an increased tendency to tuck, and a loss of stability at speed almost regardless of trail amount.  The tucking thing can get to be really pronounced at at extreme lean angles, too, almost to the point of the wheel being pulled in and "riding over" the front wheel.

If You've ended up with a head angle of less than about 25, you should raise the front or drop the rear to get back to around 25.5, IMO.

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The effect of offset is not identical when changed at the axle as it is at the clamp, simply because of the weight of the front end.  The change in trail will be identical (because that's what you're changing) but when it's made at the axle, it doesn't change the fork offset, and therefore does not change the swing weight of the front end (or only by the shift in weight of the wheel), nor does it change the effect of that weight in fallover.  When it's changed at the triple clamp, it changes both the swing weight and the trail.

I agree with everything else.

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The swing weight of the fork assembly isn't what gives the steering the lighter feel when the trail is reduced, it's the "fallover" effect, which is based on the distance of the tire contact point behind the steering axis (trail), and the weight of the bike itself.  From experience, there is no difference between a 2mm reduction in clamp offset, and a 2mm reduction in forward axle offset in the feel of the steering while in motion faster than 3-4 MPH under any condition.  It's a 2mm increase in trail either way.

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I did a quickie measurement with my protractor this morning and it looks like the rake, as modified, is around 30 degrees. I took a look in the book and it lists the stock rake as a whopping 43 degrees. I suppose that's to get the geometry right with a 17" rear and a 21" front. I put a 1/4" spacer in the shock when I revalved it and moved the lower eye of the shock to the factory-provided "lowering" position. I lowered the front end about 3/4" and with the 17" front, it has come down quite a bit.

I guess that's why converting my YZ to a supermoto didn't yield the same radical changes since the chassis started out with a 19' rear and the 21" front. That bike worked pretty nicely. A slight understeer...very slight.

This DR is prone to some serious oversteer. I have to keep widening my line through sweepers.

I agree with both of you guys in general, and while I haven't given any thought to swing weight, I'm not sure it is much of a factor with these bikes...or I'm not that sensitive to it.

Getting the rake kicked back out there will certainly get things back to more normal characteristics, but I'm embarrassed to say that I have an aesthetic issue with there being any more rake. That's why I began to contemplate the tree offset.

I ran across some pretty cool gizmos at Cheney Engineering; that presumably address this issue. They convert a leading axle to a location just below the bottoms of the forks. No big deal for a dirt tracker since the front brake would have to be re located, and it would just be a day or two on the mill, but I think machining a set of trees would be easier.

Thanks for the inputs!

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I don't think that's right.  43 degrees would be unheard of.  Just by googling a DR650 I would guess its somewhere in the conventional 28-30 degree range. 

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Must be a misprint.  As you can see, that would be a chopper.  Can't be right.

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