Abe Froman

Edit:Tile says cassette, I meant freewheel

Buddy brought over an old early 90's MTB and he asked if I'd look at if for him. It has a Suntour drivetrain with 7 speed freewheel and is all original with low miles.

It hadn't been ridden for several years. I put it in the repair stand and shift through the gears. I notice on a couple gears the chain hangs up and rides over the top of the cogs. Like its not shifting over enough.

I think no problem and adjust The deraileur as normal. Same problem. Upon closer inspection I notice that a couple of the cogs have slight grooves on the top of the teeth and the chain seems to catch in these grooves. The grooves make a step from the inside to the outside of the cog. The side plates of the chain just ride right over the top of the cogs.

It appears that the normal profile for the cog teeth is that they were originally squared off on top. The cogs have no taper to them like a couple old Shimano's in my parts bin.

Chainrings don't appear to have any noticeable wear on them.

Is this typical wear? What kind of results will I get from grinding these teeth?

Sorry no pics, couldn't get a decent shot of the problem.

wesmamyke

It's probably not wear, they were likely designed that way. Sachs brand freewheels had teeth like that, other might have also. I'd give adjusting everything another try.

HillRider

Sun Tour made early attempts to profile the teeth on their freewheel cogs to improve shifting speed and accuracy and that is what you may be noticing. Obviously Shimano, and later Campy, greatly extended and improved on this concept.

Out of curiosity, what type and width of chain is on the bike? 7-speed freewheels required a "narrow" (7/8-speed) chain.

DannoXYZ 

Problem with the Suntour cogs is they're fat and wide at the tips. Lets the chain ride up there for way too long. I've found that grinding an angled bevel on each of the teeth tips really improves the shifting performance. Use a Shimano Uniglide cog as a model and cut the Suntour cog to be similar.

Abe Froman

Thanks for the replies.

These cogs are fat and wide at the tips. It looks like they were just plain cut from flat stock, with no ramps or signs of any engineering (bevels, ramps) to improve shifting. Just this little irregular ridge at the very top of a few of the teeth. These ridges are hard to see but can be felt with a fingernail. Right smack on top of the tooth.

Chain is a suntour chain. I tried a new 8spd chain I had lying around and it was the same. Seemed to be hanging up on these ridges.

Anyway, I ended up using a die grinder and smoothed out the teeth that were rough. Shifting is great now.

FBinNY 

I do the same, but only grind the inside (toward larger sprocket) face, and also slope the tips of the teeth inward. This leaves the highest part of the tip toward the smaller sprocket where it helps grab and lift the chain, while the bevels help move in inward so it engages smoothly. I've been doing that to freewheels for race bikes since the sixties long before hyperglide came about.

BTW- the worst offender in this category was Regina which had a groove right down the center of the teeth and was notorious for it's ability to keep a chain up there. Old timers can thank Regina for forcing us to learn to anticipate shifts and never do so under load.