shim on rear dropout to increase wheel size

[Catdr]

My rear brake barely fits a small 28 tire. The brake pads do not extend to the bottom of the brakes, as opposed to the front fork that will fit a 30 tire without a problem. Tire width is not a problem. Could I fit a small shim in the rear droupout to lower the brake pads and allow the fitting of a larger tire? (Post should say tire and not wheel)

[Too Tall]

Do you have dental insurance?

[dgaddis]

That sounds super sketchy, and like a massive PITA from a practical standpoint.

Buy a new bike!!

[Gattonero]

Could work only with long, horizontal dropouts. No way with modern vertical dropouts.

There are "drop-down" brake pad holders that may be useful for you?

BDop OFFSET HOLDERS

[dgaddis]

Sounds like he needs to move the brake arms UP, not the pads down.

[Catdr]

Need to move the rim down a couple of mm.

[andrew]

Need to move the rim down a couple of mm.

Nope. You need to get a different bike. Or use a tire size that will fit.

[czernysz]

Could work only with long, horizontal dropouts. No way with modern vertical dropouts.

There are "drop-down" brake pad holders that may be useful for you?

BDop OFFSET HOLDERS

That looks handy.

[bulgie]

Could work only with long, horizontal dropouts. No way with modern vertical dropouts.

Is that a typo or did you mean to say that?
It can only work with vertical dropouts. Shimming a horizontal at the back of the slot would do nothing, and shimming it at the top of the slot (where it would have the desired effect of offsetting the wheel downward) would make the slot too narrow for the axle to go in.

Also I'm not sure what "modern" verticals means in this regard. It will work the same with third-millennium verticals as with 1940s verticals, providing the axle nut and QR nut still have enough metal to bite into after the 2 mm downward offset. This is a function of the shape of the dropout faces and the strength of the metal they're made of, not what decade they were made.

I'm waiting for the nay-sayers to explain why this wouldn't work or would be unsafe, or whatever else the problem is.

Myself, I would try it, but I have decades of wrenching and framebuilding experience, and mechanical engineering training, and above-average common sense. So I know what to look for, and how to know if it isn't going to be safe with a particular dropout. Part of me wants to say "if you have to ask, then you are not up to the job". But let's give the OP the benefit of the doubt and give some general guidelines:

First, try it without shims. Assuming vertical dropouts (again, the only way this can work), loosen the QR and lower the axle in the dropouts until you have the needed tire clearance up above, then tighten the QR. Look carefully at where the dropout is pinched between the axle nut and the QR nut. Is there still enough dropout being grabbed? Remember the main force on the axle in use will be up, trying to push the axle back up into the dropout, so there's some leeway there. Normal dropouts (anything but through-axle) always have less than 100% engagement, so obviously, dropout-axle interfaces can work with less than 100% How much less you can get away with depends on dropout design, strength of materials, how much you weigh and how hard you pedal -- maybe other factors too but I think those are the main ones.

If you don't trust yourself to make that determination, then don't proceed.

But if you think there's enough metal engaged, then the next step is to make the shims. The main requirements for the shims is that they not stick out wider than the width of the dropout -- they exist only within the slot. They need to be the same height right and left so the wheel ends up centered in the frame.

They will be held in place by the axle while riding, but you don't want them to fall out when you take the wheel out, so you should probably epoxy them in place, after you have determined that they actually do work. (You can ride the bike first without epoxy as a test.) I'd use a toughened epoxy.

If you aren't sure what I mean by any of that, or don't know how to go about making the shims, or can't be bothered to follow the directions on the epoxy container exactly, then you probably aren't up to the job and should not proceed without expert help.

Good luck, and report back with results if you try it.

Mark Bulgier
Seattle

[Gattonero]

1st advice from the workshop: don't blast "the truth"

Then, let's go back to the OP, this chap doesn't make it easy to understand, from what I get without having the bike on my workstand, he's got two problems:

The first, is a rear brake that is likely to be the wrong type for the bike, being a mid or deep-drop in a frame that would take std. drop brakes.
From this, my suggestion of using "shift pads" fitted in the reverse way, so to decrease the drop of the brake. Those brake pads would reduce the drop by almost 5mm which is relatively a wide margin.

The second problem, is tyre clearance. This seems consistent with a road frame that would use std. drop brakes, with clearance limited to 25mm tyres at most.
Moving the wheel away, thus slightly increasing the wheelbase, gives good results in most cases. Surely, where the chainstays are properly round (more likely on a track bike, tho) the clearance would be always limited. Oval or dimpled chainstays would give better results.
So we're left with the clearance between the seatstays. Given that those are not dimpled, nor do spread as much as the chainstays (as the seat cluster is nowhere near as wide as a BB shell), it needs a substantial shift of the rear wheel to gain real estate there.

And this last point brings back to the dropouts.
I cannot recall may bikes built before the 80's having proper vertical dropouts as they're seen in the vast majority of today's bikes. If we think of a shape like the venerable Campangolo 1010, we see that the rear wheel could be moved generously fore/aft. And this would be while keeping the wheel skewer and hub locknut with a good 300º of contact with the dropout itself.
We're talking of 15mm or more, which gives a lot more chances than what you will get by shimming a standard modern dropout. Let's say it all, how much can you add? 2-3mm at most? Past the 2mm point you've already lost a good portion of dropout surface where the wheel skewer and hub locknut are supposed to grip. Sure, this is not disk-brake equipped, yet it doesn't justify the risk taken.
I as a professional mechanic would never recommend to have the nominal wheel axle size (ø10.0mm) past the end of the dropout.

[Catdr]

Okay, a little more explination. I have verticle droupouts and standard reach brakes. When set up, the brake pads fit on the rims, but have 2mm or so of additional room to drop down. I want to move the rim away from the brake/brake bridge to allow more vertical clearance for a larger tire. Width of the tire is not a problem. I was thinking of using part of a spoke as a shim, and epoxy to keep it in (as described above). There would be plenty of room for the skewers to lock on the dropouts. I do not see any reason that it would not be as secure as it is without the shim. Bulgie's post made a lot of sense. The post was to ask if there was something that I had not considered or if someone had done it before.

[andrew flowers]

id trust gattonero

when you begin to drop the axle lower in the dropout you open up a large can of worms regarding leverage on the back half of the dropout

secondly tire clearance as tight as what you are suggesting is going to be awful on anything rougher than the smoothest pavement

[Justin@BlackOakVelo]

This is not recommended.

Stay safe out there folks.