Fork Uneven Height

[ozn3]

It's my first time fiddling with suspension setups and I recently had mine adjusted by a mechanic. I had my forks raised by 5mm from the stock position (flushed at the triple clamp). Bike handled when I took the bike for a spin.

Upon re-checking when I got home, the right fork is slightly higher by approximately .5mm - is this a big cause for concern or doesn't matter? Apologies for my OCD-ness..

[betoney]

4 hours ago, ozn3 said:

Upon re-checking when I got home, the right fork is slightly higher by approximately .5mm - is this a big cause for concern or doesn't matter? Apologies for my OCD-ness..

No concerns at all.

[stumpy]

i would double check the torque on the clamps bolts for sure

 

[BBB]

as @stumpy, just check that the torque of the pinch bolts is set correctly so that it isn’t that one side was set evenly but is now creeping out of position. If all is good then I would leave them as they are.

[nhchris]

This topic makes my head hurt.  Please help!

By sliding the fork tube UPWARDS in the clamps the stock rake and trail measurements are both REDUCED.   Correct?

Reducing both these measurements has to make the bike more unstable, especially at speed (less trail).    Correct?

Take a look at this series of diagrams to see what I mean.

But... does the increased weight on the front wheel offset the instability caused by the reduction of rake and trail?

And...  has anyone actually (accurately!) measured the real-world increase in weight on the front wheel caused by a 10mm raise of the forks?

Or is this fix another internet  urban legend? 

And no fair saying "I did it and it just feels better."  Of course it feels better YOU did it!!

Thanks for the help getting my bean around this topic.

Edited July 21, 2019 by nhchris

[betoney]

@nhchris - Its not an urban legend, road racers have been doing it for decades to get more precise steering and front weight bias for tire grip but you can overdo it.  Thats why everyone mentions increasing the fork height incrementally in millimeters, usually 5-10mm with several people reporting that 15mm is too much on this bike, personally I try mine between 5 and 8mm.  Additionally, almost every aftermarket shock has ride height adjustment to achieve the same thing.

Same thing applies to dirt bikes, I have been altering steering geometry on mine for years.  Its especially critical when switching different tire manufacturers, not all sidewall and tread height is the same and some will really mess up your steering/handling.  I used to switch from a 19" rear rim with a knobby to an 18" rear rim with a trials tire and the steering geometry is completely different.

 

[duhs10]

1 hour ago, nhchris said:

And...  has anyone actually (accurately!) measured the real-world increase in weight on the front wheel caused by a 10mm raise of the forks?

I would actually be curious to see these numbers if there was an accurate way of measuring.

[ozn3]

12 hours ago, BBB said:

as @stumpy, just check that the torque of the pinch bolts is set correctly so that it isn’t that one side was set evenly but is now creeping out of position. If all is good then I would leave them as they are.

bolts torqued to spec. I'm not comfortable bringing it down myself so I'll leave that .5mm for the time being  Thanks for providing some peace of mind!

[nhchris]

Thanks @betoney.    I respect you knowledge and experience.

What confounds me is the counter intuitive nature of the adjustment.  If we are seeking improved high speed stability (+100 mph) why would we ever reduce the trail value?

I think most road race bikes are equipped with steering dampers. Maybe these nullify any reduction in straight-line stability caused by moving the fork tubes up in the clamps and resulting reduced trail.

I might try this as an experiment just for the fun of it.  (Can someone tell me the torque spec for the triple clamp pinch bolts?)

 

[BBB]

Upper 19 lbft

Lower 17 lbft

It’s a fine tuning thing @nhchris. My OEM suspension was twitchy and dived too much on braking. Once I had the fork internals matched to my weight and revalved, plus an upgraded shock this removed the instability that came from the cheap OEM setup. I could then raise the forks in the triple trees to give me a  controlled sharpness that I prefer.  I now have a very responsive handling bike, without the instability, so it is possible to have a happy compromise. Without the suspension upgrades I would not have raised the forks as it would exacerbate the problems.

You can try it easily. Remove the front wheel and mudguard, loosen the lower pinch bolts completely, just loosen the upper ones and gently rotate the fork whilst pushing upwards until you have raised the fork the amount you want. Repeat on the other side and torque up the upper and lower pinch bolts to spec.

[norcal616]

It's way to easy to over think suspension by just looking at numbers on paper but ultimately it comes down to gotta try it...

but it's hard to simulate the real world conditions on paper such as nice race track vs normal roads with bumps, dips, crowns or some ppl carry higher RPMs in corners, different windscreens, riding styles, tires, frame bolt torque, motor mount torque, chain a bit longer...

I had to change my suspension settings this year by basically mainly backing the fork preload off to almost nothing(didnt even check sag)because they resurfaced/ground down alot of roads around here, if I left the previous settings in the bike was very harsh in terms of comfort/rider confidence...

 

 

 

 

[betoney]

1 hour ago, norcal616 said:

It's way to easy to over think suspension by just looking at numbers on paper but ultimately it comes down to gotta try it...

but it's hard to simulate the real world conditions on paper such as nice race track vs normal roads with bumps, dips, crowns or some ppl carry higher RPMs in corners, different windscreens, riding styles, tires, frame bolt torque, motor mount torque, chain a bit longer...

I had to change my suspension settings this year by basically mainly backing the fork preload off to almost nothing(didnt even check sag)because they resurfaced/ground down alot of roads around here, if I left the previous settings in the bike was very harsh in terms of comfort/rider confidence...

Very good points.  It has been said here before but to reiterate- ride with a screw driver and wrench to adjust your suspension, you can make changes on the fly and immediately feel the results.

My buddy and I both have '15 FJ's, both with Penske shocks and fork modifications.  On a recent road trip we swapped bikes and he was in awe at how my bike handled, we made a few changes to his, then rode for a bit and made a few more changes and finally he was ecstatic with the results.   

[nhchris]

What a great idea... I will try changing settings on the fly next ride.  Thanks for the tip.

In general, I'd like the bike to be more composed over the bumpy roads we have here in New England.

I am happy with the way it turns in, the lightness of the counter steer inputs required and the overall ride.  I am not too sophisticated a racer, but will push it.

I am running the rear shock pretty hard (5 out of 7?) and fooling with damping to smooth out the ride. Up front I set 40mm of sag on the shock tubes and again am twisting the damping to see how it feels.

At 210 lbs I know I am at the high end of the "average" rider the suspension was designed for.  (I am also trying to lose some weight but it ain't easy!)

On a recent ride I accelerated out of a toll both full wide open to110 mph in 5th. The front end felt VERY light the entire time and I wondered if the front wheel was even planted (TC was on but did not activate). It was a bit unnerving and did NOT inspire my confidence in the bike. 

But again, coming off an ST 1300, maybe this lightness of being is something I need to embrace!

At this point I don't want to sink $2,000 into suspension mods. I think I'd rather try a Versys 1000 or find a used R1200RT with low mileage.  Then wifey could occasionally ride along too.

Edited July 21, 2019 by nhchris

[betoney]

@nhchris - On the FJ (and Versys as well) you are limited to preload and rebound adjustment. 

You don't NEED to drop big dollars into suspension but at a minimum, correct springs for your weight (about $200) make a huge difference in getting your suspension feeling correct.

I am not at all surprised with your experience under heavy acceleration, think about the physics, the rear end is squating and the front end is lifting, with a stiffer shock spring that effect is lessened.

[stumpy]

20 hours ago, BBB said:

Upper 19 lbft

Lower 17 lbft

It’s a fine tuning thing @nhchris. My OEM suspension was twitchy and dived too much on braking. Once I had the fork internals matched to my weight and revalved, plus an upgraded shock this removed the instability that came from the cheap OEM setup. I could then raise the forks in the triple trees to give me a  controlled sharpness that I prefer.  I now have a very responsive handling bike, without the instability, so it is possible to have a happy compromise. Without the suspension upgrades I would not have raised the forks as it would exacerbate the problems.

You can try it easily. Remove the front wheel and mudguard, loosen the lower pinch bolts completely, just loosen the upper ones and gently rotate the fork whilst pushing upwards until you have raised the fork the amount you want. Repeat on the other side and torque up the upper and lower pinch bolts to spec.

you don't have to remove anything...with the bike on the center stand, loosen one side pinch bolts just enough that you can rotate and twist up the fork up to your desired height and then re-torque.  repeat on the other side.  it only takes about 10 mins to do this.