10 speed cassette with 8 speed mech?

So...

This week I cold-set my touring frame from 126mm to 135mm, as my wheels were knackered and I fancied upgrading from 8 speed to 10 speed.
As far as I can tell the operation was a success (from measuring the new spacing / tracking with Sheldon's basic ruler and string method)

I have a new 10 speed rear hub with a 10 speed cassette (all shimano Deore LX)... but am still using the old 8 speed Deore rear mech and old friction shifters.

I can't get the rear mech to reach the biggest sprockets on the cassette... it goes to about the 8th or 9th but then stops. I have tried the limiter screws but it didn't make any difference.

My question is:
Will getting a 10 speed rear derailleur fix this?

ie: does a 10 speed rear mech simply move a few mm more than the 8 speed ones? Or should this combo work anyway, and therefore it's the tracking or something else that's wrong?

Any ideas appreciated...
Cheers!

The overall width of an 8 and a 10 speed cassette are very very similar. I would look elsewhere other than the derailleur itself.
10 speed is 37.2 mm and 8 speed 35.4 so only 1.8mm difference. (the width of an 8 speed sprocket).

Is the rear mech reaching the limit screw? If not, then the cable may be too slack and the shifter reaching the end of its travel before the rear mech reaches the screw. Try moving the rear mech by hand to see if it will reach the limit screw and engage the lowest sprocket(s)

Cheers - the extra distance I need is more like 10mm so I maybe it's just the frame.
So (theoretically at least) the old mech should pretty much cover the distance of the new cassette?

Yes - butting right up against it at its most generous setting in that direction... nowhere else to move... guessing this is bad news for my cold-setting skills?

you could try disconnecting the cable and see if you push the mech over to below the largest sprocket.

Cheers, have just tried that but it's still stopping short...

Thanks for your help everyone, I'm coming to the disappointing conclusion that it must be the tracking after all... so here I go again, pulling everything apart and attacking my frame with a plank.

Remember, kids: measure twice, cut once.

Before you do that, I would check that the derailleur has the 40mm odd of movement you need.

You may get away with shimming it.

It's probably/more likely just a matter of setting it up right.

I can't see how making the frame wider will have any effect at all on the travel of the derailleur.

Any joy?

I can't see how making the frame wider will have any effect at all on the travel of the derailleur.

It's not the spacing I'm talking about but the tracking. At the moment the wheel is sitting too far towards the non drive side of the frame. My thinking before was that if I could sort the tracking out so the wheel was perfectly central in the frame then the RD would be able to reach further across.... but of course the RD would have moved as well so would presumably have the same limited reach....?

I should sort the track out anyway, just so it rides better... but I think you're right - I can't see that it will make any difference to the RD problem.

I haven't had time to try anything this week but will update as and when.... can't believe people are actually interested in this! Thanks @Dogtemple & @DHD

A photo of the rear derailleur and cassette in the frame may help.

Fwiw I'm using a '8-speed' xt mech with 10 speed STIs and no issues at all. Old 8 speed mechs are cheap, you could try another one to see if yours is just faulty for some reason?