Suspension config - theory/geometry question

[Slide Panda]

Ok, so I picked up a KTM 690 Enduro. The front axle mounts are a bit in front of the centerline of the forks. The 690 SMC, it's street oriented brother has the axle mount just like any other street bike - center of the axle is in line with the center of the fork.

What effect does that different placement translate to in feel and application?

[battlecry]

Sounds like it reduces the front tire trail.  It should reduce the straight-line stability of the bike and make for easier turns.

AN example of an instantaneous reduction in front wheel trail is when you race down a hill in an enduro and hit a patch of sand at the bottom.  At that moment the contact patch moves forward on the tire, reducing the trail.  If it moves way forward, the front turns on you, you fall forwards, land hard, tear your rotator cuff, damage your hand, and watch your friends pass you as you try to reach for the kill button with your good hand.

[Slide Panda]

I should have had my coffee and hit the books instead of being lazy. Yep, it reduces the trail. Which makes perfect sense as this is the dirt version and would expect to see lower speeds with tighter turns.

[Curmudgeon]

Sounds like it reduces the front tire trail.  It should reduce the straight-line stability of the bike and make for easier turns.

Axle in front of the leg? Should increase rake which = more stability. All my Maicos were like that. Also permitted larger fork leg volume for springs, oil and air. 30 degrees used to be considered a good compromise for stability in the rough. No idea of current philosophy though.

Just looked up a new Husky WR and 27 seems to be "the deal" these days...

[Slide Panda]

Reduce is right for trail. Rake is a different measure



We need to factor in offset, as that diagram is 0 offset. But the trail is still reduced when the wheel is moved forward of the axis

[Curmudgeon]

Reduce is right for trail. Rake is a different measure

We need to factor in offset, as that diagram is 0 offset. But the trail is still reduced when the wheel is moved forward of the axis

You ain't riding a bicycle and suspension on a dirt bike is a compromise.  Just to muddy the water, take the measurements with a rider on the bike. Then measure at the extremes of the suspension travel. My old bikes had ~ 8". At one point I believe they got as extreme as 13" but I believe they've settled on ~ 11". Very different than a street bike which averages 4" - 5" (unless you ride a BMW!  )

[Slide Panda]

Oh yeah.. it's going to be a process.

For even more fun I've got a set of 17" and 21/18 wheels.  Aaaaand there's triples available with more offset which would also reduce trail

[Curmudgeon]

  That really makes my head hurt!  Hope you're a "math major". Maybe the seat-of-the-pants dyno will be the best course. 

[Slide Panda]

Yeah, I'll be doing some measuring and math and seat testing. Since we can rattle off numbers all day, but the feel of a bike is subjective for the most part.

[Speeddog]

You didn't say, but I suspect the Enduro runs a 21" front, and thus would need more offset to get the same trail as the SMC with a 17".

Furthermore, there's a dramatic difference in tire width between a 21" and 17".

[Slide Panda]

Correct sir. Yes the E ships with a 21" front, 18 rear. But I've got a set of 17" wheels for SuperDoDo shenanigans. Currently that's the only modification that changes the geometry from the stock config... only he says.

The SMC ships with a triple that's an offset of ~33mm vs the ~24 of the enduro. The SMC axle is inline with the fork leg, while the E mounts it infront of the centerline