Progressive Springs / Oil Level?

[rickt]

Just about ready to install the forks on my '78 GS750C restoration project. Just for the heck of it I pulled out the instruction sheet for the new Progressive springs. They suggest a maximum oil level of 5.5 in. Both the factory service manual and the Clymer recommend 8.1 in. from the top of the fork tube for the stock springs.

Just wondering which of these measurement I should shoot for? I imagine many have faced this situation?

rickt

[Mysuzyq]

Go with the Progressive Suspension recommendation. Their springs are a lot beefier than the stock items, and displace more oil. From memory, I think they say to use the stock level if it is less than 5.5"....otherwise, use 5.5" as a max level. Some of the factory manuals specify the volume in each leg, rather than an oil height anyway.

Tony.

[rickt]

The somewhat odd thing is that filling to within 5.5" of the top will actually put more oil in the tube than the manual calls for with stock springs (8.1"). The Progressive instructions suggest that their springs displace more oil than stock. So, it would appear that the oil level in the tubes will be higher than ever when filling to within 5.5" of the top?

rickt

[Nessism]

The oil height controls how much trapped air is remaining in the forks. The air acts like a spring when the forks compress so in effect, Progressive is specifying more air-spring effect in addition to stiffer coil springs.

In my experience with my 850, the PS springs are pretty stiff. If you are a light weight rider, or don't ride very aggressively, you might want to use the factory recommended oil height. There will be plenty of oil for the damping mechanism to function properly but the forks will have a smig more travel over large impacts on the road.

Good luck.

[Mysuzyq]

The somewhat odd thing is that filling to within 5.5" of the top will actually put more oil in the tube than the manual calls for with stock springs (8.1").

rickt

The P.S. leaflet said IIRC, that you could use the factory spec. (height or volume) as long as the level was no higher than the 5.5 max from the top. It has been a few years, but I removed excess oil with a syringe-type fork tool set at 1" lower than the P.S. recommendation (6.5). The ride is firmer than stock for sure with the PS springs and this set-up, but it isn't harsh.

Tony.

[Zook]

If you have air assisted forks Progressive usually recommends running zero psi.

[49er]

If you you have ridden the bike with the stock springs you can make a judgement on how much stiffer you want the new PS setup to be. The idea is to try and replicate the ride height after the new springs are fitted.

Compare the stock and progressive springs side by side to get an idea of the differing coil designs and lengths. My 850 did have progessively wound coils but they were made with smaller diameter wire and the coils not as tightly wound as the Progressive Suspension Spings (# 11-1107). The progressive springs were stiffer and 18 mm longer in length. I measured the stock steel preload spacers and made up new ones out of the PVC sleeve material supplied in the kit. I took the 18 mm extra spring length off but then added 5 mm extra for the ride conditions that I wanted to achieve, without having to use air over the oil, as the stock system uses. This was a guestimate, but did work out!

I used the 5.5" (140 mm) suggested oil level and 15 grade oil. This has worked out well for my riding style and 170 lbs weight. I did slide the fork tubes as close to the handle bars as possible to improve the front ride height. They are now mounted 12 mm further up the tree than stock.

Your 750 will be similar and should take the same springs. You will need to do the calcs for your own weight.