Frame tracking and rear triangle spacing

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javatime
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Frame tracking and rear triangle spacing

Postby javatime » 13 Oct 2014, 9:20pm

After 30 odd years of fettling bikes I am up against a conundrum.

I recently completed an overhaul of a Dawes Galaxy - the second hand frame of unknown history was new to me, not sure of the age but with loops on the head tube for bar end shifter cable outers I imagine not terribly old although with a narrow rear triangle spacing.

The rear triangle spacing was tight, but I managed to spring in a 130 OLN rear wheel. Therefore no surprises to find on the test run that the frame was not tracked correctly. It didn't have the stability I have experienced with a number of Galaxies.

Back home I used the elastic string from both rear lugs to the head tube to check the tracking and no surprise to find it 10 mm out and rear spacing to be around 125 mm

I used a piece of studding in the rear drop outs and a couple of nuts to get the rear triangle spacing right, and checking the tracking found that has corrected the right way as well, maybe 1- 2 mm out.

But here is the strange thing - although the wheel is centred at the chainstay bridge with the axle right at the back of the lugs, it is probably 10 mm out at the top brake bridge.

I don't understand this.The wheel was built correctly by Spa a while ago, and trying several other wheels I get the same offset ?

I would have thought the dish of a traditional 5 speed block type wheel for which I assume the frame was built, would be the same as a modern 8 speed - i.e. the centreline of the tyre tread is at the centre line of the axle ?

Until I take another ride I don't know if this will affect the ride, but open to any insights or explanations.

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531colin
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Re: Frame tracking and rear triangle spacing

Postby 531colin » 13 Oct 2014, 10:20pm

Image

Check the wheel alignment as in the above photo from here...http://forum.ctc.org.uk/viewtopic.php?f=1&t=59332&hilit=+string

The wheels need to line up at the tyre contact patch, its not as important for the top of the back wheel to be in the middle at the brake bridge, just as long as you can set up the brake OK.
If the rear tyre contact patch is a few mm off to one side, I would expect the bike to steer no hands by putting the saddle under one cheek rather than sitting straight.

Other stuff......conventionally, the rim is equidistant from the left and right dropouts......irrespective of how wide it is.
As to why the wheel is out at the top, I suppose the trite answer is "Something bent"....difficult to be specific......but if the dropout slots are not exactly parallel, for example, then the wheel could be skewed when its at the very ends of the slots. Might be OK to good at mid-slot, for example.

LWaB
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Re: Frame tracking and rear triangle spacing

Postby LWaB » 13 Oct 2014, 11:10pm

It isn't unknown for rear dropouts to be at different elevations, which cocks the wheel at an angle.

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NATURAL ANKLING
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Re: Frame tracking and rear triangle spacing

Postby NATURAL ANKLING » 13 Oct 2014, 11:52pm

Hi,
As I would have to line up wheels on hundreds of motorbikes in the past (actually mostly used the tyres) the only correct way to be more precise is to get the rims parralel which each other, and the centre line of rims to be on the same plane too.
I had no garentee that the steering head was parralel too, only a ride would tell, with out removing forks.

The steering head needs to be on the same plane too.

If these are correct then nothing else matters.

From what you are saying the dropouts are at different heights / frame bent / frame not jigged correctly when made.

Place eight tins of beans on a large kitchen worktop, equi spaced and same pattern both wheels.
Place bike with tyres removed / flat (no air) on the tins touching the rims.
Get some one to hold down rear wheel onto the tins, whilst you attempt to get all tins to touch front rim :?:

This will tell you that the rims are on the same plane and parralel too.

You could hold front wheel onto tins and adjust rear skewer, but if dropouts are different heights then the error will be aparent.

This does not check the steering head and a twisted steering head will give same results as above.

Twisted steering head or malaligned wheels will put the steering head off centre with front tyre patch and will affect stability.
NA Thinks Just End 2 End Return + Bivvy
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NATURAL ANKLING
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Re: Frame tracking and rear triangle spacing

Postby NATURAL ANKLING » 14 Oct 2014, 6:24pm

Hi,
Rims have to be on centre of hubs, i e - if you turn the wheels over in frame / forks, the rims are still in same place equi distance from forks.
If they are not then rims will need trueing.
Ideally you need a dishing tool to check both wheels.
NA Thinks Just End 2 End Return + Bivvy
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.

javatime
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Re: Frame tracking and rear triangle spacing

Postby javatime » 14 Oct 2014, 9:37pm

Many thanks for your suggestions - I do like these pragmatic low tech solutions.

I will also measure the distance from the QR skewer centre to to seat clamp bolt up each seat stay - that should confirm if there is any misalignment there (perhaps one is longer than the other from when the frame was built ?)

Probably not until next weekend, but I will let you know how I get on.

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Lance Dopestrong
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Re: Frame tracking and rear triangle spacing

Postby Lance Dopestrong » 14 Oct 2014, 10:24pm

I had the same on a used steel.mtb frame I bought. Align the wheel properly.at the chainstay bridge and it would be misaligned at the brakes. I checked alignment using the Sheldon Brown method and found both rear triangles were bent off to one side. A bit of careful cold setting cured it.

Scroll down to 'checking alignment (symmetry)'

http://www.sheldonbrown.com/frame-spacing.html
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elPedro666
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Re: Frame tracking and rear triangle spacing

Postby elPedro666 » 15 Oct 2014, 7:07am

NATURAL ANKLING, excuse my lack of imagination (or perhaps excess of it!), but is there any chance of a pic of the bean tin method?

javatime
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Re: Frame tracking and rear triangle spacing

Postby javatime » 15 Oct 2014, 10:08am

Lance I think you have misunderstood. The frame is now tracked correctly (checked using the elasticated string technique) but the wheel is not in the centre of the brake bridge and the wheel is correct.

Brucey
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Re: Frame tracking and rear triangle spacing

Postby Brucey » 15 Oct 2014, 10:15am

elPedro666 wrote:NATURAL ANKLING, excuse my lack of imagination (or perhaps excess of it!), but is there any chance of a pic of the bean tin method?


I think what NA didn't make abundantly clear was that the bike is to be lain on it's side, so that each wheel rim lays sideways on four bean tin tops. (It is a cunning plan but I was wondering what he was on about to start with too!)

cheers
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Brucey
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Re: Frame tracking and rear triangle spacing

Postby Brucey » 15 Oct 2014, 10:20am

javatime wrote:Lance I think you have misunderstood. The frame is now tracked correctly (checked using the elasticated string technique) but the wheel is not in the centre of the brake bridge and the wheel is correct.


Does the wheel sit in the exact same spot when it is reversed in the frame? If so the wheel is correct and the frame is bad.

The seatstays could be curved/ set to one side (I have seen this a few times) or the seat tube isn't parallel to the head tube (again quite common) or the DO's are not set to the same height as one another. More tests required!

BTW to check the head tube/seat tube parallelism, you need to fit a long tube into the head tube and eyeball it, or use an accurate (machinist's) spirit level on the frame. You can get spirit levels that are accurate to ~0.001" per foot.

cheers
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531colin
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Re: Frame tracking and rear triangle spacing

Postby 531colin » 15 Oct 2014, 10:24am

javatime wrote:Lance I think you have misunderstood. The frame is now tracked correctly (checked using the elasticated string technique) but the wheel is not in the centre of the brake bridge and the wheel is correct.


The most important thing is that the rear wheel tyre contact patch follows directly in line behind the front contact patch and the downtube/BB....these should be in line too.
Its not good enough for just the dropouts to be aligned left to right.....if the wheel is off-centre between the seatstays, it is most likely also off- centre at the contact patch as well.....ie the wheel isn't vertical when the frame is vertical, one dropout is higher than the other, or simply at a different angle.

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elPedro666
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Re: Frame tracking and rear triangle spacing

Postby elPedro666 » 15 Oct 2014, 11:07am

Brucey wrote:
elPedro666 wrote:NATURAL ANKLING, excuse my lack of imagination (or perhaps excess of it!), but is there any chance of a pic of the bean tin method?


I think what NA didn't make abundantly clear was that the bike is to be lain on it's side, so that each wheel rim lays sideways on four bean tin tops. (It is a cunning plan but I was wondering what he was on about to start with too!)

cheers


Ahhhh, I can see it now, thanks Brucey I'd concocted all sorts of mental bean tin scaffold towers!

All I need now is to find a flat surface in our wonky old flat...

javatime
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Re: Frame tracking and rear triangle spacing

Postby javatime » 16 Oct 2014, 1:08pm

UPDATE :

Many thanks for all of the help and suggestions.

1. It is definitely the frame, I have checked with the rear wheel around the other way.

2. Using Colins string through the front wheel valve hole, and eyeballing the string over the rear wheel rim seems to show that the tyre contact areas are in line.

BUT :

The RH seat stay is definitely bent from the brake bridge up to the seat clamp cluster. A straight edge shows 6 mm of daylight at the top. If this was straight it would put the brake bridge into the correct central position with respect a centred rear wheel. The LH seat stay has avery small gentle curve as I would expect from the respacing.

The frame must have been like this when I got it, I remember having problems aligning the rear canti brake but did not investigate further at the time, and when I respaced the rear triangle the effect would have been to bend the seat stay in the opposite direction.

I can't think of a way of correcting this using standard shed equipment, whilst also maintaining the current rear triangle spacing and the string check checking that the seat tube remains central.

So I will do the 8 can check as well and see how it rides.

It has been an interesting exercise in mental geometry ! Thanks again Brucey, Colin, NA et al

Brucey
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Re: Frame tracking and rear triangle spacing

Postby Brucey » 18 Oct 2014, 12:17pm

The chainstays (rather than the seatstays) primarily determine the track of the rear end. Often when respacing the seatstays don't move with the chainstays in quite the right way, or can be otherwise damaged anyway.

If you don't mind a (slightly nerve wracking) experiment, you can correct this kind of alignment fault, regardless of its origin.

With a rear wheel fitted, lay the bike on its side with the bulge in the stays upwards. Support the frame beneath the seat pin and the end of the wheel axle on pieces of wood. Use a third piece of wood under the head tube. Pad all the contact points to avoid damaging the frame finish. Now (gently at first, using soft-soled shoes, with protective tape over the stays if necessary), bring your weight to bear onto the uppermost stay immediately above the seatstay brace, using the ball of your foot. You should be able to put a set into the stays without too much difficulty. Obviously a strong touring frame may need all your bodyweight, or even a little bouncing... :shock: but a lightweight frame may move at a fraction of the load. You can check progress by seeing how centred the brake is or by using string taped between the dropouts and the seat pin.

hth,
good luck!

cheers
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