Tire Pressure vs. Shock Preload

[Anonymous]

I am hoping to get some advice on setting up the rear suspension of my GS1100EX. It has Dunlop K491s front and back along with Progressive Suspension rear shocks. The rear shocks only have a preload setting. The front forks don't have preload adjusting anymore. The pegs broke off of inside the forks.

My problem is that the highways here are quite rough and definately not good for building up my confidence. When I have the suspension soft the bike bottoms out on straight-aways and wobbles through corners. When I have the suspension hard and the tires pumped up the back of the bike chatters through the corners and is just as hard on my back as bottoming out. All of the corners up here have long drop-offs and cable fences so I'd like to keep both tires on the round 100% of the time. #-o

The bike currently has 30/25 PSI front/back and rear shocks at 2nd stiffest position.

Thanks, Steve

 

[gspaul]

Pressure is to low for modern tyres,I know the bike manual may say otherwise.
I run 35 front, 45 rear, work's for me.
Maybe check the maker's website!
Go with the right pressure first, then play with the shock's.
If the front spring's are to soft,cut a spacer to go on top of the spring.

 

[argonsagas]

Pressure is to low for modern tyres,I know the bike manual may say otherwise.
I run 35 front, 45 rear, work's for me.
Maybe check the maker's website!
Go with the right pressure first, then play with the shock's.
If the front spring's are to soft,cut a spacer to go on top of the spring.

He is right on all points, Steve. DO check with Dunlop.

Your tire pressure is WAY down on the back.

My Metzelers run with 38/40 rear, depending on load, and 35/36 front, and they do not perform well if pressure is down.

I had my rear tire drop from 40 to 32 in one day. That made the bike twitch on high-speed curves and made me very uncomfortable.

Off the highway, refill it, and it was fine again. Curiously, I could find no reason for the sudden pressure loss, and it never leaked again.

 

[gspaul]

I have a very slow leak in the front ,I can feel when it get's down to 28 psi ,I feel like I'm drifting in the corner's.

 

[KGB]

Steve,
What is the sring rating for the rear shocks?
You may have a heavy spring mounted up to those shocks.
Progressive does offer heavier spring rates.
I'm a large person, now down to 255lbs, I special ordered a heavy
spring for my 1000S, With just me, first or second setting.
With my wife #3 setting.


As already stated, check the Dunlop web site for tire pressures.
What does it reccomend on the sidewall?, for pressure?

 

[saaz]

Pretty well for most bikes (track days excluded) 36psi front/42psi rear is a good starting point. Once you have the tyres within reason, then try the suspension.

It sounds like at the moment the low tyre pressures would make it hard to get the suspension set up properly. Try doing a search on suspension setup. You need to set up the amount the front and rear sits down on the suspension first, then adjust the damping to suit. Front fork oil being too old and a worn out rear shock would make the bike a bit scary with those tyre pressures!

 

[Sandy]

The front can be setup using air and/or different spacers to adjust the spring and different weight fork oil to adjust the dampening. If you're using a lot of spacer you may need to look at different springs. You need the right combination of both to work correctly for your riding style and weight. There really isn't much you can do with the rear if all you have is adjustment on the spring. By the sounds of how the rear is behaving, or misbehaving, you need to set the spring slightly higher than the soft position and have some way to increase the damping a bit. The way I usually set up suspension for the street is to go with a light damping setting to start with and set the springs so they are compliant on most bumps and only approaching bottoming out on large frost heaves. I then increase the damping to regain the suspension control without the suspension becoming too abrupt. The technique I usually use is to set up both ends to be close to what I want then fine tune the front. To fine tune the front I put a nylon tie wrap around each fork tube between the triple clamps and lower fork leg and slide them down within the range of suspension travel. I then set one tie wrap at the position where the suspension bottoms out and leave it there, I then use the tie wrap on the other fork tube to gauge how much suspension travel is being used. I then play with the spring rates and damping so that the suspension only approaches bottoming out under aggressive riding, braking or on rougher roads. It will have to be set for your riding conditions. I then set the rear shocks so that they sag and rebound at the same rate as the front. For this you will need shocks with both spring and damping adjustment.

Just remember that suspension is a combination of spring rate and damping control. The springs hold you up while the damping controls the rate of movement. Too much spring and not enough damping it will feel like a pogo stick. Too much damping and not enough spring and the ride will be harsh and the bike will sag too much when loaded and bottom the front under hard braking. You need a balance between the two.

Oh yes. the tires should be run in the mid to high 30's as mentioned above.

 

[Anonymous]

Thanks all, I will turn up the tire pressure some more. The rear tire says to use something like 44 PSI for highway cruising and heavy loads but I couldn't bear the handling last time I tried it.

Steve

 

[Anonymous]

Tire pressure is helping. I've decided to run the bike as stiff as possible and avoid riding in town at all costs. At 35PSI front and back with the 2nd stiffest suspension setting I have been able to comfortably ride at legal speeds. ps. The springs are the same as the stock ratio.

Tonight the suspension turned sloppy on me taking a decreasing altitude turn at an estimated 160 km/hr. I went a little wide and started to wobble but I had expected this...the turn included a left turning lane so I didn't venture into the oncoming lane. I'll be up to 40 PSI front/back with the stiffest suspension settings tomorrow. Time to make some spacers.

Steve

 

[Dink]

Steve dont forget that the suspension needs to be balanced front and rear, the only way to do this is to measure "sag".
You will need someone to assist you for about five minutes.
First place the bike on a stand (centrestand or a jack ) so it is upright, so as both tyres can be brought clear of the ground, now with no weight on the suspension at the end you are measuring, measure between two fixed points (i.e. the lower triple clamp and the top of the fork seal cover, and the top and bottom bolt on the rear shock) write the measurements down, now take the bike off the stand and you sitting on it measure again, subtract the second figure from the first and you have your current static "sag". If this figure is outside the range of 25 to 30mm (1" to 1and 1/4") you will need to preload the suspension with a spacer, BTW trick here the spacer required is normally very close to the difference between your current figure and 25mm.
I would not be surprised to find that your front end is over 25mm and your rear end is under.
Dink

 

[Billy Ricks]

Use Dink's method of setting up sag. It's the very first thing you should do when setting up suspension. Then you start working on things like tire pressure and damping if you have adjustments for it. Light compression damping with more rebound damping usually works best.

 

[Anonymous]

2.25" of sag up front. I will be putting some 1" spacers in tonight. It's kinda odd that I went through this with my GS400E and then promptly forgot about it once the bike started handling well. :roll:

Thank you, Steve

 

[Dink]

I would use a 1 and 1/2" spacer Steve.
Dink

 

[Anonymous]

Dink:

I only had enough material to make the 1" spacers and it was one heck of a time getting them in. I must be turning into a wimp. Believe it or not, I still have 2" of preload up front. I measure preload by putting the bike on the sidestand, holding the front brake, and pulling the bike up. That gives me 7" of fork tube. Then I get on the bike and it settles to 5" of tube. DOH I didn't put any air pressure back in, that's probably why I didn't gain anything! I will have to make a set of 1.5" spacers and order new preload adjusters. Mine don't have the centre parts so it is mind boggling trying to get the caps threaded in with the pressure point very close to the edge instead of the centre.

On the plus side the bike did feel like it was handling better but it wasn't fantastic by any amount. It sure isn't anything like my 400. That's good though considering my mistake of measuring preload with the pressure still in.

Steve

 

[mark m]

I only had enough material to make the 1" spacers and it was one heck of a time getting them in. I must be turning into a wimp. Believe it or not, I still have 2" of preload up front. I measure preload by putting the bike on the sidestand, holding the front brake, and pulling the bike up. That gives me 7" of fork tube. Then I get on the bike and it settles to 5" of tube.

You are having some terminology confusion, Steve. Preload is the amount the spring is compressed from it's free length when the fork is fully extended. That is, with the forks fully extended, how much did you need to compress the springs to seat the fork caps? On my 1100 I ended up with about 10mm of preload with modified Progressive Suspension springs, and it was no problem to seat the caps with that. Since you have a very soft spring rate, you will need more initial preload force to get the correct sag numbers, which will make it harder to put the caps on...

What you are calling preload is really the laden sag. You need two sag measurements - static and laden. Static is how much the forks compress under just the bike's weight (with the bike sitting vertical, off the center stand). That should be around 25-30mm. Then you need the laden sag which is with you sitting on the bike. That should be 35-40mm. You may not be able to get both the sag settings to work together (in fact, with stock springs it will NOT happen), so in that case the laden sag is the more important one.

DOH I didn't put any air pressure back in, that's probably why I didn't gain anything!

You are better off using the least amount of air you can. It causes greater seal wear, more stiction and is really just a patch for inadequate fork springs. A set of PS springs is only $90CDN (from my local dealer) and makes a HUGE improvement in front end feel.