Sag and preload etc

Ok folks im going to risk asking another question and hopefully the title wont get changed and then deleted like the last one!

So sag and stuff. Ive got zero preload on the front of my 1999 curvy at the mo and have progressive suspension (the company) springs with 10w oil and a larger than normal air gap. Now things are good but under breaking theres a lot of dive it doesnt quite bottom out but its a lot.

ive measured with the front in the air and get 127mm fron the dust seal to the yoke.

now with the bike on the flat its got 98mm so thats 29mm of sag with the bike weight on it.

now when i get on (no it doesnt bottom the forks out ) i get 87mm so is that 40mm of sag?

i know you should have about 30mm i think so was going to pop a couple of 5/8 washers in the top to give about 8mm of preload. not sure what this will do but thourght it might be worth a try.


Your thourghts on the subject please

[Luckypants]

when you say you get on the bike and you get 40 mm of sag, is that with you wearing all your bike gear (leather is heavy) and with both feet off the floor? It is essential that you measure in the riding position.

If you are measuring correctly, the 40mm of rider sag would be too much (IMHO) with linear springs, but not too sure with progressives. Your plan of adding 8mm of preload sounds good.


PS If you are using a cable tie to measure the amount of travel used, how much travel is used on a normal ride?

ima bout 10mm off the yokes on a normal comute (4 miles)!

[Luckypants]

Sounds like you are using up all your travel with nothing in reserve for bumps when braking hard. More pre-load is definitely a good idea if this is the case.

any idea how preload equated to travel?

[timwilky]

I would agree that your sag should be measured loaded and set about 30mm.

What does concern me is that you say you have more airgap than standard, what is the gap? remember the oil does not compress but the air does, you would be effectively reducing your damping with this larger air gap.

As Mike says, add your preload. but also top up the oil

Originally Posted by AndyBrad

ima bout 10mm off the yokes on a normal comute (4 miles)!

The seals will not bottow out any way near the yokes - manufacturers make sure of this otherwise you would crush the seals on bottom out.

If you've got your travel with 10 mm of the bottom yoke then you must be very close if not on bottom out.

[Luckypants]

Originally Posted by AndyBrad

any idea how preload equated to travel?

I have no experience of progressives, so cannot really say. With linears I believe the relationship is direct. i.e. 8mm more preload gives 8mm more 'free travel'. No doubt I'll be corrected if wrong.

Perhaps someone with progressive springs can comment.

www.progressivesuspension.com says give an increased air gap due to the springs being substancially thicker than the stock ones.

Originally Posted by timwilky

I would agree that your sag should be measured loaded and set about 30mm.

you would be effectively reducing your damping with this larger air gap.

Not strictly true, I think. The air gap acts as a secondary spring when the forks compress beyond a certain point. The bigger the air gap the softer the forks on bigger compressions (hard braking). It would not effect damping.

To be more accurate with the air gap and make sure it isn't causing a problem perhaps try this: set the correct air gap, insert the stock spring and measure the new air gap. Then set the air gap with the new springs in to be the same as the air gap with the stock springs in. I think this would be more accurate than "just add a bit less oil". Although the air displaced by the new springs would be more than the air displaced by the stock springs you would now have a smaller volume of air than with standard but the difference would surely be negligible.

Set preload for 8-10mm.