Misaligned frame — how much is too much?

Question

I have an aluminium frame (Scott Speedster).

There's a fairly substantial difference in the amount of space between chain stay and tyre on the left and right side. This happens regardless of how true the wheel is, so I measured the alignment of the frame using the string-around-the-frame method. The space between the string and the seat tube is a good 2-3 mm longer on the right side than the left.

I've never had any severe crashes with the bike, but I do have a trailer. If I load it up with both kids and some groceries, it weighs a good 50-60 kg, and the entire load of pulling it ends up on the left side dropout (via the QR skewer). It seems feasible that the asymmetrical load could have bent the frame.

If it were a steel frame, I could hand it in to my LBS to have it aligned (or tried it myself). But with aluminium, I really don't want to do that. These frames seem almost like throw-aways; they can't really be repaired.

So the question is: How much does it impact the safety of the bike? Does a misalignment increase the risk of the frame cracking under stress?

Answer 1

Have you recently changed the wheel, axle or had the hub serviced?

Wheels do not only need to be true, but also "dished". That is, the centerline of the rim may not align with the axle's mid point, it could be displaced to one side to allow space for the sprockets, depending on the gear count.

The axle can also have some spacers mis placed, which can cause the wheel being closer to one stay than the other (At least once I did bolt the cones and lock nuts on the wrong side after servicing the hub and only noticed when mounting the wheel to the frame).

Could your frame be asymmetrical by design?

I would try to rule out that the measurements difference that you found is not there by design by contacting the manufacturer or by measuring other frames of the same model (or asking other owners to measure theirs for you). In the MTB world there are several frames that have asymmetric rear triangle design, It would not surprise me if a couple of those had some allowance for a wide cassette.

Also, by using a string similarly, find if the whole wheel is pointing straight forwards or if indeed is pointing to one side. If the whole rear triangle is rotated towards the right, the wheel would point towards the left.

Regarding the string checking method, I find it estrange that almost never I see frames compensating for high gear count in one side versus no disk rotor on the other, yet usually the wheel points straight at the seat tube. I suspect that at least some frames have that compensation to allow for a less dished wheel.

Could there be damage from another type of event?

I really doubt your bike was bent by pulling a trailer. Since the trailer coupling is directly on the skewer, the force is transmitted almost directly from the axle to the trailer mount. When you pedal, the rear axle pushes the whole bike forward.

Completely different would be if the bike was pulled by the front, for example, if you tied a rope around the head tube and towed the bike and the loaded trailer, then yes, the force to pull the trailer would be transmitted through the frame and mostly the left stays.

However, a bent frame can result from applying load in a direction it is not meand to withstand, like, for example, high weight then the bike is leaned on its side, or sideways force when attached to a rack or even a sideways impact when in storage, etc. In many of those cases, you may find witness marks on the sides of the frame.

When did you notice the miss alignment?

If all of that still confirms that the frame is indeed misaligned, there is still a slight chance that is a manufacture defect (was it welded like that? Was the alignment jig out of true in the factory? Have you measured the frame before and found it was symmetric?)

If your frame is not misaligned as result of damage, I would not worry at all, the structural strength of the frame should be roughly intact. I have seen people ride severely misaligned bikes with no issue other than the aesthetics of it.

I would closely inspect the welds that join the stays to the front triangle. Look for cracks near the welds. Using a torch/flashlight to shine light on them from various directions while gently applying sideways force with hands. Also look for kinks in any of the tubes, rear and front triangle. In my experience, kinks are really weak points in aluminium tubes.

Another warning sign would be creaking sounds then pedalling hard or applying load to the fame. Just be sure the creaking does not come from a lose crank or bottom bracket.

If none of these signs are found, I'd be tempted to rule out damage and continue to ride the bike.

Answer 2

On the Scott Speedster it is in fact aligned asymmetrical as Jahaziel already mentioned. at least that is the case with mine. the right side is visibly wider than the left side, where the disc is, while wheel and seat tube remain perfectly aligned. Assuming that is the case with your bike as well.

Answer 3

Additional - I doubt your trailer caused any change in the frame.

1. The forces of towing are all in tension on the left-side chainstay, there is minimal rotational force and that would only be while turning the bike.

2. 60 kg sounds like a lot, but that's the overall weight and the tension on the drawbar would be less, only the effort required to accelerate the trailer

3. When rolling at constant speed the tension drops, and will be balanced by the overall drag of the trailer (rolling and air resistance, plus any grade you're going up)

Gut feeling is that your trailer would have to weigh tons to distort the bike's frame through normal usage. This specifically excludes any accidents.

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As for a fix, first, flip the rear wheel in your frame as a test. This is to check the rim is centered between the outer locknuts, relative to the frame. If yes, the wheel is fine. If no, then:

1. Depending how far the wheel is out, you really should re-dish the wheel. This will move the rim sideways so it runs better in this frame. This can be fiddly but with patience is achievable.
   I had a frame where the rim needed to be ~3mm to the left of center. Once fixed it was no big deal.

2. Quick fix is to try repostiioning the wheel as a test

   a. If rim brakes, clamp on your rear brake lever to hold that part of the rim relative to the bike. If disk brakes, disregard this.
   b. Back off your QR's tension
   c. Use your left hand where the wheel passes through the chainstays to hold the tyre centrally. Three points of contact means the QR will subtly shift
   d. Tighten the QR and confirm there's no rub at the brake and the chainstays. Do make sure the axle is fully up the dropout on one side.

This will show how much gap has been left in one dropout to get the wheel running without rub. You could in theory file the side with contact, until the axle sits home on both sides. HOWEVER this is fitting the frame to the wheel, which is backwards. Redishing the rear wheel is your solution.