Frame not straight/symetrical?/does it matter?

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

Postby rogerzilla » 14 Sep 2020, 8:24pm

4mm at the rear triangle was what Cycling Plus used to call "acceptable" in tests. Do they still do that, or do they just waffle meaningless superlatives and fighter plane metaphors for a bunch of carbon frames out of the same mould in Shanghai?

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

Postby Valbrona » 15 Sep 2020, 1:23am

If you are stuck with a carbon frame that has misaligned dropouts the manufacturer will mostly always come back and say 'it is within acceptable limits' or 'that is the best we produce them to'.

An American guy was selling little facing devices, like a cutter revolving around a type of thru-axle or similar. I believe you could have some success with these and manage to get dropouts straight and parallel to one another.
I should coco.

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

Postby 531colin » 16 Sep 2020, 9:07am

rogerzilla wrote:4mm at the rear triangle was what Cycling Plus used to call "acceptable" in tests. Do they still do that, or do they just waffle meaningless superlatives and fighter plane metaphors for a bunch of carbon frames out of the same mould in Shanghai?

OK, so lets say one dropout is 4mm higher than the other. The top (or bottom) of the wheel will be about 10mm left or right of where it should be.
Acceptable?

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

Postby 531colin » 16 Sep 2020, 9:11am

Valbrona wrote:If you are stuck with a carbon frame that has misaligned dropouts the manufacturer will mostly always come back and say 'it is within acceptable limits' or 'that is the best we produce them to'.

An American guy was selling little facing devices, like a cutter revolving around a type of thru-axle or similar. I believe you could have some success with these and manage to get dropouts straight and parallel to one another.

Its good practice to face (ISO) disc caliper mounts so they are square to the axle. Facing tools available from Park etc.
Good luck with "facing" a dropout thats an apparently "acceptable" 4mm out.

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

Postby rogerzilla » 16 Sep 2020, 9:22am

531colin wrote:
rogerzilla wrote:4mm at the rear triangle was what Cycling Plus used to call "acceptable" in tests. Do they still do that, or do they just waffle meaningless superlatives and fighter plane metaphors for a bunch of carbon frames out of the same mould in Shanghai?

OK, so lets say one dropout is 4mm higher than the other. The top (or bottom) of the wheel will be about 10mm left or right of where it should be.
Acceptable?

I believe they were talking about lateral inaccuracy. It would take a real muppet of a framebuilder to get the stays different lengths (ok, those muppets are out there) but getting the rear triangle out of whack is very easy. Luckily, on steel frames, you can reset much worse than 4mm.

It's also possible to put the BB shell in at a skew, or to get the front triangle misaligned, but this is less common due to the lugs and you can't readily straighten it later.

I reckon half of the steel forks I've had have come with some misalignment, either one blade leading the other, asymmetrically spaced, or dropout slots at slightly different heights. I fixed one that had all three faults at the same time!

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

Postby 531colin » 16 Sep 2020, 9:33am

4mm lateral offset will give a bike which won't ride no hands properly.

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

Postby rogerzilla » 16 Sep 2020, 10:58am

531colin wrote:4mm lateral offset will give a bike which won't ride no hands properly.

Well, it depends how good you are at that sort of thing! The only bikes I find unrideable "no hands" are those with very little trail, e.g. Bromptons, some expedition tourers and my Harry Quinn track bike with its 50mm fork offset. Someone wanted "twitchy" there!

In an ideal world, bikes would be perfectly straight, and a decent Taiwanese carbon frame mostly will be. I haven't known many steel frames that were really straight. As said above, it really shows up when you try to build a bike with a straight chainline and find that it doesn't work with supposedly matched components. You measure a Campag track 42.5mm chainline at the chainring and a 42.5mm chainline at the sprocket, but your straight edge (which never lies) shows the ring is 4mm outboard of the sprocket.

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

Postby 531colin » 16 Sep 2020, 12:08pm

rogerzilla wrote:
531colin wrote:4mm lateral offset will give a bike which won't ride no hands properly.

Well, it depends how good you are at that sort of thing! .......

Nonsense. However "good" you think you are, a bike where the wheels don't line up will never ride properly no hands.
Read what I wrote here.
531colin wrote:
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.)

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

Postby rogerzilla » 16 Sep 2020, 2:23pm

You're entitled to your opinion :D

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

Postby Sweep » 16 Sep 2020, 3:56pm

531colin wrote:4mm lateral offset will give a bike which won't ride no hands properly.

How much does this matter colin?
I haven't ridden without hands since I was a kid.
Or do you relate it to how well the bike rides with hands?
Will return with more info on my bike when I return to a keyboard
Sweep

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

Postby 531colin » 16 Sep 2020, 4:41pm

If the thing is wonky/bent/out of track, then something is WRONG.
If you don't know exactly what is wrong, you could be riding a frame that is, for example, cracked. I don't want to do that, personally.
Recently somebody on here was telling us about a fork where one blade had never been brazed into the crown...I don't want to ride one, thanks.
You can get used to all sorts of things....bent pedal spindles, handlebars at a funny angle, saddle pointing to one side.
You can get used to a bike which doesn't go straight if left to its own devices, or a bike which is more keen to turn one way than the other; but I don't want to. You can get used to steering flop, where once you turn the steering away from "straight ahead" it wants to turn tighter all the time.
Any of these things will almost certainly mean you are wasting some energy somewhere, but other than that it might be just a minor inconvenience .....or it could mean the thing is only held together by the paint.

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

Postby Valbrona » 16 Sep 2020, 10:13pm

If a rear wheel is out 4mm at the chainstays, the misalignment of the dropouts is much less. And this because of the magnifying effect of the wheel.
I should coco.

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

Postby 531colin » 17 Sep 2020, 7:21am

Valbrona wrote:If a rear wheel is out 4mm at the chainstays, the misalignment of the dropouts is much less. And this because of the magnifying effect of the wheel.

Sometimes yes, sometimes no.
The front wheel, frame, and back wheel should line up. Lets call this line the bike axis.
If the back wheel is vertical, 4mm out at the chainstays and at an angle to the bike axis then one dropout is in front of the other; and as you say by a lot less than 4mm.
If one dropout is higher than the other the back wheel won't be vertical.
If the back wheel is vertical, 4mm out at the chainstays and parallel to the bike axis then the dropouts are 4mm out laterally.
Other combinations are available.