Sag theory for forks

[pkboxer]

If I want to increase my sag 1" would I decrease my preload by 1"?? (just replaced my progresive fork spring with a stock BMW spring) Measuring between the bike on the center stand and the bike off the center stand - no rider weight)

I wouldn't think that it's exactly 1:1 for the amount of preload removed and the amount of sag increased and would probably depend on the spring strenght.

I just need if it's close enough, or if 3/4" less preload will give me 1" of additional sag.

Or do I need to remove a bit more preload than the additional sag that I want.

Thanks - hope this is making sense.

[Major Softie]

If I want to increase my sag 1" would I decrease my preload by 1"?? (just replaced my progresive fork spring with a stock BMW spring) Measuring between the bike on the center stand and the bike off the center stand - no rider weight)

I wouldn't think that it's exactly 1:1 for the amount of preload removed and the amount of sag increased and would probably depend on the spring strenght.

I just need if it's close enough, or if 3/4" less preload will give me 1" of additional sag.

Or do I need to remove a bit more preload than the additional sag that I want.

Thanks - hope this is making sense.

It is 1:1. The compressed spring will always be exactly the same length. You are raising and lowering the bike with the spacers, so the spacer change is exactly the distance you want to change the sag.

I think what is confusing you is that you are mixing it up with how much you would change spring length to change sag. If you were cutting off some spring length, THAT would not work 1:1.

[pkboxer]

Thanks MS, I figured it would either be 1:1 or pretty close to that, but didn't know.

[Roy Gavin]

Most suspensions work best with some preload/pre compression on the springs, somewhere between 16 mm and 20 mm is Ohlins recommendation.

So you really want softer springs, as you probably dont have 25mm preload now, never mind the 45 mm you would need.

You will also upset the balance, front to rear. 10% less rider sag on the rear works well for most folks.

Given the weight of the unsprung bits on an airhead I dont think you would want much more than 55mm rider sag front and 50 mm rear.

Shortening a spring will make it stiffer, and as you still require the same preload it will actually reduce the sag.

[ME 109]

The only sag I'm familiar with is on sports bike riders faces.

[pkboxer]

Roy, I had almost 45mm of preload that was in it from the progressive type springs that I replaced.

I put in a stock set of springs that were also a bit longer than the old set.

I was only getting 1/2" of "free sag". (I don't know if that's a real term but that was what was in the article I was reading)

Same article also said 1.5" of free sag was a good starting point.

Now I have about 16mm of preload in there and about 1.5" of sag w/o rider weight.

I'll ride it and see how it's handling.

Thanks for the input.

I

[Roy Gavin]

By sag I meant sag from wheels off ground to on ground with rider on board. So your 38 mm without a rider will probably be close to the - 45r / 50 f that works well on most road bikes.

If your progressives have 45 mm preload you have different ones to the one I bought, but even with less they were too stiff.

[Duane Ausherman]

As I understand the "sag" theory it is nonsense. Especially when it is related to the preload spacers.

[Roy Gavin]

The theory that you can make the wrong spring the right one by changing the preload hasnt much merit.

And the suggestion that sag should should be a certain percentage of total suspension travel has considerably less.

And most "experts" confuse race sag with road sag.

But at the end of the day sag figures offer a simple way of comparing the relative stiffness of springs as installed.

But balance, back to front is more important that any notional amount of sag, as a bike can be set up hard of soft and still work OK, as long as the balance, and damping, is correct.

And up until recently balance back to front didnt get much , if any, mention in most how to set up your suspension guides.