tyrion

Should I put a washer between a steel frame and an aluminum rack?

Nylon washer?

SurferRosa

I don't use them on my grocery getter, a '75 Motobécane with rear rack. My eyelets are threaded, so, I just use the 4mm allen bolts at the bottom. On top, the brake bolt isn't long enough to use a washer with the fixing nut.

dsbrantjr

If any, I would use a stainless washer. Remember that the bolt needs to be under constant tension in order for it not to loosen. A nylon washer is likely to compress or shrink over time, allowing the joint to loosen.

79pmooney

I never put washers there and in fact believe solid contact between the rack and fender eye is good. Means the bolt is entirely in shear and tension and can best do its job of holding the rack securely. (Maybe hard on the eye paint. I've always considered the paint on the inner and outer surfaces of eye and dropouts to be temporary.)

Ben

Bill Kapaun

+1
Imagine you HAD to use 1" long spacers. Think how that would "leverage down" and tend to bend the bolt every time you hit a bump.

tyrion

I actually do have 1/8" spacers in there now because 12mm bolts would touch the disc without them. I have shorter bolts on order so I can get rid of those spacers.

easyupbug 

Washers are to protect the joint surfaces from tightening, distribute the load evenly under the nut and in some applications the stress under the edges of the hex head can cause deformation. I suspect none of these apply in this application and many others.

rydabent

Yes the big point here is a washer spreads the load. A washer will keep the aluminium from deforming.

Mad Honk 

And nothing in this discussion about using a lubrication for the threads to keep from seizing via dielectric activity. Smiles, MH

Bill Kapaun

A hacksaw can shorten small bolts quickly. Or 3" cutoff wheel.
Just screw a nut on first to use as a thread chaser after the cut.
A few careful, light strokes with a fine file can remove any burrs.

easyupbug 

Good point, I see Tyrion is in lovely San Diego where galvanic corrosion can be an issue in a somewhat corrosive atmosphere. Down here in Tucson no worries, I think we have had 1.5" of rain this year!

hokiefyd 

I always use nylon or rubber washers between accessories and the frame. Mostly to protect the paint, but also as a load distributor where the surfaces are not machined perfectly, or where the angles may not line up correctly (such as with a rack mount and the fender eye). Part of my bike maintenance therapyroutine is to go over the bike periodically and check all fasteners, or at least most fasteners. I certainly do catch the accessory bolts and snug them if they need it.

It's definitely more ideal to always have a fastener under constant tension, and to not have materials that contract or expand between the fastener and the clamped work, but I've also never had anything fall off my bike and the certain downsides to using a soft washer don't seem to bubble up in practice, at least for me.

ClydeClydeson

I think a nylon washer between the rack and frame is a bad idea. The majority of the rack load is held by the clamped joint between the frame and lower leg of the rack, and this must be tight as to allow no movement when a load is present and especially when riding over a rough surface with a load on the rack. A nylon washer will not have the shear strength to hold the rack still so the load will wind up being transferred to the M4 bolt, which will not take much abuse before bending and breaking.

In short, this sounds like a problem in search of a solution, but one which causes another problem.

tyrion

Yeah I have a corrosion problem. Things corrode pretty fast where I live.

79pmooney

Actually I lied. I do use washers/spacers when they are necessary to move the rack stay out beyond the fork or seatstay. It's a compromise and the bolts usually don't last nearly as long. Bent aluminum rack stays and the scraping of paint over the steel fork/stay being the alternative. The bikes I've had to do that on aren't good cargo/truck bikes in my opinion, but if that's what I have, I do it/

Pop N Wood

A shot of WD40 or equivalent every now and then will hold off the corrosion. I routinely treat every threaded part of my bikes.

Stainless washers would work, but most racks have thick enough ferrules that you shouldn't need a washer to distribute the load.

Best way I've found to shorten bolts is a small belt sander. I have a knife sharpener with 60 grit belts. Works perfectly, seems to round the end of the threads so nuts screw on without drama. If you do cut a bolt with a hacksaw or cut off wheel, thread on a nut first so you can back it out and chase the threads before cleaning up the cut with a small file or sandpaper.

sweeks

I was thinking the same about galvanic action, which in theory could cause corrosion (dissimilar metals in an electrolye...).
The short answer is I wouldn't worry about it, but I agree that lubrication is a good idea.

easyupbug 

Stainless steel would one of the worst materials to use in this case as the combination of aluminum and stainless steel is notorious for causing accelerated galvanic corrosion.

Pop N Wood

And yet every bike I own has stainless hardware, even the 40 year old aluminum ones.