I like working on older bikes and I see these all-the-time without really understanding them. My primary question; that I’ve wondered about for awhile; is: How do you know when the rear wheel is absolutely straight. It seems that the eyeball method could never be really accurate, unless by chance; and that even a minuscule error would cause extra tire wear, at the least. What is your method to set the adjusters, and is really important?
The secondary question I have I guessed with no proof. I can see how the ability to adjust the wheelbase would be a nice feature. Lengthen it when you need extra stability, control, and comfort; shorten it on race day.

Actually the only way to do it is by eye, and it works fine. The wheel doesn't need to be *perfectly* straight.

The screws are there so that once you get the wheel straight, you can tighten up the screws. Then the next time you insert the wheel, push it in until it hits the screws and it will be in the exact same place it was last time.

Why not just take them out then?

Because of the angle of the dropouts, shortening the wheelbase also steepens the head angle a bit.

Pretty tiny changes, though. I never really saw the point, unless it was to allow framebuilders to be more sloppy...

the adjusters are justthere so when you pull the wheel back in, after mending a puncture
the wheel goes back in the same spot..

but they rust fuse in place after a while.

When you set up a bike's derailleurs you dial the derailleur's position relative to the cog set to maximize the performance of the shifting system and if you re-install the wheel in a different position this will affect the fine tuning of the rear d and change the chain tension.

The set screws ensure that the wheel goes back in the same position and that it will be properly aligned in the frame and when you are setting up the bike you can use these set screws to make very fine adjustments.

It is something you only find on higher quality frames with cast dropouts... lesser frames often use a backstop for the drive side and the wheel gets aligned by shifting the position of the axle on the non drive side.

And Forged dropouts .. have 2 Campag 1010 dropouts , 2 bikes .. as such.

This is the only answer I've heard that makes any sense. The changes in wheelbase would be to minuscule to really make an impact, and proper alignment could be more easily achieved by simply pulling the wheel to the back of the dropouts.

You can, and you'll be none the worse off.

The bolts were originally intended as a racing feature, to facilitate quick wheel changes. Once properly set, you just pull the wheel all the way back against the stops, tighten the QR down and you're good. But they also weaken the dropout -- a common dropout failure mode is cracking through those bolt holes.

Thanks all. It sounds like there are pros and cons to whether or not to remove them And, an eyeball alignment seems the only practical way to set them. I guess I'll just keep doing what I always have and that is to set them the best I can and then leave them alone.

You don't want to remove the adjustment screws because then you'll have to recenter the wheel every time you re-install it. Their purpose is to provide a single, reliable, repeatable position for the axle, so that wheel changes can be done on the fly without having to fool around dialing in the position. Vertical dropouts do the same job eliminating the need for the positioning screws.

BTW- when these were in common use, one problem is that with normal (somewhat rough) handling these would get bent where they stuck out behind the dropout. This was a problem because they couldn't be moved or removed without damaging the threads in the dropout. When I set up team bikes, I'd get them perfectly adjusted then saw off and file smooth the section outside the frame. If need be they could still be adjusted or removed from the axle end, but no more damaged threads.

Since these are just standard M3 bolts, one way to handle the too-long problem is use shorter bolts. I replaced the ones in my Surly with a pair of bolts just long enough to let me adjust the wheel alignment with very little sticking out the back of the dropout.

Same result, but the bikes came with the screws and sawing was free which trumped having to spend dough. Also 40 years ago 3x.5 screws weren't as readily available as they are today.

I didn't exactly spend much. The bolts were left-overs from some long gone Cat-Eye cyclonmeter handlebar clamps.

FWIW, you can use the Sheldon Brown technique for measuring frame alignment. Just measure from the string to the wheel rim instead of the seatpost.

For a smooth running chain, you want the wheel to be very, very close to parallel with the front chainrings. It's worth getting out a caliper and setting the wheel position by adjusting the dropout bolts. Once that is set, the Park derailleur alignment tool is the best $50ish you can spend for getting clean, crisp shifts.



The Park tool allows you to align the derailleur hanger to be very close to precisely parallel to the wheel and thus the rear cluster. After you've gone to the trouble of getting everything aligned, it's worth having the dropout adjusters to make sure that removing the rear wheel and replacing it doesn't screw up your great shifting rear derailleur.

I've noticed that Sixty Fiver always calls forged dropouts cast. Maybe it's a Canadian thing.

I'll just add that provision for fore-and-aft adjustment of the wheel is no longer necessary because modern derailleurs have B-tension adjustment screws which accomplish the same thing, hence vertical dropouts. But for a "classic" derailleur like a Campy NR or SR, you need horizontal dropouts to get everything dialed in properly.

Cast and forged are two different things process - wise but you get similar results... but even if they are forged my brain wants to say cast.

Too much time spent in the machine shop working on cast parts I guess.

Good stuff Gentlemen. A bit over my head for now. Sometimes I feel like I'm a work in progress as much as the bikes I work on.