Bike Sizing?

[biggoffy]

I have just had an odd bike buying experience!
The machine was on Ebay, a Specialized road bike and size 58cm (the correct size for me), but in the pictures the head tube looked very long(aka high)!
However the seller was a fairly local cycle retailer so I bid for the bike and won it

To cut a long story short, It turned out it was a 61cm! Therefore much too large.

Turns out that this retailer sized it by measuring the effective top tube length NOT the seat tube length (effective or otherwise) AND he insists that this is, and always has been the correct method! This is a new one on me, I have sized bikes by seat tube length for about 50 years now.

No harm done, the retailer has cancelled the sale.

Here's the question, have I been sizing frames wrongly for all this time, or has it changed recently, if so when did this happen???

Regards, Goffy.

[horizon]

All bike sizing these days is a bit meaningless. ETT measured as if horizontal is good but still requires the seat angle to make sense. Seat tube length is irrelevant if the top tube is sloping.

What seat tube does give you is the exact model in the range but you will need to know the dimensions of that model for it to be any good. I think it would help if manufacturers called their sizes after fruit or pop stars (e.g. apple, Cliff). that way you would know exactly which bike was being talked about and would have to go to the frame geometry chart.

[biggoffy]

I have always understood that the seat tube length, or now the effective seat tube length, was the industry standard by which one chose one's basic frame size?
Specialized still seem to, my size, XL is clearly marked on a sticker on the seat tube as a 58cm and this is the headline measurement on their website when comparing sizes and geometry.

This particular bike had had all such stickers removed by the first owner, leaving the retailer to do his own thing re sizing, which he did by measuring the top tube effective length.

But where did he get this idea from? He was quite adamant that this gas always been the way!

I do note though, that the bike has been relisted on Ebay as a 61cm. If anyone is in the market, it's a beauty, barely been used

[Brucey]

To cut a long story short, It turned out it was a 61cm! Therefore much too large.

Turns out that this retailer sized it by measuring the effective top tube length NOT the seat tube length (effective or otherwise) AND he insists that this is, and always has been the correct method! This is a new one on me, I have sized bikes by seat tube length for about 50 years now....
.

retailer is;
a) a crazy guy in that all manufacturers still specify frame size by some vertical measurement but also
b) quite sensible in that ETT is probably at least as (if not more) useful in terms of fitting bikes to people.

cheers

[LittleGreyCat]

Just came across this thread as I am doing a bit of bike sizing to look at potential new bikes (see Dawes thread).

I have three current bikes in various states of working (one works, one works but doesn't quite fix me despite loads of effort, one fits me like a glove but isn't working).

Seat tube no longer seems to be a reliable measure because with the fashion for sloping top tubes and consequent much longer seat posts the seat tube measurement no longer reflects the actual geometry. Perhaps a virtual seat tube measure ( perhaps 10 cms below saddle height for a road bike, more for a mountain bike) might give a realistic measure.

Real or virtual top tube lengths are still not massively accurate because it depends very much on how far the bars are forward of the headset. Which made me look again and on the three older traditional bikes (including a Dawes hybrid ladies bike) the head tube angle leans backwards so that the cross part of the handlebars sits more or less directly over the bottom bearing below the head tube. However on the more modern one (which doesn't fit me) the head tube is vertical and the bars are then set forward. So in this case the virtual top tube still doesn't give a true geometry. Perhaps the virtual top tube should be from just below the saddle to the cross part of the bars? This would more accurately reflect upper body length.

Noting that traditionally mountain bikes should be sized smaller than road bikes to give agility at the cost of stability in a straight line (I think).

Anyway, some measurements. Hope this tabulates O.K.

Bike.......................................Virtual top.....Bars forward...Together....Seat............Seat to saddle...Together
Rover 3.5 all steel mountain..........58 cms..........+11 cms.........69 cms......58 cms...........+20 cms.........78 cms
Dawes Galaxy (old)......................59 cms...........+9 cms.........68 cms......64 cms...........+10 cms.........74 cms
Scott P4 Sportster Hybrid..............54 cms..........+12 cms.........66 cms......48 cms...........+25 cms.........73 cms

The Scott is a lovely aluminium bike with more gears and better brakes which doesn't fit me. I have tried mightily with much help from this forum but it just doesn't match my body. Looking at the data above it may be a combination of a more vertical geometry and a shorter distance between saddle and bars.

Anyway, I'm not sure that I can accurately translate the sizing of a modern frame into the sizing of my old but comfortable frames, which makes buying on line a little more problematic.

My current thought, for what they are worth, are that sizing should reflect the true reach from saddle to bars, the true distance from bottom bracket to saddle, and possibly the angle between the bottom bracket and the saddle to give an idea of how relaxed the geometry is.

[531colin]

...........Anyway, some measurements. Hope this tabulates O.K.
Bike.......................................Virtual top.....Bars forward...Together....Seat............Seat to saddle...Together
Rover 3.5 all steel mountain..........58 cms..........+11 cms.........69 cms......58 cms...........+20 cms.........78 cms
Dawes Galaxy (old)......................59 cms...........+9 cms.........68 cms......64 cms...........+10 cms.........74 cms
Scott P4 Sportster Hybrid..............54 cms..........+12 cms.........66 cms......48 cms...........+25 cms.........73 cms
...........

So you say you have 3 bikes, and the frame sizes are 58, 64, and 48 cm?
If you will pardon my saying so, that doesn't seem very likely. I am about 5' 10", and I fit about a 54cm; 58 is too big for me, and 64 is about the biggest made, for somebody about 6' 5". On the other hand 48cm would fit somebody around 5' 2"

[LittleGreyCat]

...........Anyway, some measurements. Hope this tabulates O.K.
Bike.......................................Virtual top.....Bars forward...Together....Seat............Seat to saddle...Together
Rover 3.5 all steel mountain..........58 cms..........+11 cms.........69 cms......58 cms...........+20 cms.........78 cms
Dawes Galaxy (old)......................59 cms...........+9 cms.........68 cms......64 cms...........+10 cms.........74 cms
Scott P4 Sportster Hybrid..............54 cms..........+12 cms.........66 cms......48 cms...........+25 cms.........73 cms
...........

So you say you have 3 bikes, and the frame sizes are 58, 64, and 48 cm?
If you will pardon my saying so, that doesn't seem very likely. I am about 5' 10", and I fit about a 54cm; 58 is too big for me, and 64 is about the biggest made, for somebody about 6' 5". On the other hand 48cm would fit somebody around 5' 2"

This is why I included the "real" figures.
I think you make my point very well - if you use the seat tube measurement you get wildly different frame sizes especially with the newer profile frames which have a steeply sloping top tube. This is why they quote a virtual top tube in many cases.
If you look at the figures, the saddle height is much the same for all three - that is, measured from the bottom bracket along the seat tube and the seat post up to the saddle.
The Dawes (64) has a traditional flat top tube. The Scott (48) has a very sloping top tube. However bottom bracket to bum is much the same distance, as is bum to cross part of handle bars. As I noted, the Rover has an under sized frame because (as I understand it) that is the way that you used to size mountain bikes. including more clearance for your nadgers should suddenly find yourself off the saddle with your feet on the floor.

[slowster]

I've lifted the following quote from your Dawes thread, because I think it's more appropriate to comment on it in this thread.

Spa cycles looks interesting.
One of the issues is, though "try before you buy" to see if the frame geometry suits.
If you don't live within striking distance of Harrogate then this might be a bit tricky.
In my case about 230 miles each way.
Much the same issue with buying from Wiggle.

It does look as though one off brands such as Spa and Edinburgh Cycles are fine if you've already seen and ridden one but very complicated if you just want to sit on one and see if it fits.

If anything, I think that that frame sizing is now less complicated than when your Dawes Galaxy was built. The necessity of most frames then being built with horizontal top tubes because of the use of lugs, and the relatively short lengths of the seat tubes available, meant that frames were typically sized in 1/2" or 1cm increments.

Now road frames have typically 3cm increments, because once you no longer have to build with a horizontal top tube and short length of exposed seat post, the same frame will be suitable for a much wider range of heights/body dimensions, with the adjustment between different people provided by varying stem length, height of spacers under stem, handlebar width and saddle height/setback.

That makes it very simple when I look at Spa's range, because I know for certain that I would need a 54cm. 51cm would be too small and 57cm too large, and there are no intermediate sizes to confuse me.

So unless you have unusual body proportions, I would be a lot more comfortable nowadays about buying a frame/bike online.

If by frame geometry you mean handling, and especially as a result of the head angle and fork offset, that's different, but even so I would be very comfortable about expecting the handling of an out and out tourer from Spa and Thorn etc. to be appropriate for a touring bike. In fact it is the mass market brands like Dawes that I would careful about: to maximise efficiencies of scale and reduce costs they might use the same fork for their touring bikes and their more racey bikes, especially at the lower price points.

[Paulatic]

I can’t help but think you might be making life hard for yourself with all these 'real' figures. It’s certainly doing my head in trying to follow it all
The BB is a fixed point and can’t be moved. Starting there how much set back and how high is the saddle. So now you’ve fixed the saddle position what’s the hight differential and distance between the point where you put your hands.
So any bike you are looking at can those two moving points be easily put into that same configuration? Myself I always, rightly or wrongly look at the wheelbase as well I think all my bikes are within an inch and I believe give a similar handling experience.

[LittleGreyCat]

I can’t help but think you might be making life hard for yourself with all these 'real' figures. It’s certainly doing my head in trying to follow it all
The BB is a fixed point and can’t be moved. Starting there how much set back and how high is the saddle. So now you’ve fixed the saddle position what’s the hight differential and distance between the point where you put your hands.
So any bike you are looking at can those two moving points be easily put into that same configuration? Myself I always, rightly or wrongly look at the wheelbase as well I think all my bikes are within an inch and I believe give a similar handling experience.

I think my "real" figures are more or less what you are describing - as in the real distance from BB to saddle, real distance from saddle to bars.

What I was trying to illustrate is that a size based on the seat tube no longer defines the frame size - as note above that only really applied when there were horizontal top tubes.

[Paulatic]

I can’t help but think you might be making life hard for yourself with all these 'real' figures. It’s certainly doing my head in trying to follow it all
The BB is a fixed point and can’t be moved. Starting there how much set back and how high is the saddle. So now you’ve fixed the saddle position what’s the hight differential and distance between the point where you put your hands.
So any bike you are looking at can those two moving points be easily put into that same configuration? Myself I always, rightly or wrongly look at the wheelbase as well I think all my bikes are within an inch and I believe give a similar handling experience.

I think my "real" figures are more or less what you are describing - as in the real distance from BB to saddle, real distance from saddle to bars.

What I was trying to illustrate is that a size based on the seat tube no longer defines the frame size - as note above that only really applied when there were horizontal top tubes.

I’m not seeing any reference to set back. Have all those bikes the same seat tube angle?
You also appear to be measuring stem length so have they all got the same bars?
Why are your legs so much longer when you ride the MtB ?
Sorry it still looks unduly complicated.

[gxaustin]

64 is about the biggest made, for somebody about 6' 5"

My steel bike is a 63cm (25") and I am 5'11"

[531colin]

......….My steel bike is a 63cm (25") and I am 5'11"

We didn't get very far last time we set out down this road.
Here are my numbers.....
Leg (by the book up your crotch method) 86cm
BB axle to saddle top 70cm along the seat tube
BB axle to bum bone dent 73cm
My DIY bike fitting guide is linked below my signature; it has the how and why I set my riding position as I do.
At 5' 11" you are not particularly tall, yet you are riding just about the biggest bike that's made. Do you suppose the bike industry is so awash with cash that it can afford to ignore the really tall people? What do all the 6' 5" men ride?
If my memory serves, you have the saddle (on a 63cm) high enough that you have a significant bar drop, and you have an in-line seatpost so that you can reach the bars.

[531colin]

"Little grey cat"....I am a bit confused by your numbers. Do you have 3 bikes, where the bottom bracket axle to saddle top dimensions are
78, 74 and 73cm?
My dimensions are
leg 86cm (by the book up your crotch method)
BB axle to saddle top 70cm (along seat tube)
BB axle to bum bone dent 73cm
I can't see how you can ride 2 bikes where the saddle height varies by 5cm.
If your saddle is really 8cm higher than mine, I would expect you to be roughly 16cm taller than me....that's 6'4"
Below my signature is a link to my DIY bike fitting guide.

[gxaustin]

We didn't get very far last time we set out down this road.

Sorry, "didn't get very far"?

For comparison:
My measurements: Inside leg (no shoes) 89cm
BB to saddle: 797mm (NB 172.5mm cranks and SPD pedals)
Drop from saddle to bars 80mm (After I removed 30mm of spacers)
The inline seatpost is only because Brooks saddles don't have much adjustment forwards to achieve approximate KOPS fit. I have a 100mm stem btw (up from 90mm as I became more flexible).

I'm merely pointing out the variations in the body which have to be accommodated. In this context frame size can be misleading since mine vary from large woman's to extra large men's.
I've had 3 bike fits by the way and ride audax distances up to 300km. I think I'd find that difficult if I was uncomfortable on the bike.