I’m getting to know my 1985 Trek 770 frame. I notice that those little screws are screwed in pretty far limiting the rear axle from being inserted deep into the dropouts. The chainstay length is already really short and I think I could gain maybe 3/8” in crank to rear axle distance by turning out the screws. I’ll probably need to add links to the chain and maybe it would mess up the derailleur operation.

It looks like from the witness marks that the axle was once further into the dropouts than it is now.

Is there a proper adjustment for these screws other than to center the rim in the frame?

Thanks.

Pretty much all they do is help you center the wheel when you put it back in. You could even remove them if you wanted to move your axle further back.

Wheel placement in horizontal dropouts can influence shifting and the cog size that can be accommodated. Depends on the bike and rear derailleur and shifter system. My '89 Centurion Ironman with Suntour GPX group is picky about rear wheel position in the dropout.

I am not certain that it would necessarily "mess up" the derailleur but it would certainly require re-doing the indexing adjustment.

Be warned--you're lengthening your rear-center--which means an effectively long chainstay length--which means your chain may end up too short.

Is there something wrong with the way the bike works now? If not, why change anything? If so, is moving the axle back 3/8" a likely fix?

If you have friction shifting, none of that shifting chatter makes any difference. I would be surprised if the chain needs to be adjusted either. I've read about these things here, for nearly 10 years, and have never had to adjust chain length for this reason. Dumb luck perhaps.

Here’s what I’m thinking...with the triple the chain alignment would better although the short distance I’d gain may be insignificant in that regard. The most important fix would be more room for a larger tire. I brought home a Bontrager 28c tonight to see how it fits in the frame. I’ll back out those screws when I try the larger tire and see how it goes.

Thanks for all the responses.