Bent frame?

[graymee]

I'm almost 100% certain that my MTB frame is not straight. I turned it upside down and the centre of the tyre on the rear wheel did not line up with the hole in the centre of small bracing bar between the chain stays just behind the BB.

How would I go about confirming this and how would I straighten the frame? It's an aluminium frame by the way.

[Valbrona]

It could be that your wheel is not dished correctly, meaning that the rim does not sit central between hub flanges. To check wheel dishing just flip the wheel around - the rim should sit in the same position each time.

It is often the case with frames that sometimes the dropouts are not aligned parallel to one another which gives the effect which you are seeing. You need dropout alignment gauges to check alignment and these are also used to make corrections in alignment.

[531colin]

Above image is from a fairly comprehensive series here....http://forum.ctc.org.uk/viewtopic.php?f=1&t=59332&hilit=+string

[graymee]

Thank Colin and Valbrona. I built the wheel myself, don't have a dishing tool so the wheel could be wrongly dished.

If the dishing is correct I'll use Colin's method to check out the frame. What are my options if the frame is bent? Could I possibly straighten it myself, would I have to go to a bike shop or is it a case of live with it or bin it?

[CREPELLO]

Do you know if the frame's been crashed?

AFAIK, aluminium frames can't be reset.

If it's a budget alu frame, it could very well be out of true - plenty of frames are, by a few mm (If it's not your wheel dishing that's at fault )

[Tigerbiten]

How's the handling/tyre wear.
Unless you all get a speed wobble or are going though tyres very fast, I don't think it makes a lot of difference if the frame is slightly off true as you'll compensate for it as you ride.
If you always end up hurting after a ride it could be a bit more 50/50 if the bikes causing it by being out of true. You'll need to ride another bike which is correctly setup for you to test this.

[graymee]

I have come off the bike a couple of times but as far as I'm aware nothing so big that it would bend the frame. I work away from home and I've left my bike there whilst I'm on a weeks leave. I won't be able to check whether it's the wheel dishing until after work a week tomorrow.

Handling seems ok and I've not noticed uneven or rapid tyre wear.

I just wondered, after reading about cold setting a frame to take a wheel with a larger OLN, if something similar was possible in this case.

[531colin]

If the frame is a bit out, you can often get the wheels back in line by filing the dropouts, or re-dishing the wheel to suit the frame, or both.
In my experience, an error at the front end is much more likely to mess up the no-hands riding than an error at the back end.
The frame needs to follow the front wheel, but the wheels only really need to be in line at the tyre contact patch......it doesn't matter anywhere else, as long as you can adjust the brake to suit.

[graymee]

I'm more and more thinking that it's likely to be the dishing of the wheel that's causing the problem. I rebuilt the wheel after breaking several spokes not long after buying the bike. When I removed the cassette to replace another broken spoke I noticed that a few of the drive side spokes had flat spots, I think because the cassette had been fouling them, possibly another indication that the dishing is wrong.

[Bicycler]

I just wondered, after reading about cold setting a frame to take a wheel with a larger OLN, if something similar was possible in this case.

Steel frames can be cold set, Alu ones cannot.

[sreten]

Hi,

Wheel dishing and bent frame don't mean the same thing.

A dished wheel will be offset from the centre line, a bent
frame the wheel will be angled from the centre line, and
to boot steel frames can be bent to fix dishing issues.

Aluminium frames can have offset rear stays to reduce
rear wheel dishing, compared to the triangle centerline.

rgds, sreten.

Washers on a wheel can move it across, away from a stay.
My cheap road bike has rear washers, presumably for this.

[gregoryoftours]

I just wondered, after reading about cold setting a frame to take a wheel with a larger OLN, if something similar was possible in this case.

Steel frames can be cold set, Alu ones cannot.

Alu frames can be cold set to a certain degree, I guess it's because they are alu alloy not 100% alu, but it will weaken them somewhat and is a risk.
Have a look at this video of the people at the Orange bike factory bringing the back ends of their full suss aluminium bikes into line - from about 10mins 10sec in - https://www.youtube.com/watch?v=mlIYEdRFQu4
- Also bike shops regularly bend alu derailleur hangers back into line. It's going to be weaker but it may be possible.

[squeaker]

Have a look at this video of the people at the Orange bike factory bringing the back ends of their full suss aluminium bikes into line - from about 10mins 10sec in - https://www.youtube.com/watch?v=mlIYEdRFQu4

Mmmm, nice big hammer

[niggle]

I'm more and more thinking that it's likely to be the dishing of the wheel that's causing the problem. I rebuilt the wheel after breaking several spokes not long after buying the bike. When I removed the cassette to replace another broken spoke I noticed that a few of the drive side spokes had flat spots, I think because the cassette had been fouling them, possibly another indication that the dishing is wrong.

This is certainly a possible cause of the misalignment, follow what Valbrona said to check, i.e. reverse the wheel in the dropouts and if the wheel is now misaligned by the same amount but to the other side then the problem is wheel dishing. If it is still misaligned to the same side the problem is the frame.

To adjust the wheel dish loosen all the spoke nipples on the side it is misaligned to by a quarter turn and tighten all the opposite ones by the same amount, recheck and repeat if necessary.

[graymee]

Gregoryoftours - Thanks for posting that link to the video. I love Guy Martin videos, he has a passion for engineering like myself. I admire the skill of the guy aligning the frame, his percussion adjustment to get the dropouts level was almost calibrated.