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Measuring rear suspension static sag Measuring rear suspension static sag
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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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Photo by Alessio Corradini,
on the Salar de Uyuni, Bolivia,
of two locals



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  #1  
Old 8 May 2008
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Measuring rear suspension static sag
Hi all,

I know the theory about the weight of bike plus rider should use up about one third of suspension travel, and to check this you should measure between a point on the frame and one below it on the swing arm with A) bike supported so that the back wheel is off the ground, and B) with the rear shock fully compressed. Take one reading from the other and that is the range of travel, you want to use up a third of it as static sag.

My question is has anyone got a method that they have used to sucessfully compres the rear shock?

I have contemplated removing the saddle and using a ratchet strap between the swing arm and rear of the frame - could this damage the frame?

Thanks for your thoughts

Mark
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  #2  
Old 8 May 2008
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compress the shock!?!
If you have a yamaha, probably full rear swingarm course is available here. If it's a different bike, and you can't find this info, compress the shock as you propose is not practical, requires a lot of strenght and could damage frame (or your body if something goes wrong).

Take the shock out of the bike, unmount the spring, put the shock back on the frame with screws without spring, take your measurement, and reassemble all.
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Old 8 May 2008
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Quote:
Originally Posted by Mark View Post
Hi all,

I know the theory about the weight of bike plus rider should use up about one third of suspension travel, and to check this you should measure between a point on the frame and one below it on the swing arm with A) bike supported so that the back wheel is off the ground, and B) with the rear shock fully compressed. Take one reading from the other and that is the range of travel, you want to use up a third of it as static sag.
Mark, sag is measured from the axle to a point vertically above it - subframe.
First you measure this distance withe the bike on its stand or with the rear wheel off the ground. Then with the shock on its standard setting, stand it on its wheels, sit on it and bounce up and down a couple of times. You should be sat with your feet on the pegs, hands on bars and then measure the distance again. This will give you your sag measurement. You need a couple of helpers to do this.

Each bike/shock will have its own sag measurement. Lets say its 50mm You adjust your shock spring to give you that distance. You should adjust the sag to be the same with you and your load on the bike.
Some bikes have a three position spring adjuster to achieve this but is doesn't give you much control. Thats where the Ohlins/Wilbers come in. They have a threaded adjustment to give you a bigger adjustment range and you can get a spring that will suit your weight range.

Taking a 3rd of your total shock travel wont hack it. Most rear springs are progressive these days.

Once set, you can play with compression and rebound

Have fun,
John
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Last edited by Redboots; 8 May 2008 at 21:09. Reason: spelling
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Old 12 May 2008
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Thanks for your replies.

The spring on the original XT600E shock, and it's Wilber aftermarket replacement both appear to be linearly wound to my eye (i.e. not progressive), but I seem to remember reading somewhere that the linkages produce a progressive effect.

The instructions supplied with the Wilber shock do say to set the preload so that one third of suspension travel is used up by weight of bike plus rider, hence my original question. They do not mention a distance in mm for setting the sag. I have found many sites on the web for setting up sport bike suspension that state that with just the weight of the bike on the suspension it should sag #1 to #2 mm, with the rider it should sag #3 to #4 mm and if you cannot set up the preload to get inside both of these ranges then the spring strength is wrong for you.

Unfortunately I have been unable to find advice for bikes with our longer travel suspension, and I was hoping that someone here might be able to give this info.

I will try emailing Wilber, even though I bought the shock 8 years ago from an Australian agent.

Thanks

Mark
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Old 12 May 2008
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rear travel
Ok here you find requested magic number, Stammbaum XT600 3tb

Rear travel is 200 mm so you should have about 60 mm of sag, measured as per Reedboots instrucions: first time bike all up, second time your weight over it, handlebar near a wall, feets on footpegs, a friend taking measurements
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Old 12 May 2008
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Perhaps methods and terms for setting Sag differ
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Last edited by mollydog; 26 Mar 2009 at 19:34.
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Old 12 May 2008
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Smile Saggy
Quote:
Originally Posted by mollydog View Post
First: there are TWO sag measurements to be made.
Ahem. Think that there are three.

Measure
1. Wheel(s) off ground.
2. Bike on wheels with no load.
3. Bike on wheels with rider in riding position and any load.

The difference between 1 & 2 is Static Sag.
The difference between 1 & 3 is the rider/race sag.

Static sag is important because you can "top-out" the shock under hard braking if there is not enough.

Rider sag, apart from stopping the rear shock "bottoming-out", also maintains the attitude of the bike fore and aft. Too much sag and the bike will look like its dragging its arse and in doing so, change the steering head angle and make the bike handle like a sock of hot poo.


John
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Old 12 May 2008
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Static sag did NOT change. Go figure. But race sag did.
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Last edited by mollydog; 26 Mar 2009 at 19:34.
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Old 13 May 2008
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Quote:
Originally Posted by mollydog View Post
If I recall right, I am pretty sure Paul Thede (not Keith Code!!! DOH!)
Sorry, never heard of them...

Quote:
Originally Posted by mollydog View Post
...suggests using the measurement between #2 and #3, not #1 and #3 to find Race sag.
I'll go back and look ..... anyway, the above is how I've set up my dirt bikes for 20 years or so. Probably wrong!
Never set a dirt bike up. I think Mark will be using it mainly on roads...

Quote:
Originally Posted by mollydog View Post
PS : and how does one adjust static sag? And what effect would that have?
As far as I know, static sag is established by the manufacturer of the bike. This will normally be around the 25-35mm mark... Even my Valkyrie was only 40mm if I remember right.

The only way to adjust static sag would be as you say, via the springing/linkage.
Some people say that you should also add the panniers/load before you set the static sag. I prefer the load to be added as part of the "rider/race" sag measurement.

On my recent trip I ordered a Wilbers shock for my R100GS to take a 75kg rider with 45-55kg load. The spring on the shock was supplied for that weight range and once the rider/race sag was set it proved AOK, keeping the back of the bike at the right height and with no handling problems.


John
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