weird sprocket ratio?

[Jmcb]

I picked up a 78 GS550. I Instintly realized the rear sprocket had packed it in and would need to be replaced. I looked up the stock sprocket teeth count. Manual says 50 teeth on the rear and 15 on the front. However my bike has a 42 tooth sprocket on the back and stock 15 toothed one on the front. What would be the benefit (if any) of running a 42 on the rear but keeping the front one stock? Should I keep this setup as is? Or woud it best to return the rear sprocket to the stock 50T?

 

[Steve]

With only 42 teeth on the rear, the ratio is about 15% different. That means that your engine will turn about 15% slower. If someone were riding next to you on their stock bike at 5000 RPM yours would be turning 4200 RPM. Definitely a bit more "relaxed" sound, but those smaller engines like (need) to rev, so you may be hurting more than you are helping.

I have not checked the parts fiche, but I have a feeling the '81/'82 550T might have had a sprocket with fewer than 50 teeth, but probably not as few as 42.

Your acceleration will be less. You might think your fuel economy might be better, but you might be running below optimum speed and need to downshift more often to get the engine speed back where it will do something, which will offset the economy gains.

You could safely drop 2 or 3 teeth, but I would not go 8.

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[chuck hahn]

with stock teeth count my calculator says its a 3.33 to 1 ratio

 

[Sandy]

The change from 50 to 42 is a large change in gearing. One question that hasn't been asked is what are you going to be using the bike for? Is it just mostly city commuting or lots of touring and mostly highway use? Stock gearing is always a compromise between the two especially on smaller bikes. If you're just commuting the stock gearing (15/50) is probably just fine. If you're going to be doing a lot of highway driving then probably dropping 3 teeth or so on the rear might be all you need to bring the revs down a bit. As Steve mentioned if you drop more than that the bike is going to feel dead and in top gear you're going be doing a lot of down shifting on hills or passing. Before you go swapping sprockets check to see what sort of wear you have on what is there. If the chain and sprockets are worn you will need to replace both sprockets and the chain. If that's the case, instead of dropping 3 teeth from stock on the rear I'd probably just increase 1 on the front ....gives pretty much the same gear ratio.

 

[cowboyup3371]

Stock for his 78 as well as a 81/82 550 is a 50-tooth in the rear

 

[Steve]

Stock for his 78 as well as a 81/82 550 is a 50-tooth in the rear

The "L" model specifies a 49-tooth sprocket. Probably to compensate for the 16" wheel.

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