Mavic GP4 ERD help needed!

[foz]

So I'm measuring up for my next wheel build (GP4 rims on record hubs), and I've come across a problem...
I've measured both of my rims in 3 different places, and come up with the same ERD of 618mm at each place. But all the sources I can find online say that GP4s have an ERD of 615mm.
I'm using the exact same measuring technique that I always use, and that always gives me numbers that coincide with what I find online. So either I'm doing something wrong this time or my rims are larger than normal!

These are the second generation GP4s, purple/yellow label, but the shallower version with the narrow brake track.

Can anyone confirm or deny my measurements? I think I'll take them down to the LBS this afternoon and have them measure them too, see what they say.

I can't order spokes until I get this sorted out, as between 618 and 615 there's a 2mm difference in the spoke lengths required!

cheers, Andy

 

[orbeas]

GP4

According to Spocalc website
Gp4 first model is 615mm
GP4 second model is 612 mm
Mavic GP4 new angular (ERD is Mavic's Nipple Seat Dia + 3mm for nipples)
Hope this helps

 

[foz]

Thanks. I have the first version with the purple/yellow label, which is slightly shallower than the second version with the same label. Everything I've seen says 615mm, but all my measurements say 618mm... I'll take them down to the LBS on monday and see what they measure them as.

cheers, Andy

 

[Holiocornolio]

Re:

I've just taken delivery of some later,deep profile brake track GP4 and was looking for guidance on erd. Came across this thread. Surely an erd of 618mm on an ertro 622 mm rim would have to mean essentially a flat tyre bed and 2mm rim depth? That can't be right can it? Tyre bed is definitely a little bit concave.

 

[jim haseltine]

With sprint rims the ERTRO 622 is the tyre bed. Just measured a 2nd gen GP4 (purple/yellow label, bought in 1999). Overall diameter of the rim is 632 and the tyre bed is 5mm below that so 622mm (632 - 2*5) bed as it should be - note that this is the diameter at the deepest part of tyre seat, not the top edges of the eyelet bells, that's another couple of mm. The end of a spoke nipple is 5mm below the tyre bed (approx 6mm below the top of the eyelet bell), so ERD from my calculations is 612mm.

Spoke length calculations have been doing my head for months so I've given in and invested in a threading tool.

 

[Holiocornolio]

Re:

Thanks. I'm going to thread a couple of spokes up later and measure that way. Will post findings.

 

[Holiocornolio]

Re:

So my purple and yellow label,deep brake track GP4s are measuring Bob on 615mm ERD. They weigh in at 480ish grams each., So a bit portly by modern standards. When laced onto the hubset I have (DA7400) these will be the heaviest but costliest wheels I own. Thank god they'll look fast at least.

 

[foz]

Re:

Wow, is it really 9 years since I built mine? I haven't used them much due to a general lack of riding but they're built on record 10 speed hubs and have veloflex pave 24mm tubs, they're great wheels, especially when the road surface is anything less than smooth. If you use them as intended, you'll be glad they're a little on the heavy side!

 

[Holiocornolio]

Re: Re:

Wow, is it really 9 years since I built mine? I haven't used them much due to a general lack of riding but they're built on record 10 speed hubs and have veloflex pave 24mm tubs, they're great wheels, especially when the road surface is anything less than smooth. If you use them as intended, you'll be glad they're a little on the heavy side!

Sorry for bringing your post back from the dead. Can you recall what spokes you used in the end? I'm trying to decide between Sapim Race all round or a mix of race and d light. They're long spokes and I want wheels that are comfortable but not too flexy.

 

[foz]

I'm pretty certain they'll be DT competition - they're usually what I use. Good all round combination of strength, weight and cost.