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Deliberately bending a bicycle frame?

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stinkwheel
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PostPosted: 21:18 - 21 Feb 2013    Post subject: Deliberately bending a bicycle frame? Reply with quote

I've decided that I can no longer cope with the cyclocross tyres I've been using on my wholly inappropriate mountain bike. The pressure you need in them to prevent pinch-flats means it's painful doing any speed over hard ground and grip on wet stones is non-existant.

So. "proper" MTB tyres it is. Unfortunately, they JUST foul the chainstays. If the wheel was entirely true and dead-centre, they wouldn't touch at all but I'd give it 5 minutes befoe that's no longer the case with someone my weight.

Can't get narrower tyres, these ARE the narrow ones. I could shave bits off some of the knobs but this seems self-defeating and the tyres are worth more than the bike.

What would fix it is if I could put a bit of a bend in the chainstays where the tyre runs past. A few mm is all it would need. The question is how? It's a steel frame.

I thought about bolting an old hub between the dropouts to keep them in place and jacking the chainstays apart but my car jack is too wide to fit in the gap. Maybe I could even lever them apart with a fencepost and a bit of brute-force?

Anyway. Also photowhoring. Note 29" MTB tyres on 700c road wheels.
https://i48.photobucket.com/albums/f216/stinkwheel/CIMG0873_zpsc3b17a98.jpg
https://i48.photobucket.com/albums/f216/stinkwheel/CIMG0874_zps591014f5.jpg
https://i48.photobucket.com/albums/f216/stinkwheel/CIMG0876_zps86d31fbb.jpg

Anyone know if that gussett plate bewteen the chainstays you'd attach a mudguard or calliper brake to is structurally essential?
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.
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The Shaggy D.A.
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PostPosted: 22:07 - 21 Feb 2013    Post subject: Reply with quote

I'm wondering whether you could fashion a turnbuckle to use as a spreader, from a couple of bolts and some threaded tube, along the lines of one of these :-

https://www.ebay.co.uk/itm/Turnbuckle-/321074997250
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Currently : Royal Enfield 350 Meteor
Previously : CB100N > CB250RS > XJ900F > GT550 > GPZ750R/1000RX > AJS M16 > R100RT > Bullet 500 > CB500 > LS650P > Bullet Electra X & YBR125 > Bullet 350 "Superstar" & YBR125 Custom > Royal Enfield Classic 500 Despatch Limited Edition (28 of 200) & CB Two-Fifty Nighthawk > ER5
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G
The Voice of Reason



Joined: 02 Feb 2002
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PostPosted: 22:18 - 21 Feb 2013    Post subject: Re: Deliberately bending a bicycle frame? Reply with quote

I was thinking along similar lines, but a bit of threaded rod - maybe two.

A couple of bits of steel to act as large washers; wind them out with bolts. Add some heat with a blow torch maybe.
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Pete.
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PostPosted: 22:28 - 21 Feb 2013    Post subject: Reply with quote

I would not try spreading those tubes so close to that spreader plate. I would either remove the plate and re-weld after easing the tubes out or make a tool to crush the inside of the tubes in whilst preserving the outer wall. Another option is increase the wheelbase slightly by moving the rear wheel back.
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stinkwheel
Bovine Proctologist



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PostPosted: 23:04 - 21 Feb 2013    Post subject: Reply with quote

I've had another engineering type suggest flattening the inner wall of the tube. Presumably this makes structural sense.

Thinking along those lines. If I were to remove that plate, heat things up nice and toasty then slowly hammer a wedge in there from the top then the bottom (I think I've got some oak about somewhere), it ought to both flatten and slightly spread the area in question without disrupting the shape of the outer part of the tube.

I'm not even sure how structural that plate is. Might even be brazed on.

Mind you. I just found an article on Sheldon Browns website where he widens the distance between the dropouts on bicycle frames using a big lump of 2x4 braced against the seatpost. Perhaps I'm being too technical. If I do a similar thing but with a hub bolted in place... Thinking
https://sheldonbrown.com/images/frame-spacing-l.jpg
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.
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G
The Voice of Reason



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PostPosted: 23:07 - 21 Feb 2013    Post subject: Reply with quote

Presumably you could cut the plate, then weld in a bit of metal either in the hole or to replace the whole thing.
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Pete.
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PostPosted: 23:09 - 21 Feb 2013    Post subject: Reply with quote

My experience with such things is that one side will usually give first, and the weakening that initial give creates causes that side to deform more. I would look for a more controlled method.
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132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy
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smegballs
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PostPosted: 01:35 - 22 Feb 2013    Post subject: Reply with quote

stinkwheel wrote:

Mind you. I just found an article on Sheldon Browns website where he widens the distance between the dropouts on bicycle frames using a big lump of 2x4 braced against the seatpost. Perhaps I'm being too technical. If I do a similar thing but with a hub bolted in place... Thinking
https://sheldonbrown.com/images/frame-spacing-l.jpg


I was talking to my cyclist brother about spreading dropouts. I was going on about using a bit of threaded bar and spacers/bolts to give a controlled spread. He the proceeded to call the idea "cute" and said it was generally just done with a big chunk of wood... Shocked
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stinkwheel
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PostPosted: 18:37 - 22 Feb 2013    Post subject: Reply with quote

FOund a couple of bits on the web about this.

Getting a more common thing to do as people build "fat" bikes.

It appears bicycle frame builders are rough as buggery. Looks like it's simply a case of making a dent in the tube where the tyre is fouling by clamping a bit of dowel into it hard or smacking it with a hammer.

The more finessed ones make a longitudinal dent.
https://www.trophyoftexas.com/about/fp313%5B1%5D.jpg

Some even put two dents, one each side of the tube.
https://2.bp.blogspot.com/_F3s93ZJ2Et4/TQb5uFW0ohI/AAAAAAAAANA/nT4qkUwcsSg/s1600/DSCN3735.JPG
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.
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haroman666
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PostPosted: 01:52 - 24 Feb 2013    Post subject: Reply with quote

stinkwheel wrote:
It appears bicycle frame builders are rough as buggery. Looks like it's simply a case of making a dent in the tube where the tyre is fouling by clamping a bit of dowel into it hard or smacking it with a hammer.


Dunno if you've seen this already but even the big MTB bikes that need to be strong get pretty man-handled during manufacture: https://youtu.be/mlIYEdRFQu4?t=10m10s

I guess it's a case of bending as much as you need, without deforming the metal so much it becomes too weak... But I doubt you'd know you'd done that until you're face down in the mud Laughing
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