OEM spring rate for an 86 GSX-R750G

[tabby59]

Hi fellas. New here, so bear with me as I learn the ropes. Yes I read the fucking rules. I am an original owner of an 86 G.

It has a host of mods like they tend to get as the years go by. Anyhow, I would like to know the OEM spring rate for the 86 750.

I have the 1100 rate as 6.1 Kg/mm {340 lb/in}. The reason I'd like to know is I am using the 1100 shock linkage on my bike with an Ohlins shock.

The Ohlins is the SU 5274 shock set with the stock 1092-24 spring that comes with it. {8.15 Kg/mm   457 lbs/in}

I have a sneaking suspicion that the leverage ratio of the 1100 linkage is different than the 750's linkage. With the stock linkage and the Ohlins,

ride compliance is very supple. With the 1100 linkage, it is far too firm. Firm enough that when heeled over and my knee skimming the deck, it will

skitter if it hits pavement ripples. It never did anything like that with the 750 linkage, and this is with Michelin Power Supersport tires mounted, scrubbed and at Michelin's recommended track day pressures. I did some minor rebound/compression damping adjustments but to no avail. My thinking is, if the 750 {a lighter bike} has a higher OEM spring rate than the 1100's OEM spring, that would confirm to me that there IS a different ratio in that linkage. The to get the 1100 linkage to work better on my bike, I'll have to go a similar spring rate as the 1100. Like many, I'm using the 1100 linkage to quicken the steering response.

[gray711]

This is of interest as I've mucked about with various combinations of 750 & 1100 linkages & swinging arms.

I'm going back to stock arm & linkage though I do have K forks so it turns in quicker anyway with the reduce height. Take it you've dropped your forks already..

With you you on the firmness tho & no matter how much adjustment it was never enough ....

[tabby59]

I tried dropping the forks a 1/2"  but when I had .90 fork springs in, I would drag the lowest socket head cap screw on the starter clutch side. {Track days} So I try to avoid that.

With my Ohlins I did make an eccentric upper bushing that allows me an additional +or - 3/4" of ride height. 

[clivegto]

 

With my Ohlins I did make an eccentric upper bushing that allows me an additional +or - 3/4" of ride height. 

Have you got a pic of the upper bushing ?

Edited September 15, 2015 by clivegto

[tabby59]

No, but I'll dig up a post from another forum that has the dimensions and post it here.

[tabby59]

Camshaft had asked for this info, or pictures for the eccentric bushing I had made for my Ohlins shock. I'm sorry I don't have any pictures available yet, but I'm sitting here with my dial calipers, so I'll measure things up. Please understand that these numbers are being generated with those calipers, which isn't the most accurate way to measure. Also, you can or may have to alter the dimensions for your own application. This is just another way to alter ride height in the rear. And yes there is enough movement of the shock with this bushing that you may have to grind some clearance on the shock linkage arm to clear the spring.

1. I.D. bore of shock body after factory bushing has been pressed out.   1.028"
 1a. Width of shock body at bore.  .75"
---------------------------------------------------------------------------------------------------
2. O.D. of eccentric bushing.   .910" {1" dia. mild steel stock that was available at the time}

3. Width of bushing   1.182"
On this dimension, measure the actual width of the frame that the bushing will be fitting into. You want them to be as good a fit as possible, so the frame won't be crushed when you are tightening the bolt. This insures that the bushing will not rotate in use.

4.Drilled hole diameter through the bushing.  .390"  

5. Drilled hole of bushing,   offset from centerline.    .200"
------------------------------------------------------------------------
6. Delrin bearing O.D. 1.027"  {teflon impregnated Delrin}

7. Bearing I.D.  .911"

8. Bearing width.  1.182"  {See #3 above}
------------------------------------------------------
9.Delrin thrust washers {2} O.D.  1.2"
These are to keep the shock body centered between the frame.

10. Washer I.D. 1.028"

11. Washer width.  .21"
-------------------------------
I think I read somewhere that the linkage ratio on the 86 GSXR750 is something like 4:1. Which means with my .2" offset that should be about .8" at the rear axle. My rough tape rule measurement showed 1", but there may be some variability in there, and a tape rule isn't very accurate.
You may be able to get a little bit more offset if you don't make a Delrin bearing like I did. Then make the bushing O.D. say 1.027". If you go that route you may want to use some manner of oil impregnated sintered bronze for the bushing material so wear of the shock body bore won't be an issue. 

After I made and installed the bushing {in tallest setting}, and after measuring the differences in tire diameters from the 18" rim set  to the 17" set, I pretty much effectively raised the back/lowered the front so I did get an approximately 1 degree drop in front end rake from OEM 26 deg. to 25 deg. I wasn't shooting for a specific  number, just wanted to make sure  that the switch to 17s went well with no drawbacks.

Edited September 15, 2015 by tabby59
missing "is"

[tabby59]

OK, here is a little of what I've found out. I haven't found the OEM spring rate for the 750. What I did confirm was that the 85-87 750 and the 86-88 1100 indeed do have different leverage ratios in their linkages. What the actual ratios are, I don't know,,, and I'm not a mechanical engineer. I did some cross referencing with Ohlins shocks.

They were nice enough to send me some info.

1986-88 GSXR1100   Shock -  SU 635, Spring - 1095-21

Rate for the 1095-21 spring:  7.65 Kg/mm  {428 lbs/in}

1985-87 GSXR750     Shock - SU 5274,  Spring - 1092-24

Rate for the 1092-24 spring:  8.15 Kg/mm  {457 lbs/in}

So yes, the lighter 750 DOES use a heavier spring than the 1100. That would indicate a different ratio in the linkage.

If you make the swap, put the 1100 linkage in the 750, and it doesn't feel right or a lot different, now you may have an idea why.

I am going to try the 1092-21 and 1092-19 springs to see if they will be a better combination for this bike.