Setting sag and preload on new fork springs?

[MCRIDER007]

Sounds like you're in the ballpark....... interested in your new spacer calculation and sag.

I spent about 40 minutes on the phone with Oso last night trying to figure out why he was having so many problems....which I originally thought were being caused by excessive preload due to a spacer that is still much too long. Excessive preload would result in a very low bike sag reading and would also make the fork cap very difficult to install.

However, he said the fork cap went on very easy. The bike sag was 20mm but additional sag when he sat on the back was only 2-3 mm....opposite of what I thought he had described in this thread. To further confuse the issue, adjusting the external preload didn't seem to change the sag numbers at all and his spacer reduction of 52-53mm seemed to be very close to what I had calculated based on the difference of spring length and rates.

At that point the only thing that I could thing of that would explain his sag numbers would be a hydraulic/air lock caused by excessive oil in the forks. He had used the OEM oil height of 92mm but the RT springs are much longer and he had also installed the RT spring guides...which together are probably displacing much more air than the OEM springs, making the "air spring" excessive. I think you mentioned that RT used to recommend a 130mm oil height which seems much more appropriate. We decided the best course of action would be change the oil height to 130mm, leave the current spacer length as is, reinstall and remeasure sag, and hope the external adjustments will give him the sag he wants.

 

[RaYzerman19]

Agreed, get the fork oil level down to at least 110...... I wasn't thinking 92 would cause hydraulic lock, but maybe......

Spacer if cut too short, would that not allow so much sag that adjusting preload isn't doing anything...... thus getting false sag readings? I'd try adding some washers (20 mm worth) if the next trial isn't showing much better. It's all I can think of short of something binding.

 

[yamafitter]

I think Race Tech uses 310 mm springs and I have measured the spring length of my GP Suspension springs and they are only 260 mm. I have the spring buffers installed in my forks and my preload spacer is 143 mm if that is of any help to you.

Being an old fat guy I am in the process of going to 1.05 kg fork springs. The 34-45 mm rider sag mentioned earlier is a little excessive but is dependant somewhat on the rear sag to keep things balanced. The idea is to keep the bike ride height somewhat level as to not affect handling. Rear sag is measured vertically and since fork sag is measured parallel to the fork tube it is a little bit more due to the angles involved if ride height front to back is to remain somewhat constant statically.

I believe you will find that if rear sag is somewhere between 15-25 mm then front sag should not exceed 35 mm

 

[sath182]

Measured from the axle straight up my stock '13 is 31mm hard and 37 soft with me (174lb) on board. I guess you might measure the front sag from the fender straight up to a point on the bottom of the nose.

 

[Oso]

I went with FJR Commander's advice and just reduced the oil level to 130mm. I also noticed that burrs on the cut end of the spacer prevented it from moving smoothly on the damper rod assembly. I cleaned those up reassembled the forks on to the bike and promptly broke the head off the caliper mounting bolt. If I get the calipers back on today I will check my new measurements.

 

[Oso]

Okay, everything back on the bike except the plastic. Preload set at 4th line. I bounce the forks a few times to even them out. Set the bike over on the kickstand, push the zip tie down to the fork seal, gently mount the bike, lift off the sidestand, lightly touch the wall for balance and put all my weight on the bike. The zip tie has moved about 21mm up the fork. So, I think that still means I'm only getting 21mm of sag after removing 38mm of oil. I cut the spacer length exactly at top of washer to top of fork length minus the height of the top bolt. Something is still not right or am I not measuring sag correctly?

How far down should the top cap go on the threaded rod? I back the nut all the way to the bottom then thread on the top cap and it goes just about down to the top of the nut. I then bring the nut up tight to the bottom of the top cap. At this point there is a little bit of play in the spacer.

 

[RaYzerman19]

That is the correct way to install the top cap, however instead of play in the spacer, you should have your desired preload (you were going with 20 mm?). Although not usually stated as such, you have negative preload (a space that must be taken up before the springs offer any resistance. When you measure sag, it should be from your zip tie, with the forks fully extended. As I expected, removing oil did not affect sag.

Add some washers to add preload (the real fix is new spacers, or add back in the appropriate amount of your cutoffs from the spacers), and remeasure sag.

 

[MCRIDER007]

You should start your sag measurement with the bike on the center stand (not side stand) and the forks fully extended. The first measurement is the sag when the bike is standing upright off the center stand, the second sag measurement is with you sitting on the bike.

It sounds like you are getting 21 mm of sag from your weight whereas you were only getting 2-3 mm before you reduced the oil level.

 

[Oso]

Okay, I have re measured everything. Preload set at the 4th line. I measure from the bottom of the triple clamp to the top of the fork seal. Here are the numbers:

Bike on centerstand, front wheel jacked up off floor = 152mm

Bike weight only = 117mm

bike and rider = 97mm

From my understanding of sag, this gives me 20mm. RT recommended 40mm

When I put the top cap back on it was set at 2.5 lines of preload. From a previous post, are you saying there should be no up and down movement of the spacer when everything is put together? Does where the preload is set effect how far down it will go onto the threaded rod?

 

[MCRIDER007]

I think your understanding of sag is incorrect. The terms vary but I would say you have 35mm of bike sag and 20mm of rider sag or 55mm of total sag. Turn your external preload adjusters clockwise (fewer lines showing) and it will reduce the bike sag.

The RT recommended 40mm sag is total sag....bike and rider, my personal preference is 45-50mm total sag. If you have 4 lines showing on your external preload adjusters you may have enough to lower the sag to to 45mm. I would go to zero lines showing, remeasure the sag and then test ride the bike. If the steering is too slow then you will have to use a longer internal spacer or add washers internally.

 

[RaYzerman19]

OK, I'm getting it now. Actually, your sag is 152 - 97 = 55mm. Makes sense if your spacer is too short, which it is, since you cut off 50mm. Back off the 2.5 lines of preload, you are 67.5 mm of sag (roughly, it's not necessarily linear due to stiction, etc.).. Your 117 number is off too since the bike is falling down to take up the gap in your spacer to nut, and because you have zero preload in your calculations.

If you re-do your spacer calculations per the instructions, you'll find you should be much longer. As I mentioned, with RaceTech springs I usually cut off 32-33 mm not 50, but that depends on the length of the springs. But just guessing to get that 55 down to 40 you have to add 15+mm to your spacers. If it were me, I'd add 20mm (if you measured while set at 2.5 lines). You'll have enough adjustment, and even if you don't you can add a washer or two. As I mentioned earlier, if you want to simulate before you re-do your spacers, stack in 20mm worth of washers and recheck your sag.

 

[escapefjrtist]

Outta likes, but it sounds like '007 and Ray have you on the path to proper set up. Assuming RT is using the same springs, follow Ray's recommendations above for your spacer length.

IIRC, my spacer length ended up with ~1/2 of the threads showing on the dampener rod w/o the spring compressed. Spin the nut to the bottom of the threads and you'll end up with a tight spacer. Reassemble and use the external preload for fine tuning.

Good luck!

--G

 

[Oso]

I picked up some copper pipe for new spacer material and dialed my preload down to 1 line showing. This gave me 35mm of sag. I'm going to test ride that before tearing down the forks again. Thanks all who contributed advice.

Oso

 

[MCRIDER007]

I picked up some copper pipe for new spacer material and dialed my preload down to 1 line showing. This gave me 35mm of sag. I'm going to test ride that before tearing down the forks again. Thanks all who contributed advice.Oso

Sounds like you can adjust your front preload to achieve between 35 and 55mm of sag with the current spacer (which I think is an ideal adjustment range). Try measuring the rear sag and add 10mm for your front sag. There isn't any "correct" sag, use whatever you like the best for your riding style.