(5th gen) Suspension expert recommends 15wt oil?

Hi everyone.

I know this is debated a lot but:

I'm replacing my VFR fork springs with some heavier ones (K-tech 9.5) and having read as much as I could find on the forums and FB groups, I emailed K-tech to ask which oil they recommended to go with the springs.

Their reply was Motul 15wt, with a 150mm air gap. This surprised me because most VFR owners seem to think that 15wt would be too hard, and that the VFR forks have "small holes" which need lighter weight oil.

K-tech are experts and must surely know what works well? Is the VFR a special case or something, where their normal rules of thumb might not apply?

I'm about 240lbs without gear, no pillion, no track days, just want comfortable road use.

Thanks
Greg

 

I have tinkered with forks on a number of bikes, and tried different weight oils and modified damper components. In my view K-tech may be advising that higher weight of oil to control the rebound in the stiffer compressed spring when that is released, compared to the energy released when the stock (more weedy ) springs are released. The damper components in the stock 5G forks tend towards light shim stacks but small ports, so with the thicker oil I think you will find the compression damping too stiff and a bit jolting on big bumps. But that is only my assumption. Changing fork oil is not a big task, so try the 15W and see what you think, it is easy enough to switch back to something lighter.

 

Thanks Terry.

Yeah I was wondering if that's why they recommended a larger air gap than normal (to soften the suspension a little)..?

Greg

 

The air gap will only change the bottoming resistance. Because the oil is incompressible, as the suspension moves it compresses the air gap on the top of the oil. A larger gap means the pressure rise is less than with a smaller gap. You should only notice that when the fork is near bottoming. I reckon the larger gap has been recommended because the spring rate is higher so less air "assistance" is needed.

 

What should the air gap be on a bone stock 5th gen? thinking of changing the oil to new motul 10wt.

 

My recollection is 130mm