Static Sag Rates?

[bmccrary]

Ok so I was looking into the amount of sag that a street bike should have. I have done this before for my XC bikes, but I felt that the numbers will be completely different between dirt and street.

Everything that I have found so far as been between 20 and 40mm of sag for both front and rear. 25mm being for track use and 30+mm for street use. Now we cant adjust the sag that easily in the front end, it requires cutting the spacer or making a new spacer to lift it up.

The rear would require simply chaing the preload settings (1-7) on the shock.

So for those that have done this, what preload rates are you guys running? Preload settings in the rear is not what I am looking for. As I just found out, our springs rates can vary a good deal depending on how much use and abuse has been put on them. Looking for solids measurments in mm. Also besure for the rear end to clairify where you were taking your measurements from.

Back to the front end.

Lets say the bike is 5mm to high ( too much preload) Would this solution be cured by simply cutting 5mm off the spacer? Would that spacer make a direct change? If the bike had too much sag, a new spacer would have to be made. So what would be a good material to use for this spacer. Mild steel, stainless, 6061 Aluminum? I have the ability here at school to make nice clean cuts with a large horizontal saw, so this would not be tough. I just want to get some feed back before I start hacking.

Also, I realize that adjustments will need to be made for two up riding as well. If need be, I could make a seperate set of spacers for two up riding since they are very easy to instal and can be done while the forks are on the bike.

Input, thoughts?

-bryan

 

[Wolfman]

I have racetech Springs & Gold Valve Emulators on the front, Ohlins shock on the rear, am running a rather tight 25mm of static sag at both ends, works for me.

:thumbup:

 

[DefyInertia]

I responded to your rear shock spring thread ealier with a lot of information.......

No, a 5mm cut won't create a 5mm change in rider sag (it could but this would be random). If you're cutting spacers, you can use PVC to get the correct length before cutting the metal once. Any metal should work but maybe others can chime in as I just used what RaceTech sent me. Chances are you need to UP the preload in the front which you can do by adding shims on top of your tubular spacers.

I run ~30mm front and back. Take some measurements and report back.

 

[bmccrary]

Sorry nate,

I didnt see where you had posted up any info. I will go back and check on that. Later tonight I will try and get some measurments. Good idea with the PVC pipe test spacer. With the higher weight oil I have a feeling I will need to cut some material.

Thanks guys!

-bryan

 

[bmccrary]

Ok so I took my measurements just a few mins ago. The center stand makes things easy, lol. Now I was just in street clothes, no gear or bags. Ill need to try again with some weight on the bike. Hopefully tonight.

I measured to the rear from the end of the sing arm flat, going up to the top of the bung for my givi rack. The amount of sag was 36mm in the rear based on where I measured. The preload setting was on the 5th notch at this point.

For the front, I measured from the bottom of the lower tripple tree, to the bottom of the black plastic fork gaurd, as it was a straight edge. For the front I have 45mm.

I know these settings will change if I put on my gear and if I add some bags. I might throw both my tail bag and tank bag on tonight and put a couple pairs of jeans in them to get some weight and see how it does. Based on what I have read, so far the rear might need to be a bit stiffer, Ill try a couple different preload settings. Then for the front, it looks like I will have to add some spacers.

-bryan

 

[bmccrary]

So tonight I am going to try again. However I made my mind up on how I will do it. I do not like how the front and rear are supposed to be the same sag amount when their amount of travel is different.

I think what I am going to do is figure out how much travel each end has, front and rear. Then determine the amount of sag and multiply by (1/3) or less but not (1/4) that way I can get a precentage. I think that will be more accurate for the bike and not so general.

So after Fluid Dynamics tonight, I plan on spending some time, while I have it, figuring some of this out.

Stay tuned.

-bryan

 

[bmccrary]

Oh Boy!

So I got some measurments taken tonight. Be ready to read some math.

I put all my gear on and put both the tank bag, as well as the tail bag on and filled them up with jeans in order to add some weight to the bike.

First: I placed a zip tie around the fork leg just above the dust seal. Then with the help of my large friend and a wall we bottomed the front end. Then I took a measurement from the bottom of the zip tie to the bottom of the black plastic fork gaurd, as it was the only clear and straight reference point.

The front end has 135mm of travel using those points.

Using the idea that the bike should sag between (1/3) or 33.33% and (1/4) or 25% the amount of travel, then at (1/3) I should have a 45mm of sag and at (1/4) I should have 33.75mm.

Once on the bike, the new distance came out to 85mm. 135mm - 85mm = 50mm sag. Taking the inverse of the travel over the sag and multiplying by 100 gave the percentage.

(((135mm/50mm)^-1)*100)= 37%

So I am a bit soft. Time to go to lowes and pick up some 1.5 inch O.D. pvc pipe and start playing with different length spacers before cutting a new metal one. The steel spacer was measured to be 0.046 inches think.

Now for the fun part, the rear end.

I softend the suspension preload and bottomed the bike out with the help of some tie-downs. By subtracting the measurement from my reference points when the bike was on the center stand (535mm) from the lowered number (427mm), the travel was 108mm. Using the same theory as above with the (1/3) to (1/4) the rates should be 36mm and 27mm respectivily.

At preload setting 6. The measurement was 495mm which gave me a sag of 40mm. Calculating to percent came out to 37%

This isnt too bad, but is still too soft.

So I went to preload setting 7. The new measurement was 500mm which came out to 35mm sag. Doing the same inverse function as before but replacing the sag number ended up with 32% which is < 33.33% for street use.

(I knew I liked the stiffest setting best on the bike. The butt never lies)

Here is the scary part...

I had the smallest guy in the group that was helping me who weights 130lbs, hold my tool box, ~20 lbs and sit on the back of the bike with me to represent two-up riding. This is a close approximation to my g/f riding along with saddlebags and tails bags attached.

The new measurement was 465mm with came out to 70mm of sag! 64.8% of my travel! This would explain the light feeling of the front end when riding two up...

To fix this:

On of my friends here at school works for Ohlins on the school breaks and such. The Hendersonville shop isnt far from his home town. After talking to him today between classes, it looks like he might be able to get me a good deal on a new spring for the rear.

Time for more calculations.

When I had the spring off the shock the other week I took quite a few measurements.

The preload adjuster moves a total of 3/8 of an inch between settings 1 and 7. Multiply that by the spring constant or stiffness which I tested to be 680 lbs/inch came out to be a 255 lb/in total increase, totaling 935 lb/in.

Divide 3/8 by 7 and each setting decreases the spring length by 3/56 of an inch. Multiply that by the spring constant and mine came out to be 36.42 lb/inch. So each preload adjustment simulates 36.42 lb/in

So going by the assumption that I might be in need of a 900 lb spring. I did some more math.

Multiplying the 900 by 3/8 again yields and increase of 337.5 which totals 1237.5 lb/in.

Subtract 1237.5 from 935 (The max spring settings of the 900lb spring and the 680 I have now) and you 302.5.

Now based on my adjustments off the solo sag. Each preload adjustment seemed to raise the bike 5mm by increasing the spring 36.42 lb/in.

So divide 302.5 by 36.42 and you end up with 8.3. Multiply that by 5 and the increase would 41.52mm. Which would in theory bring my sag back up to within range. While at the same time allow me to run the preload towards the bottom when running solo.

I could probably get away with around a slightly softer spring, but we will have to see what my friend can get together at Ohlins.


Hopefully this made since. I have several pages of notes and math sitting here in front of me. I tried to get it all in this post.

-bryan

 

[rgatling]

bmccrary,
You must be a future engineer, because that was the best "data filled" post I've ever read on any forum! I'd be interested in your findings after you change the rear spring.

Thanks for sharing, and for taking the time to write all that!

 

[dako81]

How much do you weigh?

 

[bmccrary]

185 lbs. Call it 200 with gear on and after a big lunch... just for rounding.

-bryan

 

[urbanj]

I have it on 7 as well and I'm 200lbs. Thanks for verifying the setup. Can't wait to test out the r6s forks though and see how bad the rear is.

 

[dako81]

185 lbs. Call it 200 with gear on and after a big lunch... just for rounding.

-bryan

Yea, I just wanted to know for a ref. so everyone can sort of judge what they'd need once you get yours all figured out.

Thanks!

 

[bmccrary]

Yea, I just wanted to know for a ref. so everyone can sort of judge what they'd need once you get yours all figured out.

Thanks!

It can be used a guideline I suppose, but the only issue with that is that obviously the spring rates vary from spring to spring. They should be close, and shouldnt matter that much... however it wouldnt hurt anyone to go ahead and take the measurements on their own equipment.

Still waiting on the spring from my friend at ohlins. However, the bike is the last thing on my mind right now. Except for the fact that it has been in the mid 60's lower 70's all week....

-bryan