1980 GS750E Fork Questions

[Guest]

I need to find out a few things about my front forks for 1980 GS750E.

1. Oil capacity.
2. Upper spring length.
3. Lower spring length.
4. Recommendations for stiffer front end such as heavier weight oil.

Thank you in advance,

2Stroke (aka David)

[RichDesmond]

You're best bet is to junk the stock springs and put in a good set of straight-rate aftermarket springs. They will make a huge difference in how the bike feels.

[2stroke]

You are correct my friend. I just changed the oil and they are still as mushy as ever. I have a set of air caps from another bike I tried and they did not fit.

Thank you for your reply,

2Stroke (aka David)

[Steve]

I will second the "new springs" idea. Not sure about the amount of oil it will take, but it's better to to add to a level, rather than a quantity. With the springs removed and the fork tubes fully collapsed, a level of 6 or 7 inches from the top usually works quite well.

As for springs, go to sonicsprings.com, look at their spring rate calculator. It takes into account your bike's weight, your personal graviational attraction and your riding style to make a recommendation. It might not hurt to upgrade that one step. I think the recommendation for my bike was the 1.0 springs, but since I have a frame-mounted fairing, I went with the 1.1 springs. Have not regretted the choice. Also, don't be tempted to use thicker oil to "firm things up". 10w oil will probably do the job just fine. The springs are what are supposed to keep the bike from diving when stopping, not the oil. The oil just keeps the wheel from bouncing.

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[2stroke]

Thank you Steve, I will check out the website and appreciate the advice.

[Steve]

I has been somewhat common practice to use 15w or even 20w oil to prevent diving while braking. If you understand the simple physics of suspension, you will find that it's the strength of the spring that prevents the dive, the viscosity of the fluid only slows down how fast it reaches the bottom.

Depending on you, your gear and your bike, typical springs might range from 0.9 to 1.1 (I think that's kg/mm). When the springs are installed, you will need to set some preload to maintain the proper "sag", or how much the bike drops when your weight is applied. That is typically about 15-20% of total travel. Since most of our bikes have 5-6" of travel, you are looking for 1-1.5" of sag. On my bikes, that has been achieved by cutting the spacer tube so it is even with the top of the extended fork tube. When the cap is installed, the spring will be compressed a bit, giving the proper amount of preload.

I read a paper some time ago that was written by the suspension manager of a major race team. His philosophy was to use the lightest viscosity oil that would keep the wheel on the track. Too-thin an oil would let the wheel bounce too much, but too-thick an oil would not let the wheel move enough. Our needs on the street are not quite as demanding, and certainly not as repeatable as hitting the same bumps on every lap, I have found 10w to work very nicely with the new springs.

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