Noob Hello and Q on Retro MTB Sizing

Hi Folks,

Newbie here based up in the north of Scotland, planning a retro project in the next few months (just building up the "War Chest" at present). Have spent far too much time in the last few days surfing round on eBay and Gumtree, my head hurts and my S.O. is not very impressed.

I have a question about sizing. At what point in the 90s did mountain bike sizing start to diverge from the road stuff? I see some early MTB specs with like a 21" / 54cm SP classed as medium?!!

I have a 2000s era GT Avalanche which I love, but at 18" is probably on the limit for me (5'7", 30" trouser leg). It's comfortable for long or urban rides, but not great on the technical stuff where ability runs out and the bawsack-TT interface results.

My project bike will likely be used for work commutes, fire road jaunts etc. so I think I'd size it similar to the Avalanche. In fact I'd really like a GT of some sort, rigid fork, rim brakes... (any advice there? not too clued up on the lineage, but assuming a Zaskar would be a bit OTT!).

Any advice appreciated.

I'm off to rake around in the For Sale section now! :mrgreen:

Try a 16" GT frame. They are always bigger frames than other manufacturers.

It's always differed from the road stuff. In order to avoid catastrophic bollock/crossbar incompatibility incidents MTB s have always been a little lower at the crossbar (or virtual crossbar for sloping tube designs) than their road brethren. The big difference was that not every punter or bike shop fully realised this back in the day, and it probably wasn't until the nineties that the message had finally sunk in.

Check the top tube length rather than the seat tube length.

Find a bike you feel comfortable with and measure the horizontal effective top tube. Do the same with any prospective purchase!

Effective horizontal top tube length is the only meaningful measurement, seat tube length is just crotch clearance!

Its a difficult one as some frames have higher bottom brackets and some measure centre of bottom bracket to centre top tube and others to the top of the seat tube! As a broad guess I would say a frame that measures 17.5"-18" to the top of the seat tube will probably be in the right ball park (unintended "ball" pun!). I would go for the smallest frame you can get away with and use the seatpost to get the optimum fit. Top tube length is a start also but remember you can also change the stem length to suit your arm length or desired ride position. In general I have found "retro" bikes to have a more "head down, arse up" position and bikes post 2000 offer a more upright ride position in general especially full suspension bikes.

Hope this helps and welcome...if you think you are spending a lot of time online now...just wait till the retro bug takes full hold!

All the best

Doug

was8v":2ud5dlrj said:

Effective horizontal top tube length is the only meaningful measurement, seat tube length is just crotch clearance!

Click to expand...

im a smidgen under 6'4" and trust me when I say it ain't the top tube length that prevets me extending me pins and stops me riding a 17.5" frame.

I'm 6'4 and ride 19-20" sized frames. But I have quite long legs so the toptube/ballbag interface isn't so much of a problem.

I can barely ride a 19". I could just manage it, but its too cramped and to get the seat high enough means yards of exposed seat tube and broken frames from my 250lbs levering on it.

Logic.Al":26rygdz1 said:

I'm 6'4 and ride 19-20" sized frames. But I have quite long legs so the toptube/ballbag interface isn't so much of a problem.

Click to expand...

I'm 5'10" and also ride 19-20" frames, I have an 18" I'm riding now but find it a bit too small after a bit.
All measure CTT of course.


Which means it's up to the individual, as a guide though for mid 90's to late 90's the Kona one is pretty good.


If you want more debate on anything size wise, a search will bring up many threads.

(and general frame dimensions they base it on CTT sizing and effective top tube lengths. Inseam is actual, not trouser. So I would say a Kona 17"

Chopper1192":36jq2fib said:

The big difference was that not every punter or bike shop fully realised this back in the day, and it probably wasn't until the nineties that the message had finally sunk in.

Click to expand...

You'd think so, but the "high end" bikeshop round here sold a bloke I met last night a hardtail at least 1 size, probably 2 too big for him. About 3 inches of seatpost only! The guy said it was fine on roads, but too big offroad.

I'm 6'1 and ride 19" centre to top