1981 Gs650g rear shock replacement options

[Guest]

So my stock rear shocks are crazy bouncy (like hit a bump and bounce for a block). They are 13.5 inches and I found some 11.5 inch ones from a junk Yamaha from a similar year and I like how it feels with the bike lower. The only issue is that the 11.5 shocks are very stiff. My question is can I just find some softer shocks that are shorter as long as they mount up properly or are there other factors I will need to consider, and where should I start looking?

[Steve]

If at all possible, do NOT use shorter shocks. Shaft-driven bikes have issues with the u-joint running at different angles. I do have shocks on the back of my wife's bike that are one inch shorter than stock, but we have not had them on there long enough to notice any problems.

When you are shopping for new shocks, keep in mind that there are several cheap options (usually on eBay) for about $100, but they will be like the ones you experienced, VERY stiff. next step up is Hagon, for about $215. Contact Dave Quinn Motorsports, talk to Dave. He will get some information from you about your bike, your weight, how much you ride double, etc., to determine what damping rate you need, then select the proper springs. You can mention that you would like shocks an inch shorter. (That is what we have on my wife's bike.) Next up the price ladder is Progressive Suspension, which will run about $300. Here, you can also specify length and spring rate to customize to your application. You can continue up the ladder, spending more and more money on your suspension, it all depends on how "special" you want it to be.

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[dackcommabig]

Thanks for the info Steve. I didn't think about the driveshaft angle that does make sense but I would think that my stock shocks that bounce like crazy would be worse for the the u joint then the stiff lower ones that I have on due to the angle staying relatively the same?

[Steve]

Occasional deviations from the "ideal" angle (which is NOT zero) are normal and expected, but running virtually all the time at greater angles are not good for the u-joint.

I don't remember the source, but I saw an article that detailed what I think were called "cyclic oscillations". If you compare the input and output speeds of a u-joint, you will find them exactly the same at all times when the two shafts are in-line, but they start changing when the angle increases. I just found a similar article in the Wikipedia entry. Scroll down to the "Equation of motion" section for details beyond what I can give.

Basically, what happens is that the speed of the output shaft changes in relation to the speed of the input shaft twice, every revolution. In this case, the 'output shaft' is the driveshaft on your bike. As its speed is changed, it will try to change the speed of the final drive unit and the speed of the bike. Fortunately, inertia is on our side. The weight of the shaft is rather small, compared to what it is trying to move. All it ends up doing is taking up the clearance between the pinion and ring gears in the final drive. UNfortunately, what that does is beat those gears to death.

Although those gears are rather robust and seldom fail, it probably won't take very long to destroy them with what would appear to be a constant jack-hammering by the driveshaft. The "ideal" angle is just a couple of degrees, for shafts that are under constant power, that is to prevent the bearings from wearing into a single spot. However, start increasing that angle, you will get into the speed deviations that kill the gears.

Back to basics for a moment. You say you "like the way it feels with the bike lower". Is that because you are accustomed to a low bike or do you have shorter legs?

A couple other things to consider when installing shorter shocks. 1) You will change the angle of the steering head. Yeah, it's not much, but some riders are capable of telling that the handling changes with a different steering angle. 2) You will be decreasing the cornering clearance, which might affect how you ride. If you use the bike to go bar hopping or bike nights, it won't be much of a problem, but if you use the bike for the fun it is capable of, you will be limiting its abilities.

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[tom203]

Someone had replaced my 650L shocks with 13.25 inch versions (unknown maker). I replaced them last year with some used Showa versions off a 550 that are about 12.5 inches long. I ride solo and they work well for me- shaft angle looks good.
lots of threads on here where folks find their new-designed-you shocks ride too stiff, so be careful.

[BigD_83]

I accidentally replaced the EMGO Shocks that were on my 650G with a set of Hagons that were an inch shorter than stock. I've been very impressed with the improvement in feel, performance, and ride quality. I've only had them for a year or so, and haven't yet noticed any problems with the shaft drive.

I really don't like the "stance" of the motorcycle, from a purely esthetic point of view. I do find the seating a little more cramped now, mainly in my hip flexors. Stretching the thought out, I could see taking another inch off really affecting handling. I did notice the front end feels lighter, but I was able to ride it hard at John Day last year, and only in one corner did I wonder if the longer shocks would have made a difference.