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Suspension: Setting Sag

14K views 16 replies 6 participants last post by  tomekzski 
#1 ·
I've been trying to learn as much as possible lately about suspension, and correct sag is the first step in setting up suspension, so in light of our new suspension and brake section, I thought I'd share what I've learned.



What is sag?:

Sag is how much your suspension is compressed while not moving. Race sag, (also called rider sag, or actual sag), is how much your suspension is compressed with the rider on board, in full gear. Static sag (or free sag) is how much your suspension is compressed under the weight of the bike. The sag specs for front and rear are very important to how your bike handles. They affect steering geometry, indicate whether you are using the correct spring rates, and much more.


Measuring sag:

You will probably need a helper to get accurate measurements, but the process itself is very simple. Pick arbitrary points on the fork, and the axle to tail, and take your measurements. You should be in full riding gear, and place most of your weight on the footpegs, in your "attack position".

Race sag = spec fully extended - spec with rider.

Static sag = spec fully extended - spec under bike's own weight.

Measure measure race and static for front and rear.


Setting sag:

Now that you know what you have, it's time to set your sag correctly. This is done by changing your preload settings. Changing preload on the rear is done with the large nuts on the spring. Some forks have preload adjustments, but other's don't. In that case, you need to make correct preload spacers for the springs. Obviously that requires you to tear down the forks.

Always set your preload to achieve correct race sag, never set for static sag. Once you have correct race sag, the subsequent static sag will tell you if you have correct spring rates.

Always set rear sag first!!!!!!!


I finally found good info on what sag should be for different types of bikes, here it is:


Offroad and MX bikes:
Front Race sag: 22-25% of travel (65-75mm on most bikes).
Rear Race sag: 30-33% or travel (95-100mm on most bikes).
Front static sag: 25% of race sag
Rear static sag: 30% of race sag


Street bikes:
Front Race sag: 28-33% of travel (30-35mm on most bikes).
Rear Race sag: 28-33% of travel (30-35mm on most bikes).
Rear static sag: 0-5mm

Roadrace bikes:
Front Race sag: 23-27% of travel (30-35mm on most bikes).
Rear Race sag: 23-27% of travel (30-35mm on most bikes).
Rear static sag: 0-5mm


Once you set your correct Race sag, go back again and measure everything. If race sag is correct, check your static sag. If your static sag is too little, your spring is probably too soft for your weight. If your static sag is too much, the spring is probably too stiff. This may sound counter-intuitive, but for example, if your static sag is too little, that means you had to apply too much preload to achieve the correct race sag, and vice versa.

Another method of course is too use online spring rate calculators, that is especially helpful with forks. It's really difficult to get accurate measurements with the high levels of static friction on fork tubes.



Springs are the most important part of your suspension, and only then can you work with dampening to get really good performance. Sag is the key to correct spring rates and preload.



If anyone has anything to add or has questions, dont be shy.
 
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#2 ·
have you tryed setting up your bike to this yet. and if so can you feel the hugh diff when ridding
 
#3 ·
This is only the first step in properly setting up suspension for your weight.

My fighter project has been on the back burner for a while, but right now I'm playing with a KDX220 and a KTM 300.


I need to install some stiffer springs in the KDX forks, and take a hair of preload off the rear. I think it will be spot on after that, only then can I play with clickers and such.

On the KTM I need to take some preload off the rear and then I should be good. I think I'll prob put some 15wt or heavier in the KTM fork.


I've been riding offroad alot and I'm reaching the skill level where I'm noticing the suspension needs work badly. The KDX is way too unstable at speed and the front bottoms easily and dives too much. These are the symptoms I noticed, and after a sag check, you can easily see that this is a function of both the weak fork springs and too much rear preload. The KTM feels pretty good, but the rear never hooks up and feels stiff. I'll take some preload off so the race sag is correct, and then I'll check the static and ride it again and see how it feels. I think front compression dampening is weak too, but I can't diagnose that stuff until sag and spring rates are correct.



With bikes it's the little things that matter, esp with suspension and brakes. The KTM was unrideable until I put fresh brake pads on and installed fresh fluid and bled the system. About $30 and 15min of work changed that bike 110%.


oh, here's an even better example of the little things making all the difference:

The first full day I rode the KTM, I was out for a couple hours,(dry, rocky terrain) and noticed that the front end felt soft, like it was diving too quickly under braking, BUT, was still deflecting off every rock and scaring me every 5 sec. This didn't make sense to me, since a soft frontend shouldn't deflect like that. I rode back to camp and found 19lbs of pressure in the front tire!!!!! Not sure how that happened, but I lowered it to 14lbs and it was a totally different frontend.
 
#4 ·
yeah i know it take time to get everything adjusted out. but yeah it should become a nicer ridding bike. but im going to have to reset the whole suspension system on my bike when i get 1k motor in there. and this new area is going to be hella tight to post and get help from
 
#7 ·
i gotta question for ya what about a hugh lb loss of gain. wouldnt you have to maybe change springs. cause im thinking even with the loss of around 100 lb with the plastics and subframe. but i dont know how much more the 1k motor is going to be then the old 6. so any idea of how to adjust the spring size to the weight gain. or loss
 
#10 ·
dude. do you really think you have lost 50kg for weight?? good on ya.

you will need to set it up and then see what happens, if its not right and to soft get springs, if not and you like the way it rides than you dont need new springs, its hard to say as every rider and bike is different. the way you may like it may be very different from how i like it.

put the motor in and just see what it feels like
 
#11 ·
It's not just a feel thing, but that could be part of it. Look at the specs I put up for sag on streetbikes and roadrace bikes. Once he finished the engine swap, he needs to set the race sag correctly. if the bike is alot heavier, it's possible that with correct race sag he will have little or no static sag at all. In that case, too much preload was needed, and stiffer springs are in order.
 
#12 ·
sorry my question is. is there a weight to spirng chart or something that would be close
 
#13 ·
Dude, reread my sag post. There are spring rate calculators online, but you're better off setting sag first, and seeing if you're in spec with your current springs. Since you're doing an engine swap there is no magic spring rate calculator because your bike will have a new, custom weight. Set your race sag, and see where your static is, and then you will see if you need different springs.


One more time:

Set your race sag, and see where your static is, and then you will see if you need different springs.
 
#14 ·
ok i will set the sag when i get the motor in and post then
 
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#16 ·
+1, may have forgot to mention that, but this is esp true with forks. ^^^^
 
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