Old GS handling limitations

[Big T]

Get your new front tire and sign up for a ERC

[RichDesmond]

Sorry I wasn't clear on this. 68mm was the total travel in the rear with the Ohlins shocks (measuring from the base of the shock body to the bump stop with the shaft fully extended) and I was 32mm of sag if I remember correctly.

You need to measure your rear sag properly. Follow (carefully!!) the instructions here:

http://www.racetech.com/page/title/Sag

You absolutely need to have another person. Do it slowly, and do it a few times. My standard practice is 5 measurements, throw out high and low and average the middle three. Repeat the process without you on the bike also. Post the numbers and we'll go from there.

I can't over-emphasis enough how important good, accurate sag numbers are in setting up a suspension. They are very easy to get wrong. Probably half of the sag numbers I get from customers are physically impossible, which makes for very frustrating troubleshooting dialogs.

[glib]

You need to measure your rear sag properly. Follow (carefully!!) the instructions here:

http://www.racetech.com/page/title/Sag

You absolutely need to have another person. Do it slowly, and do it a few times. My standard practice is 5 measurements, throw out high and low and average the middle three. Repeat the process without you on the bike also. Post the numbers and we'll go from there.

I can't over-emphasis enough how important good, accurate sag numbers are in setting up a suspension. They are very easy to get wrong. Probably half of the sag numbers I get from customers are physically impossible, which makes for very frustrating troubleshooting dialogs.

Well then you'll love this. I broke the spring retainer on one of my Ohlins. I ordered it from the only place I could find it which is in the UK so it'll be at least two weeks before I get them back on the bike.

I did however, follow the instructions you shared. I'm happy to do it again too to confirm my results but now I can't find where I got the range numbers. What is the correct range for the rear and where is it found?

Lastly, the shop manual says rear wheel travel is 108mm but I measure 68mm travel on the Ohlins and maybe 75 on my Progressives. What's the deal?

[bonanzadave]

I was left wanting in the corners.

It will always be that way on an old GS. Start saving your money for a modern performance bike.

[RichDesmond]

Well then you'll love this. I broke the spring retainer on one of my Ohlins. I ordered it from the only place I could find it which is in the UK so it'll be at least two weeks before I get them back on the bike.

I did however, follow the instructions you shared. I'm happy to do it again too to confirm my results but now I can't find where I got the range numbers. What is the correct range for the rear and where is it found?

Lastly, the shop manual says rear wheel travel is 108mm but I measure 68mm travel on the Ohlins and maybe 75 on my Progressives. What's the deal?

The shock is not mounted out at the end of the swingarm, so it has less travel than the axle does.

Rear sag is measured vertically from the rear axle, as outlined in that R-T article.

Total sag (what Race-Tech calls "race" or "static" sag) should be around 32-35mm. Bike sag (R-T calls it "free" sag) is critical too, but I can't give you a simple number on that, it depends on how much you weigh and and the bike's weight. Likely in the 10-15mm range for a GS, but I need the weight numbers to be sure.

[Sandy]

Thanks for sag guide Rich, I'll measure mine when I get the bike back together from winter maintenance. Question though, on frame 5 shouldn't that be ((L2+L3)/2)

[glib]

The shock is not mounted out at the end of the swingarm, so it has less travel than the axle does.

Rear sag is measured vertically from the rear axle, as outlined in that R-T article.

Total sag (what Race-Tech calls "race" or "static" sag) should be around 32-35mm. Bike sag (R-T calls it "free" sag) is critical too, but I can't give you a simple number on that, it depends on how much you weigh and and the bike's weight. Likely in the 10-15mm range for a GS, but I need the weight numbers to be sure.

As I said, it will take me some time to recheck. I'll probably have my Works shocks charged by then, but I really don't think I was off by much if any--probably because I did follow those directions. However, regarding the position of the shock vs axle, 68mm of travel at the shock would translate to 79mm at the axle and 75mm at the shock would translate to 87mm at the axle. To get 108mm of travel at the axle would require 93mm at the shock.

If the Ohlins have only 68mm of travel, 32mm of "race" sag would be half of the available travel. Could that be right? I thought that must be why I was bottoming out over tracks or coming out of a quick dip. I weigh 204 btw.

[RichDesmond]

Thanks for sag guide Rich, I'll measure mine when I get the bike back together from winter maintenance. Question though, on frame 5 shouldn't that be ((L2+L3)/2)

That's this line:

"The actual sag is calculated by averaging L2 and L3 and subtracting it from fully extended."

[glib]

With new Sonic fork springs, emulator, fork brace and the right fork oil, forget the air you don't need it.

I was about to go out and move the forks but the tubes have holes that will leak once moved out of the fittings that trap the air. Even ten mm will move them out from under the seal. Not sure what to do now. I'd have to find a way to seal the holes.

[RichDesmond]

As I said, it will take me some time to recheck. I'll probably have my Works shocks charged by then, but I really don't think I was off by much if any--probably because I did follow those directions. However, regarding the position of the shock vs axle, 68mm of travel at the shock would translate to 79mm at the axle and 75mm at the shock would translate to 87mm at the axle. To get 108mm of travel at the axle would require 93mm at the shock.

If the Ohlins have only 68mm of travel, 32mm of "race" sag would be half of the available travel. Could that be right? I thought that must be why I was bottoming out over track or coming out of a quick dip. I weigh 204 btw.

I think your math is probably off. There's more to it than just the ratio of swingarm pivot-shock mount distance to swingarm pivot-axle distance.

That said, could they be the wrong shocks for your bike?

[glib]

I think your math is probably off. There's more to it than just the ratio of swingarm pivot-shock mount distance to swingarm pivot-axle distance.

That said, could they be the wrong shocks for your bike?

Actually you are right. The forward angle of the shocks adds travel to the axle and would bring the numbers pretty close to stock with the Progressives. The Ohlins do have shorter travel. Would that justify less sag? Otherwise there is significantly less travel before bottoming. Would that be a spring stiffness issue? I believe they are SU 142s. Should be correct for the bike but maybe for a lighter pilot.

[RichDesmond]

Actually you are right. The forward angle of the shocks adds travel to the axle and would bring the numbers pretty close to stock with the Progressives. The Ohlins do have shorter travel. Would that justify less sag? Otherwise there is significantly less travel before bottoming. Would that be a spring stiffness issue? I believe they are SU 142s. Should be correct for the bike but maybe for a lighter pilot.

All of the above. If you have less actual axle travel then you need both less sag and stiffer springs.
It's important to work from axle travel, not shock travel. Depending on the mounting points dual shock setups can have either (slightly) rising rate geometry, falling rate, or be almost linear.
If you pull the springs off the shocks and then reinstall them you can measure axle travel, having that number would help. I typically use 28-31% of travel when setting total sag.

[glib]

All of the above. If you have less actual axle travel then you need both less sag and stiffer springs.
It's important to work from axle travel, not shock travel. Depending on the mounting points dual shock setups can have either (slightly) rising rate geometry, falling rate, or be almost linear.
If you pull the springs off the shocks and then reinstall them you can measure axle travel, having that number would help. I typically use 28-31% of travel when setting total sag.

Perfect timing for that suggestion because last night I decided I'd go today and get my Works shocks charged with nitrogen (hard to find someone to trust even with such a simple task), so I took off the springs. I just went out and mounted one with no springs and measured travel several times. The axle travel from extended shock to bump is 101mm (Works shock travel is 70mm--just 2mm less than the Ohlins). So I need 28-31mm of sag.

What is the consequence of too little sag? I don't think I've seen that addressed.

[RichDesmond]

Perfect timing for that suggestion because last night I decided I'd go today and get my Works shocks charged with nitrogen (hard to find someone to trust even with such a simple task), so I took off the springs. I just went out and mounted one with no springs and measured travel several times. The axle travel from extended shock to bump is 101mm (Works shock travel is 70mm--just 2mm less than the Ohlins). So I need 28-31mm of sag.

What is the consequence of too little sag? I don't think I've seen that addressed.

Depends on what's causing it. If it's springs that are too stiff then you have restricted travel and forces larger than they need to be being fed into the chassis. If it's due to excess preload then you'll get harshness at the top of the stroke, as it takes more force to get the suspension moving again after it's topped out.
Either situation compromises ride quality and traction.

[glib]

Depends on what's causing it. If it's springs that are too stiff then you have restricted travel and forces larger than they need to be being fed into the chassis. If it's due to excess preload then you'll get harshness at the top of the stroke, as it takes more force to get the suspension moving again after it's topped out.
Either situation compromises ride quality and traction.

I guess it's just a hazard of setting up used parts but I have to guess at the initial preload setting (the wire clip) and it's not an easy adjustment once installed.