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Right!

I've read everything I can find and I still don't understand the purpose of preloading the rear suspension on a motorcycle. Will someone please explain it to me?

Warning- I will ask awkward questions.

The purpose of pre loading the spring (which I assume is what you refer to) is to have the correct amount of sag. That is to say, when you sit on the bike, the rear suspension will sag giving you a nominal ride height. Having sag is very important as it allows for the damper to have rebound control as well as bump control.

*Prepares for questions*

First of all, thank you for indulging me with this.

Now, as I understand it, preloading the spring means compressing it, i.e. squishing it up a bit so that it doesn't squish as much when I sit on the bike, yes?

So if we squish it up a bit, won't that make the bike sit lower before I get on?

Correct yes. With the rear wheel off the ground your rear suspension will be at full droop and therefore zero sag. With the bike sat on its wheels a small amount of static sag is required, for the previously mention rebound damper control.

Now, with too much pre load, when you lower the rear wheel back onto the ground (applying the weight of the bike onto the wheel), you might end up with zero static sag. Too little pre load, and you will have too much static sag.

Always best to go with dynamic sag measurements though, ie have the rider of the bike sat on, as of course dynamic sag would be different if Laurel sat on the bike compared to Hardy.

Right. This is where I get lost.

Say I weigh 100kg and if I sit on the bike it goes down two inches, but if we preload 50 kg it goes down an inch, and when I sit on it it goes down another inch what have we gained?

 

1" of static sag, where before you had 0.

zero static sag is bad, because say for example the road moves away from the tyre (braking hard, a crest, or a hole as examples) the wheel wants to move downward, but is unable to as the suspension is already at full droop. This means your unloading the tyre, which isn't so much of a problem, until of course the load comes back onto the tyre and presents you with a not so nice surprise.

So static sag is the sag you gain by preloading/compressing the spring?

You should certainly gain static sag with preload, but you should mainly preload to ensure dynamic sag is correct, static sag is normally a bi product of this.

Right.

What is static sag?

What is dynamic sag?

Static sag is the amount the suspension compresses under the bike's weight in comparison to its full droop measurement.

Dynamic sag is the amount the suspension compresses under the weight of the bike and the rider.

Right. So if I don't preload the suspension I will have both static sag and dynamic sag.

Why should I preload?

Was pretty sure this thread was going to be about something else.

Still thinking about it, when there is no rider the suspension must be virtually topped out (because the spring is pushing up) which will allow almost no room for rebound damping, but when I sit on the bike I create dynamic sag and, ergo, 'room at the top' as it were.

So it seems to me that the main function of preload is to prevent bottoming out? yes?
 

So- More preload will increase ride hight with rider seated?

More preload will increase ride stiffness?

Butt out Kendall. This thread is for grown ups who eat their greens. You won't find any sprouts hidden in our pockets.

Static sag can be a reasonably small measurement yes. But it does need to be there. If you have none then the spring rate is likely to stiff for the weight of the bike.



The main function is to be able to set dynamic sag correctly. Nothing to so with bottoming out, that's what bump stops are for.



More preload will increase ride height with rider seated yes.

More preload should not have any effect on ride stiffness. The spring has a linear stiffness rate. To increase ride stiffness, you need to fit a spring with a higher spring rate.

You should run at least some preload, to ensure the spring maintains contact with it's platform. With no preload at all you risk the spring floating away from its platform which will prevent the damper from controlling the wheel as it's designed to do.

So if I am going to adjust and take measurements, it has to be with dynamic sag.

Now, say I sit in the bike and it goes down an inch, do I just increase preload until it only goes down say, an eighth of a frazzle?

If eighth of a frazzle is your required sag then sure. Every bike has a unique optimum sag.

I'm trying to understand what constitutes optimum sag though.

Optimum sag will lie somewhere between having too much pre load and too little. Sometimes found through personal preference, often set by the manufacturer.

Buy a speedway bike, be a lot easier  :D

Interesting stuff tho.

So what you are saying is I have to know how much sag I need to set the preload, and I need to know how much preload I need to set the sag?


How much sag is too much?

How much sag is too little?

If it's bike specific then how do I measure for my bike?

I need some reference point I can actually use.

YouTube: http://www.youtube.com/watch?v=TMDCEU98_9w

This is in Australian but you get the jist

I have really warmed to this thread since Tikay was told to butt out of a conversation about optimum sag.

I hope he was wearing the plum jacket at the time.

Thanks for all your efforts with this Pinchop. I understand it better than I did.