How important is frame size.

[531colin]

I'm 5'8" and leggy ..............

Oooooohh! stop it, you naughty person!

Of course, the problems of a short torso (unless offset by long arms) are the same as the problems of short stature.
Everybody is so paralysed by the modern monoculture of 45mm offset and 72 or 73 degree head angle, but there is such an easy solution.
.....a slacker head angle and more offset gives good steering, I have one (tourer) with the best part of 60mm offset, and less than 71 deg. head....you can have toe clearance and a short top tube.....but not with easily-available carbon forks.

[531colin]

Here you go, a tale of 3 bicycles......



The old black Bob Jackson I had built many years ago, its 531 lugged construction, its 23 1/2" (for a 5'10" man ) because in those days the forward extension bit of quill stems was resolutely horizontal, and it was the only way I could get the bars high enough. I had it built with a short top tube at 22", if it had been 23 1/2" top tube, I don't think I would have reached the brakes in the days before compact drops. It was my best bike for years.
The blue Orbit happened along when I wasn't looking for a bike, but it was too good to miss, so I bought it and stuck it in the loft. Its 56cm (or 22") and I assumed it was sloping top tube, because most bikes were by then...however, that one was horizontal top tube and 1" quill stem. Then my winter hack got into such a state it wasn't worth sorting out, so (with something of a heavy heart) I downgraded the old BJ to the winter hack and got the Orbit out of the loft. I rode it for a while as original, (quill stems with a rise had by now appeared) before I got a new pair of forks made (to slow down the handling a bit) and now its 1" Ahead....its 22" top tube as well, but my toe can clip the 'guard because it was built short and steep (short offset, steep head angle....my new forks are 20mm longer, to take a degree off the head angle)
The Spa Ti bike is a complete indulgence, I almost have a bike for each day of the week now! Its 54cm seat tube, but the top tube slopes up 6cm, so the front end is actually a shade higher than the front end of the huge BJ. High bars and standover clearance, toe clearance and sensible reach.....whats not to like?....

[Vorpal]

I'm 5'8" and leggy ..............

Oooooohh! stop it, you naughty person!

[kylecycler]

the sloping top tube was commonplace in about 1890.

Things go in, uh, cycles....

cheers

Further to the other examples you posted, Brucey, here are the American long-distance cyclists Thomas Allen and William Sachtleben in China in 1892. Don't know the make of the bicycles but what struck me is, they look like weirdly modern compact frames, considering they're well over 100 years old.

The photo is from this sad story, about the 'lost' round-the-world cyclist Frank Lenz:
http://www.smithsonianmag.com/history/t ... e=1&no-ist

[531colin]

.......... The frame design kludges appear to come from trying to use 700C wheels in every (small) frame size and then trying to keep a short wheelbase (for why????) in the larger ones..............

............ there is slight overlap on my Dawes tourer, with 64cm frame and laid-back angles, but not on my Giant Rapid, a road-style frame with steeper angles, much shorter wheelbase and which is only the 58cm size.

I have often wondered why they seem to want big tall bikes to have the same wheelbase as the little 'uns.
For race bikes, is there some advantage to be had from light weight, stiffness, quick turning, or something?
For touring bikes, I am sure short wheelbase is a disadvantage....you need long chainstays to keep heels and panniers out of conflict, and tourers with 35mm tyres and mudguards need a lot more space to clear your toes than race bikes with narrow tyres and no guards.
It also bumps a lot less if you aren't sitting right on top of the back wheel (ask a tandem stoker), and when you are descending some backroad with a dodgy surface, adverse camber and blind bends, its very reassuring to have the front wheel well out in front, not tucked underneath you.

[samsbike]

Here you go, a tale of 3 bicycles......



The old black Bob Jackson I had built many years ago, its 531 lugged construction, its 23 1/2" (for a 5'10" man ) because in those days the forward extension bit of quill stems was resolutely horizontal, and it was the only way I could get the bars high enough. I had it built with a short top tube at 22", if it had been 23 1/2" top tube, I don't think I would have reached the brakes in the days before compact drops. It was my best bike for years.
The blue Orbit happened along when I wasn't looking for a bike, but it was too good to miss, so I bought it and stuck it in the loft. Its 56cm (or 22") and I assumed it was sloping top tube, because most bikes were by then...however, that one was horizontal top tube and 1" quill stem. Then my winter hack got into such a state it wasn't worth sorting out, so (with something of a heavy heart) I downgraded the old BJ to the winter hack and got the Orbit out of the loft. I rode it for a while as original, (quill stems with a rise had by now appeared) before I got a new pair of forks made (to slow down the handling a bit) and now its 1" Ahead....its 22" top tube as well, but my toe can clip the 'guard because it was built short and steep (short offset, steep head angle....my new forks are 20mm longer, to take a degree off the head angle)
The Spa Ti bike is a complete indulgence, I almost have a bike for each day of the week now! Its 54cm seat tube, but the top tube slopes up 6cm, so the front end is actually a shade higher than the front end of the huge BJ. High bars and standover clearance, toe clearance and sensible reach.....whats not to like?....

Lovely pic, out of interest what are the different HT lengths?

[531colin]

Head tubes.....
The old black BJ is a monster 190mm, the blue Orbit is 120, the Spa is 160mm.
One number isn't quite the whole story, I can probably get a 40mm tyre in the Orbit and the Spa, but the BJ is limited to 28 if you want clearance.

[Tonyf33]

For race bikes, is there some advantage to be had from light weight, stiffness, quick turning, or something?

That seems like quite an odd question from someone such as yourself
In order, yes, yes, yes and probably yes (guessing that the 'something' are other tangibles that racing bikes have in terms of differences over other types of bikes for the same purpose?)

[irc]

I have often wondered why they seem to want big tall bikes to have the same wheelbase as the little 'uns.........

It also bumps a lot less if you aren't sitting right on top of the back wheel (ask a tandem stoker), and when you are descending some backroad with a dodgy surface, adverse camber and blind bends, its very reassuring to have the front wheel well out in front, not tucked underneath you.

Me too. At 6ft3 I'm sitting nearly on top of the back wheel. Since tandems with a huge wheelbase have acceptable handling why do big frames not have chain stays a couple of inches longer. No heel strike issues. Loaded panniers more between the wheels rather than over the back. Rider weight slightly more evenly split front/rear. Pump pegs behind the seat tube where the pump doesn't take the place of a bottle or get in the way when handling the bike by the top tube.

Other than the weight of 2" of chain stays and seat stays and 4" of chain any disadvantages (for touring)?

[531colin]

I'm only 5' 10". My favourite bike has 460 chainstays and 1090 wheelbase. Doesn't fit too well in the bike spaces of some trains, but its really stable.

[531colin]

For race bikes, is there some advantage to be had from light weight, stiffness, quick turning, or something?

That seems like quite an odd question from someone such as yourself
In order, yes, yes, yes and probably yes (guessing that the 'something' are other tangibles that racing bikes have in terms of differences over other types of bikes for the same purpose?)

You can make a nonsense from anything written, simply by taking it out of context.
Still, perhaps you can explain?
Why is it that designers of race bikes want the wheelbase of a big tall bike for a 16 stone man to have as close as possible the same wheelbase as a bike for an 8 stone youngster?
Why do they steepen the head angle on tall bikes, and slacken it on small bikes, altering the trail on the different sizes? Is wheelbase more important than steering geometry?
Why is a tall person sitting over the back wheel with the same length chainstays as a short person? Is wheelbase more important than weight distribution?

On this thread http://forum.ctc.org.uk/viewtopic.php?f=1&t=95631&start=45 there are people telling me that bike design is so accurate and specific that if somebody buys a bike a size "wrong" and corrects it with stem length, then the weight distribution will be wrong, and the bike handling will be compromised.....which seems to me to be the opposite of what I see in the geo. tables, where wheelbase is kept constant at the expense of other design parameters.

[RogerThat]

It's good to see that Specialized 2015 are making size specific geometry for small medium and large frames, with different weight /gauge/length rear triangles for taller and shorter riders. This isn't a custom thing they're doing on their top end race bikes, but rolling it out across the entire range. They're also fitting short stack and deep headset bearings to small/tall bikes to optimise the front end too. About time!?

S Works Tarmac:

https://m.youtube.com/watch?v=97Z3_1J3m1g

[robc02]

why do big frames not have chain stays a couple of inches longer.

Some do, but this is usually because of their intended use (typically touring or cyclocross) rather than to accommodate taller riders. My LHT has 460mm chainstays, some other tourers have 440mm or thereabouts as do a number of cyclocross bikes. Only from around mid to late 1970s did very short chainstays become the norm on race bikes.

An additional benefit of long chainstays is there is slightly less chain misalignment at the extreme combinations (big-big or small-small) of derailleur gearing.

[gloomyandy]

Hi Colin,

Why is it that designers of race bikes want the wheelbase of a big tall bike for a 16 stone man to have as close as possible the same wheelbase as a bike for an 8 stone youngster?
Why do they steepen the head angle on tall bikes, and slacken it on small bikes, altering the trail on the different sizes? Is wheelbase more important than steering geometry?
Why is a tall person sitting over the back wheel with the same length chainstays as a short person? Is wheelbase more important than weight distribution?

I'm no frame designer but I do find it an interesting subject. So looking at an example of a race bike (in this case the Cervelo R5 - I have one of these so it is of interest to me):
http://www.cervelo.com/media/docs/R5-52 ... 7f01-0.pdf
This frame comes in sizes from 48 through to 61. In this case the rear part of the frame is very similar for all sizes with a seat angle of 53 degrees and a rear centre length of 405 for all sizes (so no steepening of the seat angle on the smaller sizes), at the front the smaller two sizes have different head tube angles (70.5 and 72.2) and sizes above this have an angle of 73.5. The smaller bikes have a different fork having a different offset (53 on the small frames, 43 on the large ones). The front centre distance ranges from 575 to 620. Which corresponds to a reach change range of 360 tp 405. I don't really see much evidence of an attempt to keep the wheelbase the same for this bike? The change in steering geometry seems like an attempt to allow for smaller reach on the small bikes while avoiding toe overlap (not unreasonable), the wheelbase seems to grow with the reach. Are you suggesting that the wheelbase should grow more as the reach grows? The only odd thing is that the rear centre distance does not get larger with a larger frame, but given the effort that has gone into making the rear chainstays very rigid it may be that they think the stiffness of this area is the more important aspect. So would you say that this frame has been compromised to maintain the wheelbase?

[MartinC]

...................Why is it that designers of race bikes want the wheelbase of a big tall bike for a 16 stone man to have as close as possible the same wheelbase as a bike for an 8 stone youngster?...................

I guess it's because the perceived advantage is in having chainstays and wheelbase as short as possible. Both are determined by the wheel size not the size of the rider so they end up the same for all frame sizes.