Bent Frame

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Bent Frame

Postby Yojimbo » Mon Oct 13, 2008 9:56 am

So I think the old bike I picked up must have been in a crash or something, the frame seems slightly bent. Nothing major, just noticed that the wheels/head/seat tubes don't quite line up right.

It's rideable - at worst I figure it's robbing me of a little efficiency - so I was wondering if it was worth trying to straighten it (steel frame). Like I said it's hardly an issue, but would having the rear wheel running at a minor angle mean extra wear on the drivetrain, tyres etc?

Cheers.
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by BNA » Mon Oct 13, 2008 3:56 pm

BNA
 

Postby familyguy » Mon Oct 13, 2008 3:56 pm

Do you mean that if you hold the seat tube vertically, the head tube is twisted to one side?

That would mean your wheels are not running in line. Generally spells trouble at speed. The stress on the frame may not be an issue, but I'd fix it if I could, even to make the bike track straight. Take your hands off the bars on a bike like that and you're likely to put it on its side before you can react.

Naturally, this is personal opinion...others may differ, but I've had a bent frame that was unrideable.

Jim
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Postby Yojimbo » Tue Oct 14, 2008 9:51 am

familyguy wrote:Do you mean that if you hold the seat tube vertically, the head tube is twisted to one side?


Not exactly. the main triangle is fine - head/seat/top/down tubes all line up correctly - it's more a problem with the rear forks. I was replacing the tyres, and after reinstalling the back wheel I could get it either in line with the rest of the frame, or centered between the chainstays, but not both.

I haven't noticed any problems with balance or efficiency, but as a noob I don't really have a point of reference. I've currently got the wheel pretty straight in the direction of travel, just the centering could be better.
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Postby familyguy » Tue Oct 14, 2008 12:09 pm

Seems like the rear dropouts are not aligned. You can try this to bring it back into line.

Scroll down till you see the headings "Spreading the Frame" and "Checking Alignment (Symmetry)".

Just work one side 'in' and the other side 'out'.

WARNING: DO NOT try this if the frame is not STEEL!!!

Jim
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Postby Yojimbo » Tue Oct 14, 2008 2:16 pm

As always, Sheldon to the rescue. (RIP :cry:)

However, I think that might be a bit beyond me at the moment, probably just end up making things worse. If it turns out to be a major issue I may reconsider.
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Postby RobS » Wed Oct 15, 2008 12:35 am

It can be a fiddley job to get right. Sometimes you can just pull in the vacinity of the axle mounts and all is OK. If you grab onto different points you can skew things because the seat stay and chain stays are different thickness.

If you have specific problems then you have need to work out how much the seat stays need shifting and how much the chain stays need shifting - then work out how you are going to do it. I've place planks of wood on each side of the the head tube and seat tube to get a reference line. The brake mounts and axle mount points are good reference points.

And yes, one bad pull and you have created more work for yourself! Work in small steps.
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Postby tha-wanderer » Thu Oct 23, 2008 1:52 pm

well, usually once you damage something, you can never get it back to its original form/strength..
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Postby rustguard » Thu Oct 23, 2008 11:12 pm

Just a question; but have you checked the dish on your rear wheel, this is much more common to be the problem. and much more easily fixed.
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Postby Kid_Carbine » Fri Oct 24, 2008 12:42 am

Run a string line alignment test on the frame. It takes only a few minutes & will tell you if the rear triangle is laterally in alignment.

Remove rear wheel
Run a string from the head tube down to the rear dropouts, then across, through two points that are exactly the same both sides, then back up to the headtube again. Cross the ends of the stringline so that they wrap around the head tube & provide a reference point that is the same for both sides up front. Get someone to hold the string taut, or tape it there.

Measure the distance between the seat tube & the string on both sides. That's where you will pick up any lateral misalignment.

In a frame with rear facing tips, just run the stringline through where the axle goes but in a road frame, select two points where you can run the string through that are identical both sides.

Tools required. String, tape [optional] measuring device.
Guesswork is for fools.

Check also [as has already been writen] that the rim is equidistant from the locknuts. The rim must be centered between the axle locknuts & the frame 'square' & if so, then the forces of nature are in balance again.

The beauty of a steel frame is that as long as you don't roll it into a ball of twisted metal, it can ALWAYS be fixed. You only need decide if any repair is economically viable. If it can be made, it can be fixed.
Carbine & SJH cycles, & Quicksilver BMX
Now that's AUSTRALIAN to the core.
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Postby xc_bike » Mon Oct 27, 2008 8:12 am

rustguard wrote:Just a question; but have you checked the dish on your rear wheel, this is much more common to be the problem. and much more easily fixed.

Good question,
Also maybe the wheels in this frame are not original and the rear axel length has changed. This can make a difference some 6,7 speed 125mm axel frames should not have rear stays pulled apart to fit a longer axel. Result is wheel shall be off center. 8/9 speed longer. If its steel chances are it should be the shorter older length.

Also check the wheel is in straight in the frame. Perhaps take it to a bike shop and ask if it is bent. They may notice something you missed. Sometimes a fresh opinion does the trick.
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Postby Yojimbo » Mon Oct 27, 2008 9:50 am

rustguard wrote:Just a question; but have you checked the dish on your rear wheel, this is much more common to be the problem. and much more easily fixed.


Good point, but I'm pretty sure it's the frame. I'll check though, and do the string test.

Thanks guys.
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Postby fixie » Tue Oct 28, 2008 8:17 pm

The beauty of the string test is that it will give you a clue if the drop outs have been spread correctly or just pushed out to accommodate the axle width. The bike will be quite rideable but not nice. I once rode a bike that had the back wheel at least one cm offset from the front. I thought it was slow up hill and when it rained and I saw the track(s) I made I realised why. :D

I still have that bike and it is a favourite, but beautifully aligned. :D
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Postby rustguard » Sat Nov 15, 2008 1:01 pm

so was it the frame or the wheels? or another problem? or still dont know?
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Postby Yojimbo » Mon Nov 17, 2008 9:57 am

rustguard wrote:so was it the frame or the wheels? or another problem? or still dont know?


Dunno, hehe. :D

I've been busy with exams and whatnot, and as I said it's not a major disaster so fixing it is kind of low on my list of priorities at the moment.

I'll get around to it one day...
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