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FlyingFinn 01-19-2009 10:29 AM

Suspension Wisdom
 
I'd like to suggest a place for non bike-specific discussion about suspension tuning, how suspension works and how it affects the handling of a motorcycle.

I'd like to keep this fairly generic and leave out questions like "how do I adjust the ride height of a DRZ400?"
Rather focus on things more like "What does it do to stability and handling if I lower the the ride height in back and front by 10mm?"

Also, lets try to keep this in topic and not post pictures of you cute dog or hot girlfrind here, ok?

Over the years I've spent my time on playing with Ohlins and Wilbers shocks, turning the clickers, adjusting the preload etc. Changing springs and fork oils, playing with the oil level and messing with cartridge emulators.
As well as studying the relevant literature.

So I'm starting to form some sort of idea of what's going on but I feel I still have way more questions that answers.
I'd like to discuss those questions, and questions from other in here so we all can hopefully learn something new.

Here's my latest head scratcher.
I'm setting up a street bike R for my GF. She weighs 2/3 of what I do.
I've selected correct springs for her weight and adjusted the preload (both for F and R), so laden and unladen sag is spot on for her.

To tune the damping I ride her bike up and down a twisty and bumpy local highway that's as if made for suspension testing.

But how should I "compensate" the damping settings for our weight difference?

In other words, if springs are set correctly for 120lb rider and a 180lb rider is tuning the bike, will the the same rebound settings that are best for the 180lb rider also be the optimal settings for a 120lb rider?

My personal quess is that since the rebound damping controls the energy stored in the springs, then the rebound setting is mostly set to match the springs, not the rider's weight.

--
Mikko

mark1305 01-19-2009 02:15 PM

I worked for a large track day provider for a few seasons, and we had a suspension vendor at most events. A really good one. I watched him do setups that netted a typical rider 1-2 seconds off lap times.

Other than his vast experience and a whole book full of info from Ohlins (he's a legit Ohlins vendor), the thing I came away with and used on the Ducatis I worked the track with was simple but effective.... After getting springs and preload set properly as verified by the unloaded and static sag, stand next to the bike and with both hands on the top rear of the tank, suddenly push down with about 2/3 of you rweight and release. Both ends of the bike need to rebound evenly. And the rebound travel should take somewhere between 1/2 and 1 second. Closer to the 1/2 second, but a perceptable damping of rebound speed. I've tried it, and it gives a very good baseline setting for rebound.

Compression is a little more trial and error for me, so I'll defer to someone else on that.

FlyingFinn 01-19-2009 03:30 PM

Quote:

Originally Posted by mark1305
...stand next to the bike and with both hands on the top rear of the tank, suddenly push down with about 2/3 of you rweight and release. Both ends of the bike need to rebound evenly. And the rebound travel should take somewhere between 1/2 and 1 second. Closer to the 1/2 second, but a perceptable damping of rebound speed.

I've done that too but I guess I don't have accurate enough eye to judge the rebound. Both ends evenly and in under a second... I have hard time "seeing it".

But that tip also kind of tells me that the rebound really is set to match the springs, not so much per riders weight.

--
Mikko

Frank Warner 01-19-2009 04:23 PM

If you think of

1) the bike and rider remaining at a fixed height
2) the 'road' moving up and down under each wheel

Then the suspension is simply controlling what the wheels do - trying to keep them in contanct with the road most if not all of the time...
Too little compression damping and the wheels when the road moves up will keep the wheels moving up even when the road moves down.
Too much rebound damping will have the wheels leave the road when the road moves down ...

View this way the damping is effected by the spring strength and the weight of teh wheel togther with anything attached (brakes, swing arm, etc).

This is a simplistic view .. does not deal with too little rebound or too much compression. But it gives you an idea of what goes on..

Jonex 01-20-2009 06:51 AM

One thing to remember is you'll likely want very different settings for a track bike vs. a street bike.
On the track, you're going a lot faster so you need the suspension to be faster which means rebound settings which would be harsh on the street. Too little rebound and the bike might be too busy. Too much and traction and tire life suffers.

For compression damping, I like to start with not enough and then gradually dial out handlebar wagging.

skwidd 01-20-2009 07:16 AM

Quote:

Originally Posted by FlyingFinn

To tune the damping I ride her bike up and down a twisty and bumpy local highway that's as if made for suspension testing.

But how should I "compensate" the damping settings for our weight difference?

In other words, if springs are set correctly for 120lb rider and a 180lb rider is tuning the bike, will the the same rebound settings that are best for the 180lb rider also be the optimal settings for a 120lb rider?

My personal quess is that since the rebound damping controls the energy stored in the springs, then the rebound setting is mostly set to match the springs, not the rider's weight.

--
Mikko

You are already well into the stroke of the suspension without the input from the road, and the bike will not react properly for her if you try to fix what you feel is too much or too little movement.
Most adjustable suspension systems are position sensitive, look at where the sag is set proper for her and the difference when you sit on it.
She may not even feel the bump that you're trying to adjust away, because she has much more range of travel in the suspension that is properly set for her weight.

FlyingFinn 01-20-2009 11:53 AM

Quote:

Originally Posted by Sonex522
On the track, you're going a lot faster so you need the suspension to be faster which means rebound settings which would be harsh on the street.

Not claiming to understand all the interactions but the above statement doesn't make sense to me.
Faster speed -> need suspension to allow tires react quicker to road surface irregularities does make sense.

But to make "suspension faster" you would use LESS rebound damping, not more. Right?

For track use I can see how stiffer springs might make sense to reduce the change in bikes attitude and reduced ground clearance when either heavy on the brakes on way banked over. The stiffer springs (less suspension travel?) would in turn require more rebound damping to keep movements controlled.
But is springs are kept same for street vs. track setup, I don't quite get why more rebound damping would be good on track IF the rebound is properly set for street use.

In other words, the way I see it a suspension that's comfortable (compliant, responsive and doesn't "wollow") is also good for fast riding. No?

--
Mikko

Frank Warner 01-20-2009 01:23 PM

Quote:

Originally Posted by Sonex522
One thing to remember is you'll likely want very different settings for a track bike vs. a street bike.
On the track, you're going a lot faster so you need the suspension to be faster which means rebound settings which would be harsh on the street.

You do realise that the suspension has differnet responces to the rate of change? And that can be adjusted so you don't neeed to perform any adjustments when going from low speed cruising to high speed race track stuff? Ok... on most shocks those adjustments are internal .. but they are there and should be adjusted when your shock is serviced (say every two years for maintance reasons) .. if adjusted cortrectly you can sedt the low/medium/fast rates up to exactly what you want .. leaving the extenal adjustment for luggage adjustments only ..

Jonex 01-21-2009 06:38 AM

Correct, typically on the track you'd use less rebound damping to speed up the suspension so your tires remain on the ground. Traction is better when the tires touch the pavement:D .

This type of discussion gets all complicated when you start throwing in things like expensive, very adjustable shocks which can deal with a wider variation of suspension deflection rates. Typical OEM suspension just plain sucks and has a wide variation in adjustment and capability, so "it depends" is the real answer here. There is no "always", but there are some thumb rules such as: increased spring stiffness tends to reduce traction, increased rebound damping tends to reduce traction, zip ties on the forks can help when setting pre-load so you use the most of available suspension travel and avoid bottoming, low budget steering dampers can cause wallowing, etc. etc.

I like to keep it simple, and knowing that I prefer a more compliant suspension, I just set the sack to about 1.25" and run as little damping as possible - just enough to keep the bike from getting out of shape in the bumps and have predictable sliding. But, I've always had screen door closers and slinkies for suspension so my choices of knobs to fiddle with have been limited. Back in 1989, I raced a GS500 on bias ply tires with stock suspension. I had to run max shock preload to keep from bottoming in turn one at Blackhawk farms. I had to carry the brakes way into the corners to keep the front end bottomed to prevent pogo sticking - basically the front tire WAS my front suspension. It doesn't get much worse than that. I felt all decadent when I got a bike with damping adjustments, and happily cranked all the kick out, only to find out the thing pushed like a Dodge Dart with bald tires.


Quote:

Originally Posted by FlyingFinn
Not claiming to understand all the interactions but the above statement doesn't make sense to me.
Faster speed -> need suspension to allow tires react quicker to road surface irregularities does make sense.

But to make "suspension faster" you would use LESS rebound damping, not more. Right?

For track use I can see how stiffer springs might make sense to reduce the change in bikes attitude and reduced ground clearance when either heavy on the brakes on way banked over. The stiffer springs (less suspension travel?) would in turn require more rebound damping to keep movements controlled.
But is springs are kept same for street vs. track setup, I don't quite get why more rebound damping would be good on track IF the rebound is properly set for street use.

In other words, the way I see it a suspension that's comfortable (compliant, responsive and doesn't "wollow") is also good for fast riding. No?

--
Mikko



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