Frame not straight/symetrical?/does it matter?

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Sweep
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Frame not straight/symetrical?/does it matter?

Postby Sweep » 12 Sep 2020, 7:00pm

I have noticed lately, basically because I have been fitting wider tyres, 38/40, that on old favourite 90s steel hybrid of mine I have converted to a very decent tourer, that the rear tyre is significantly closer to the chainstay at one side - at the end closest to the seat-tube.

On the bike's drive side a 40 wide tyre is almost (but not quite, and I have no safety concerns) touching the chainstray, whereas on the other side it is quite some distance away.

Does this mean there is something wrong with the frame?

Is it likely to affect riding/handling?

I cannot say that I have noticed anything in the couple of years (I bought it sercond-hand) I have been intermittently riding it, loaded and unloaded.

I have checked a similar bike of mine (same pedigree/manufacturer) but not the same model, and that doesn't have this characteristic.

If it helps folk to help me I can supply measurements.

And pics - though may be difficult to show it in a pic.
Sweep

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Paulatic
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Re: Frame not straight/symetrical?/does it matter?

Postby Paulatic » 12 Sep 2020, 7:21pm

Can it not just be the dishing of the wheel? Needs moving over a bit.
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Sweep
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Re: Frame not straight/symetrical?/does it matter?

Postby Sweep » 12 Sep 2020, 7:28pm

Paulatic wrote:Can it not just be the dishing of the wheel? Needs moving over a bit.


I'm not uber technical and am only (being on the fringes of starting wheelbuilding adventures) just starting to get my head round the concept of dishing.

But the characteristic is evident on two wheels that have been slotted in and were both professionally built - one by Spa, one by Hewitt. Wheels seem essentially true, no issues with brake blocks.
Sweep

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cycleruk
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Re: Frame not straight/symetrical?/does it matter?

Postby cycleruk » 12 Sep 2020, 7:30pm

Paulatic wrote:Can it not just be the dishing of the wheel? Needs moving over a bit.


Yes, initially turn the wheel round in its dropouts to determine it is the frame and not the dishing or even the tyre being off-centre.
You'll never know if you don't try it.

peetee
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Re: Frame not straight/symetrical?/does it matter?

Postby peetee » 12 Sep 2020, 7:33pm

Flip the wheel round and see if the problem is still obvious. If it’s closer to the opposite stay the wheel needs re-dishing. If it’s still as it was it’s the frame... assuming you have vertical dropouts.
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cycleruk
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Re: Frame not straight/symetrical?/does it matter?

Postby cycleruk » 12 Sep 2020, 7:46pm

Sweep wrote:I have noticed lately, basically because I have been fitting wider tyres, 38/40, that on old favourite 90s steel hybrid of mine I have converted to a very decent tourer, that the rear tyre is significantly closer to the chainstay at one side - at the end closest to the seat-tube.

You mention chainstay but how does the wheel sit between the "seatstays" at the brake bridge.?
You'll never know if you don't try it.

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Sweep
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Re: Frame not straight/symetrical?/does it matter?

Postby Sweep » 12 Sep 2020, 7:53pm

cycleruk wrote:
Sweep wrote:I have noticed lately, basically because I have been fitting wider tyres, 38/40, that on old favourite 90s steel hybrid of mine I have converted to a very decent tourer, that the rear tyre is significantly closer to the chainstay at one side - at the end closest to the seat-tube.

You mention chainstay but how does the wheel sit between the "seatstays" at the brake bridge.?


thank as ever folks for the very helpful (and impressively fast) as always help- keep them coming - will do some more checking (and add measuring) tomorrow. Will also try some pics, though fear will be hard to get across in a pic.

Bike departs for a mini tour(3 days) next week.
Sweep

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531colin
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Re: Frame not straight/symetrical?/does it matter?

Postby 531colin » 12 Sep 2020, 9:07pm

Heres how to tell if your frame is straight using a bit of string....https://forum.cyclinguk.org/viewtopic.php?f=1&t=59332&hilit=string
I think its worth doing the whole procedure from front to back, then you know exactly where you are.
I think there is a hierarchy of "does it matter or not".
If the frame is structurally unsound (cracked) it matters.
The front tyre contact patch needs to be pretty accurately lined up with the headset bearing. If it isn't the bike will steer oddly, particularly no hands.
Its fairly important that the front and rear tyre contact patches line up, and that this line passes directly underneath the middle of the bottom bracket. For example, if the front contact patch and bottom bracket line up, but the rear contact patch is parallel to that line but 4mm to one side, you will have a bike which will only ride straight no hands if you ride with the saddle under one cheek. If the front contact patch and bottom bracket line up but the rear contact patch is at an angle to that line, you might have rather strange handling.
If the front and rear contact patches line up with the bottom bracket but the wheel isn't exactly in the middle of the seatstays, then I'm prepared to forgive that with an old frame that is otherwise OK, as long as I can set the brakes up. (Not a new frame, obviously!)

As somebody already said, if the rear contact patch is parallel to a line between the front contact patch and the bottom bracket, this can be corrected by altering the wheel dish; the alternative is to file the dropouts. The choice is made on the basis of where the wheel lies between the seatstays.

gregoryoftours
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Re: Frame not straight/symetrical?/does it matter?

Postby gregoryoftours » 12 Sep 2020, 10:12pm

I have a bike with the same issue. The rear wheel is dished correctly, vertical dropouts, even gaps at the seat stays but uneven at the chainstays. It doesn't noticeably affect handling but I've wondered if it might cause the rear tyre to wear faster.

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531colin
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Re: Frame not straight/symetrical?/does it matter?

Postby 531colin » 13 Sep 2020, 9:15am

gregoryoftours wrote:I have a bike with the same issue. The rear wheel is dished correctly, vertical dropouts, even gaps at the seat stays but uneven at the chainstays. It doesn't noticeably affect handling but I've wondered if it might cause the rear tyre to wear faster.

When I have managed to put a bit of wear on a (rear) tyre without disturbing it for puncture repair or whatever, it always seems to me that the wear pattern matches what I would expect from the road camber.
Its pretty easy to find out exactly whats wrong with your frame, as above.

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Sweep
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Re: Frame not straight/symetrical?/does it matter?

Postby Sweep » 13 Sep 2020, 10:24am

Many thanks for the replies folks, particularly colin with his expertise.

Am about to leave on the bike for a few days.

On my return will provide more info and measurements, carry out some of the checks reccommended and in particular Colin's string experiment and then report back.
Sweep

rogerzilla
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Re: Frame not straight/symetrical?/does it matter?

Postby rogerzilla » 13 Sep 2020, 2:18pm

Most steel frames are a bit out. 4mm is usually considered acceptable by manufacturers. It only becomes a problem when you go and buy a nice track groupset of matched components, and the chainline is atrocious. Fixies, singlespeeds and hub geared bikes show up the problem.

Valbrona
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Re: Frame not straight/symetrical?/does it matter?

Postby Valbrona » 13 Sep 2020, 5:20pm

The dropouts are most probably not aligned correctly. Dropout alignment gauges are used to both check alignment and then correct any problems.
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531colin
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Re: Frame not straight/symetrical?/does it matter?

Postby 531colin » 14 Sep 2020, 6:58pm

rogerzilla wrote:Most steel frames are a bit out. 4mm is usually considered acceptable by manufacturers. It only becomes a problem when you go and buy a nice track groupset of matched components, and the chainline is atrocious. Fixies, singlespeeds and hub geared bikes show up the problem.

4mm is an interesting number. 20-odd years ago Orbit made several bikes with the right rear dropouts deliberately offset 4mm to the right. This gives you a very worthwhile reduction in the dish of the rear wheel, with spoke tension more even on the different sides of the wheel. Its also how I know that having the rear wheel 4mm offset from the axis of the bike means that you can only ride no hands if you put the saddle under one cheek; because thats what happens if you fit a regular wheel to an offset Orbit, or an offset Orbit wheel to a regular frame.
(People get strangely anxious about frames with offset back ends; its all quite straightforward, really. You can either build the wheel in the frame, or using a dishing stick and an 8mm spacer. (8mm spacer for 4mm offset). If you use the big ring of a triple much, the chainline is actually better. If the dropouts were to be offset 4mm to the left, it would be "interesting" trying to build a 135mm 9 speed wheel; the driveside spokes wouldn't have much of a bracing angle.)

AndyA
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Re: Frame not straight/symetrical?/does it matter?

Postby AndyA » 14 Sep 2020, 8:05pm

Sweep wrote:On the bike's drive side a 40 wide tyre is almost (but not quite, and I have no safety concerns) touching the chainstray, whereas on the other side it is quite some distance away.


It's very easy to damage a frame from the tyre rubbing a chainstay. If a spoke snaps or your wheel is buckled while on tour then you'd have to stop and repair it immediately or you risk writing off your frame.
If you've got less than 5mm between the tyre and chainstay then I'd fit smaller tyres. You can use a 5mm allen key to check, I often use allen keys as extra large feeler gauges to check things like this.

Onto your frame, there are quite a few possibilities. The chainstay could have been joined to the bb closer to the centreline than the other, the chainstay could have a slightly different bend to the other, the chainstays could be a slightly different length. The first two you can't do anything about but the third can be sorted by filing the inside of the left dropout. This is quite hard to get right!