Frame alignment ... how much tolerance?

[PedallingNowhereSlowly]

Spend some time trying to work out whether the 2009 Specialized Sirrus Comp I picked up is worth saving, considering it needs lots of new parts.

I checked the frame alignment using the string method (run from one rear drop out, around the head tube and back to the other rear drop out).

It measures 38mm between the string and the seat tube on the drive side and 43.5mm (5.5mm difference) on the non-drive side.

I re-tied the string as tight as I could and re-checked about three times and got consistent measurements. And I then repeated the process, running the string through lugs that go through the lower part of the seat stays, eliminated the dropouts themselves.

Results consistent. The rear drop outs OLD of 135mm is spot on, so I don't know whether these frames are meant to be slightly asymetric, or whether both sets of stays are out of alignment. Does it matter if they are off by 2.75mm? Got to make the decision on whether I scrap it or fire the parts cannon and bring it back to life. Given what it needs, it's marginal. Thoughts?

 

[biggs682]

Apart from the worn out parts does it ride straight and true ?

 

[PedallingNowhereSlowly]

Bike wasn't rideable when I collected it. The only way to find that out would be to build it up and see.
I know some bikes/most bikes are never that well aligned from the factory, and I've read about tolerances of 2mm.

I think, having slept on it, this one isn't worth risking. Not unless I keep in mind I may end up hunting out another frame at the end of the process.

 

[Chris S]

I know it's a very low bar but Raleigh frames and forks used to be straightened by eye after welding. 2.75mm would have been well within the tolerance.

 

[PpPete]

I know it's a very low bar but Raleigh frames and forks used to be straightened by eye after welding. 2.75mm would have been well within the tolerance.

Likely with the steel frames....

But think the Cirrus is aluminium so bending is "not" recommended.

 

[silva]

One would expect that frame welders as a all day job for the same (set of) design(s) have purpose-built and checked as aligned constructions that allow to fix the tubes in the right positions before welding ends together, so measuring everytime not needed?

About that OLD (135 mm), question, with my earlier bikes, I had to pull the forks a tiny bit away from eachother in order to get a rear wheel in.
With the original frame of my avatar bike, I could just drop (bike upside down) the wheel between the forks and tensioning the bolts moved these abit together.
This frame broke behind the bottom bracket:

As extra info: that tube has a frame slot with 4 sunk head stainless bolts in it, in order to pass a belt in the case of a belt drivetrain instead of chain - it has never been used, though the bolts worked themselve loose, what I solved by putting silicone on the threads and after tensioning around the heads in order to avoid galvanic corrosion, and since they didn' loose anymore.

I got a replacement frame, and now getting the wheel in is like with earlier bikes - pulling forks away.
So it looks like the forks of the original frame where welded more than 135 mm away and tensioning wheels "bended" tube(s) towards 135 mm, which might have cause metal stress that after years riding developed a crack that then grew further.
All theory but it's the only explanation I can think of.
Question is what is the "common" case, with my earlier 4 bike frames and the current replaced, getting wheel in = pulling alittle bit open, with the broken frame, that was not, and never, needed.

 

[PpPete]

One would expect that frame welders as a all day job for the same (set of) design(s) have purpose-built and checked as aligned constructions that allow to fix the tubes in the right positions before welding ends together, so measuring everytime not needed?

Regrettably "what one would expect" from Specialized is not always delivered. I once spent hours trying to sort out a Specialized wheel that kept on losing spoke tension and going out of true. After much wasted time and many expletives I found they had used a rim with natural "pringle" to it, and horsed it straight with massively unequal spoke tensions before delivery.

 

[sevenfourate]

One would expect that frame welders as a all day job for the same (set of) design(s) have purpose-built and checked as aligned constructions that allow to fix the tubes in the right positions before welding ends together, so measuring everytime not needed?

Heat distortion. Which can vary greatly from Welding a number of supposed identical components. Measured, clamped use of a manufacturing jig included or not.

Often expected and normally manipulated one way or another post welding. But if noted frame welder isn’t tasked with, doesn’t have time for - or gets no extra pay to straighten now heat affected frame:

Then customer gets whatever comes out the jig…..

 

[fritz katzenjammer]

If you can’t see any kinks or cracks in the tubes and it tracks well ride it.

 

[PedallingNowhereSlowly]

If you can’t see any kinks or cracks in the tubes and it tracks well ride it.

I gotta buy a wheelset before I can ride it and see how well it tracks :P

 

[fritz katzenjammer]

I gotta buy a wheelset before I can ride it and see how well it tracks :P

Oops, sorry about that. I’ve got lightly used spares galore from all the stuff I’ve worked on and don’t tend to think of these things.