Bicycle sizing advice
 

Hi there. I'm going to provide some numbers and I'd be very grateful if some of you could consider them and tell me what you think about the kind of frame sizes I "should" be riding?

Or, more specifically, the kind I should be buying - because I have no usable bike at the moment.

I am 6ft (183cm) and my inside leg measurement is 35-36" (89-91.5cm)

I have had three bikes in recent years, all cheap second-hand things which I just tried and decided fit me alright. Their measurements are as follows:

1)
Centre of BB to top of seat tube (flush with top tube) = 24" (61cm)
Top of top tube to the floor, vertically = 33.25" (84.5cm)
This is an old racing bike, very nice to ride and feels like it fits me well.

2)
Centre of BB to top of seat tube (0.5" above the top of the top tube) = 24" (61cm)
This is a Dawes Galaxy from the late 70's most likely, and again it fit me well and in fact I completed 130mi on it two weeks after purchase, with no training (10mi commuting per week) and no experience over 30mi on a bike - it fit well and I had no discomfort.

3)
Centre of BB to top of seat tube (1" above the top of the top tube) = 22" (61cm)
Top of top tube to the floor, vertically = 31.5" (80cm)
This is a junky old hybrid I bought for £10. It fits alright - in the past when I've had to ride this as my primary bike I have got used to it and the others seemed wrong and uncomfortable. And vice versa.


When I was in a bike shop recently I couldn't remember how big my Dawes was, and I said I think maybe 24" - the guy was shocked and said no, no, that must be far too big for me. Perhaps 21" would be more suitable.

Anyway, as I've discovered, bikes with a large BB-to-ground clearance like 3) above can end up with a fairly similar "size" overall to bikes lower to the ground but with a longer seat tube, like 1) and 2).


I will be buying something second hand and may not have a chance to ride it before I pay (there aren't many nice bikes for sale around here). Do you think I'd be safe going for a 24" Galaxy again?

Thanks

p.s. the bike I will be getting will be used for commuting sometimes (a few miles per day), occasional touring and camping trips, and very occasional intercity rides of 100-150mi/day.

p.p.s
I rode 1) and 2) with the seatpost out to a "medium" distance. Medium to low. I haven't measured it.
With 3) I have the saddle much higher, though, it looks very high.

When speaking of traditional steel frames, there are several approaches.

The simplest is the old "straddle the top tube and look for an inch of crotch clearance". This has fallen out of any favor it ever had, but for recreational cyclists it can work just fine.

Research from the late 70s/early 80s developed into a few formulas that are still useful. One is that your actual inseam (measured precisely rather than assumed via pants size) multiplied by .65 gives you your frame size center-to-center. So by the formula, you should be riding a 58 to 59 cm (23 inch) frame. That calculation is for racers, though, and it was/is common for recreational riders and tourists to go a little bigger, to get their handlebars a little higher.

These days, everything is "compact" with sloping top tubes, and everything is much, much smaller. This is doubtless why the shop worker was shocked at your 24" frame and thought you should be riding a tiny little 21" - the assumption these days seems to be that pro racers ride the smallest possible bikes, so everyone else should too. I think a 21" frame is much, much too small for you.

IMO if you are comfortable on a 24" frame then there's no reason to go smaller. The only possible concern you should have is crotch clearance: if you like to jump off the saddle and land with both feet flat on the ground, you should ensure that your naughty bits won't collide with the top tube. If you never jump off the saddle and land with both feet on the ground, then you don't have to worry about crotch clearance.

If I were you I would not go smaller than 23" with a traditional steel frame, and would probably stick with the 24". As a tourist and distance rider, using steel frames, I ride a 24.5" frame - and I am 6'1" with a slightly smaller inseam than you. I just make sure to never jump off the saddle...

In my case, I'm all legs with a short torso, so the top tube length is the most important factor for my bike fit. If I bought bikes based on my inside leg, I'd be stretching for the bars. Seatposts slide up and down, so I completely ignore seat tube length. However, top tubes can't be changed.

So I can cheat a little and use just the top tube measurement. I know I need a top tube (center-to-center) of 565mm, ±5mm (22¼" ±¼"). I take along a tape measure and if the top tube falls between 560mm and 570mm, I know I can make the bike fit. I might need a longer seatpost and a few spacers under the stem, but I can make it fit.

BTW, this is for traditional level top tubes. On a sloping top tube I use the "virtual" or "effective" length by holding the tape measure level and measuring back to the center of the seatpost, looking for the same 565mm.

IMO your inseam will tell the tale for proper bike size.

If ya bust the "boys" gettin' on/off the bike it's to big for ya!

I'm confused. #1 would appear to have a higher BB. Maybe you meant top tube clearance instead.

I doubt Dawes Galaxies changed much, just be careful of the Dawes Galaxy that Bikes Direct sold up until a couple of months ago, they might be different from the ones of the 70s and 80s.

There's also a current Dawes Galaxy available in the UK, and I assume parts of Europe (?) that may be slightly different from ye olde ones.

Go to the Rivendell site and read what Grant Petersen has to say about sizing; www.rivbike.com . I think one of his observations was that 95% of cyclists are riding improperly sized bikes.

rtciv, Measurment to the top of the seat tube don't help much, sorry. I have a 33.5" inseam and presently ride a 56 cm roadie and a 23" (~58.5 cm) touring bike. Top tube length is a more critical measurement IME than standover and my two are within 1/2".

You're probably fine with something in the 58-62 cm range, but hands-on testing and an assesment by someone knowledgeable is the best route.

Brad

Test ride stuff, go to bike shops, then the numbers have a meaning from your experience.

Thanks for the replies, everyone.

I still haven't found a bike... I didn't realise at the time how incredibly lucky I was to find my Dawes Galaxy for £70 just a few years ago! Deals like that don't come around very often.

I am reluctant to spent £1000 on a new bike when I know there are thousands of great machines out there somewhere, which I could have for £50-100. But how do we find them? Hmm

Good observation about BB height affecting the fit. Measured by the seat tube, my Bianchi is 22" and the Diamondback is 20.5", but since the latter's bottom bracket is higher from the ground, both bikes have about the same standover height.

BTW, I'm 5'8". So it definitely surprises me that your bike shop guy wanted to put you on a smaller bike than mine! :lol: