Dura Ace 7400 Chain Alignment

[elite504]

Re:

I'm leaning towards bent crank arm. Rear triangle would have to be out massively for interference such as that.
Large-n-fromage suggestion is right though, or, if you have access to a long straight edge, put it so it touches the head tube and the seat tube on the drive side, and measure the distance bwtween the dropout and the straight edge,
Now do the same for the non drive side, and it should be the same. or a couple of mm out max.
Large's method is detailed at the bottom here:
http://www.sheldonbrown.com/frame-spacing.html

If the rear triangle is out, with a correctly dished rear wheel, there is usually a fair abount of misalignment of the tyre under the brake bridge (not centred) or not centred in the chain stays.

I guess checking the crank arm, can you get a straight edge across the chainring face, and kid of eyeball the trajectory of the chain, and see if it looks like it's going to hit.

With a correctly aligned rear triangle, your chainline (dictated by the BB axle) should be at 43.5mm thus:
http://www.sheldonbrown.com/chainline.html

But if you go to a narrower BB axle as would typically be required for DA7400, your line will be worse...

 

[cce]

Re: Re:

Thanks so much for the responses

- will check crank arm isn't bent
- can do a string line to double check the frame is straight too
- OLD is standard 126mm, not cold set. Wasn't an option given the 753 tubing as you said.

Again thanks, can't believe there's this many people who are as enthusiastic as me about old frames

Cheers

but you have an 8 speed hub in there... is this one you've customised?

 

[v11boy]

Re:

Checks so far

- crank isn't bent
- chain line is in keeping with Sheldons guide 43.5mm
- 8 speed is an 'upgrade' from 7 so it could simply be this? I was under the impression that they could be swapped although I was only looking at the spacing of the rear stays!

Need to check the frame still when I get a bit of time

Thanks again for all the assistance

Dan

 

[v11boy]

Re:

Putting a spacer on the pedal for the time being, will see how it runs..

 

[Robster914]

Re:

Hi Dan, I am running a near identical set up on my perthus. 8 speed shimano 600 tricolor hubs. Only difference is I have Shimano 600 cranks. No clearance prob at all. It may be a trick of the photos but the drive side crank looks bent. I have checked one of my bikes with Dura ace 7400 crank and it should be straight if you lay a ruler from centre of bottom bracket to pedal on the outside of the crank arm. If your crank isn't bent then can only be the frame.

 

[cce]

Re:

How does it behave with an unmodified 7 speed wheel in there? Looking again at the pics, the small sprocket looks pretty close to the dropout.

 

[jackosbournesnr]

Re: Re:

Putting a spacer on the pedal for the time being, will see how it runs..

Should be okay, but the Q factor geeks will go mental

I'm still convinced there's an issue with the bottom bracket tapers and having just done a bit more research on your behalf (don't worry, I won't charge) it does seem to be a known issue with earlier 7400 chainsets when mating them with cartridge bottom brackets.

This and a 130 OLD hub horsed into a 126 back end will easily give you this issue.

Is it a 130 hub?

 

[Robster914]

Re:

Hi, have just hopped in garage to re-look at mine.
8 speed same as yours.
OLD 126mm for rear hub
Shimano 600 cranks with original bottom bracket.
170mm crank length
About 7mm clearance as crank passes chain in top gear..

Something peculiar going on with yours I'm afraid, I hope it is a cheap fix
Regards
Rob

 

[v11boy]

Re:

Hello,

The hub is 126mm, I've previously used a 130mm hub on an older Peugeot frame (not a 753 by the way, a cheapo one) and ended up breaking the stay off the bottom bracket so I'm careful with hub spacings now!!

The drive side crank arm isn't straight if I put a straight edge from the centre out toward the pedal...

When I looked I ran a straight edge against the flat part of the outer chain ring to get a kind of datum and the end of the arm looked like it was following that same angle.. now you've said your 600 crank is totally flat i will double check what I did was correct.

If it is a bent arm I don't fancy attempting to bend it back, cast aluminium which is 30 years old isn't going to behave well. I may just have to live with it or get a new crank if that's the case.

Thanks again all

 

[v11boy]

Re:

Hi Rob
7mm seems a lot compared to this! Maybe the arm is bent.... I'll check again. Thanks for looking at it
Dan