How important is frame size.

For discussions about bikes and equipment.
User avatar
531colin
Posts: 16083
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: How important is frame size.

Post by 531colin »

OK, a couple of geo. tables to play with.....
Cervelo........I'm not clever enough to post it up
http://www.cervelo.com/media/docs/R5-525d891c-6c23-49e2-bf5e-7a1aeec17f01-0.pdf
Specialised, I found Tarmac Elite......can't do that one either.... :roll:
http://www.specialized.com/us/en/bikes/road/tarmac/tarmac-elite#geometry

With 700c wheels and short riders, its a known problem to get toe clearance and the bars close enough. They have both gone for 575/6/7 mm front centre, Spec. have the 2 smallest bikes the same at 576, Cervelo have 575, 577, 575 for the 3 smallest. Cervelo then step out 12, 17, and 16mm front centre increments for the remaining sizes, Spec. increase them by 7,8,14, and 7mm. ....I confess I didn't wade through the video looking for a reason why one increment in frame size needs a 14mm increment in front centre, while those either side get 7 or 8mm. The 61 Cervelo has 8mm longer front centre than the 61 Spec., whereas the 54 Spec. has 8mm longer front centre than the 54 cervelo.....hardly dramatic differences, and in opposite directions.
Chainstay is 405 for every size Cervelo, Spec have them all 405 except the 2 biggest, which get 2mm or 5mm longer chainstays.
For the steering geometry, most of these bikes have 43mm offset forks, generally paired with 73.5 deg head, although Cervelo also pair it with 73.1 deg, and Spec. with 74deg, which I think would have me in the ditch. Spec. also fit a 45mm offset fork (2mm offset difference? can it be worth it?) which they pair with 72.25 and 73 deg. Cervelo use a 53mm offset fork with 72.2deg and 70.5deg. I'm currently working with 60mm offset and 70.5 deg on a tourer, so 53mm offset is going to make a pretty lazy-handling race bike.
Top tube length seems to increment somewhere around the 16mm increase for each size for both bikes. With the Spec., some of this is due to really steep seat tubes in the smaller sizes which will be nullified if the rider pushes the saddle back. The Cervelo all have 73 deg seat, and the range of top tube lengths is much greater than the range of front centre, which necessitates the surprisingly large range of head angles. Despite these different approaches, the top tube lengths in both makes are the same within a few millimetres across all the sizes.
So, a bit of a summary......
Frame size range is 110mm for Cervelo, 120mm for Spec.
Wheelbase range is 45mm for Cervelo, 43mm for Spec.
Top tube range is 81mm for both, however.......
Spec. do the usual fiddle of steep seat tubes in the small sizes to get a short top tube in the geo. table, reach range is only 22mm.
Cervelo keep the same seat angle and slacken the head angle significantly to get a short top tube in the small bikes, reach range is 45mm.
So, as I have posted times without number, nothing else varies anything like as much as the range in the height of the frame.

I think the 2 most important things are how it steers, and whether you can be in balance riding the thing. I think the small bikes are compromised....Spec. have done the usual steep seat tube angle, so you need lots of layback to keep your weight off your hands, so they aren't as short as the geo. table suggests. Cervelo have instead compromised the steering, which is a new one on me, and the big Spec. is going to be awful twitchy.
My bikes get ridden up and down the steep, rough lanes in the Dales. Going up, the difficulties are keeping the front wheel down, and keeping it upright, so its long chainstays and stable steering, thank you very much. Going down, the difficulties are keeping the back wheel down if you need to brake hard, and negotiating gravel, broken tarmac, blind bends and adverse camber, so its stable steering and the front wheel out in front rather than underneath, thank you very much.
I suppose the chain-gang to the cafe on roads with reasonable surface and engineered gradients and sight lines probably places less demands on bike handling, but these are sold as the bees' knees, and I don't want the design of my bike to restrict my choices of which roads I can enjoy. (sorry....its late, and I'm feeling jaded, having waded through all that and failed to find any pearls of wisdom.)
Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: How important is frame size.

Post by Brucey »

531colin wrote: ....sorry....its late, and I'm feeling jaded, having waded through all that and failed to find any pearls of wisdom....


ah, but that is the outcome you expected, right? :wink:

At least Cervelo seem to be trying something a bit different in the smaller sizes, which is something....

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
gloomyandy
Posts: 1140
Joined: 16 Mar 2012, 10:46pm

Re: How important is frame size.

Post by gloomyandy »

Hi Colin,
thanks for all of that effort, I think I followed most of it!

I'd like to add that being a short rider, I have a 51 Cervelo and a small Planet-x pro carbon. The pro carbon plays the steep seat tube trick, so I my saddle is a fair way back, the head angle is 72 degrees and despite this there is still a lot of toe overlap (so I suspect it is not using a longer offset fork). For me there is no toe overlap on the r5 and the saddle is nicely in the middle of the rails (for the same position on both bikes). The r5 is a much nicer bike to ride and handles steep downhills very well (A lot of my riding is around the South Pennines), much better than the pro carbon which at high speed can at times be downright scary.

Having said that I've ridden many thousands of miles on both of them, with no real issues. My other bikes include a touring bike based on a no suspension 29er mountain bike frame, a steep angled tt bike and a very cheap tandem all of which are different again! Maybe all this proves is that people can adapt pretty well to different bikes!
User avatar
bigjim
Posts: 3244
Joined: 2 Feb 2008, 5:08pm
Location: Manchester

Re: How important is frame size.

Post by bigjim »

I got rid of my modern bikes with short chainstays and sloping toptubes. I now ride old steel frames. My bikes range from 23", 23.5", 24" and 25". I've been riding the smaller frames through the winter. last week I took out the 25" frames. The difference was amazing. I was climbing quicker, the bike seemed more lively and I was a lot less tired at the end of the ride. I have no idea why this is but I'm now considering selling the smaller framed bikes. I'm 6'2" with a 36" inside leg and weigh 14stone+. No, I'm not fat except for the belly that contains my emergency store. I have a short torso so always fit a short stem but don't find it alters the steering and I like the bars high.
Re sloping toptubes. How could you ever give your girlfriend a lift home? Or wife? About 5 years ago, one evening, I rode to a restaurant a few miles along the cliff path near Palma. I took an old borrowed bike. My wife sat on the toptube there and back. Love is. Good job it was horizontal. :)
Re frame dimentions. I quote.
For road bikes in the range that is available, the longer the
chainstays the better the bike handles in all but 10mph turns. Tandem
riders can vouch for that on fast descents. The trend toward short
bicycles is not handling related but rather a desire to build lighter
bicycles. The concept is bolstered by the allusion to "quick"
steering and that the fastest bicycles are the TTT bikes, that
coincidentally depends on the riders being as close together as they
can get. Hence short bicycles. That doesn't mean the bicycle is
fast, only that a four man team is fast.

Chainstay length is primarily a comfort effect of sitting directly
over the rear wheel or not. Secondarily, a short wheelbase makes
weight transfer on braking less advantageous and least of all steering
motions more disruptive to straight line riding.

The only thing wrong with long chainstays is that they are not in
fashion and your peers will sneer at you for being unconventional,
using all sorts of pseudo technical allusions to support their point
of view.

Jobst Brandt
MikeF
Posts: 4339
Joined: 11 Nov 2012, 9:24am
Location: On the borders of the four South East Counties

Re: How important is frame size.

Post by MikeF »

RogerThat wrote: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
Sizes and geometry here
Specialized Tarmac 2015.jpg
without the sales patter :wink:
"It takes a genius to spot the obvious" - my old physics master.
I don't peddle bikes.
User avatar
531colin
Posts: 16083
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: How important is frame size.

Post by 531colin »

Thats interesting.....Mike F's geo.table is exactly the same as this one i found...http://www.specialized.com/us/en/bikes/road/tarmac/tarmac-elite#geometry
....apart from seat tube length....... :?
User avatar
531colin
Posts: 16083
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: How important is frame size.

Post by 531colin »

And just to pick up on Brandt, when you are braking hard enough to lift the rear wheel, which is better....
a) to have long chainstays, ie the back wheel further behind your bum..or
b) to have long front centre, ie the pivot point further in front of your centre of mass?
User avatar
bigjim
Posts: 3244
Joined: 2 Feb 2008, 5:08pm
Location: Manchester

Re: How important is frame size.

Post by bigjim »

531colin wrote:And just to pick up on Brandt, when you are braking hard enough to lift the rear wheel, which is better....
a) to have long chainstays, ie the back wheel further behind your bum..or
b) to have long front centre, ie the pivot point further in front of your centre of mass?

Why would you be braking hard enough to lift the rear wheel? Surely thats incorrect handling? Can't remember that ever happening. But there again I'm heavy. :)
Brucey
Posts: 44521
Joined: 4 Jan 2012, 6:25pm

Re: How important is frame size.

Post by Brucey »

531colin wrote:And just to pick up on Brandt, when you are braking hard enough to lift the rear wheel, which is better....
a) to have long chainstays, ie the back wheel further behind your bum..or
b) to have long front centre, ie the pivot point further in front of your centre of mass?


Too much of b) and you will push the front into a skid before the rear wheel lifts. Most race bikes are far away from that; every single one I've ridden will lift the rear wheel if you brake hard enough.

You need a prodigious rear load to cause a front wheel skid to be possible, even on a touring bike. But having longer chainstays pushes the CoG backwards (the weight of the rear wheel at least goes backwards) and this makes braking better in most cases.

Short chainstays are popular on race bikes for one very simple reason; stiffness to weight ratio. If the stays are just 20% longer they might flex laterally by over 70% more if they are built in the same gauge tubes. If this sounds like they might need to be much heavier when longer if the flex is to be kept constant this would be right. Whilst a whippy main triangle is no bad thing, all the signs are that a laterally flexy rear triangle confers no benefits, only losses.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
User avatar
bigjim
Posts: 3244
Joined: 2 Feb 2008, 5:08pm
Location: Manchester

Re: How important is frame size.

Post by bigjim »

Whilst a whippy main triangle is no bad thing, all the signs are that a laterally flexy rear triangle confers no benefits, only losses.

At my level I don't think I'd notice. I got rid of my race bikes because of the lack of all day comfort. My steel bikes with longer chainstays are way more comfortable and I don't notice any lack of speed or more effort required when out of the saddle and climbing. In other words. I'm just as knackered either way. :)
User avatar
531colin
Posts: 16083
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: How important is frame size.

Post by 531colin »

gloomyandy wrote:Hi Colin,
thanks for all of that effort, I think I followed most of it!

I'd like to add that being a short rider, I have a 51 Cervelo and a small Planet-x pro carbon. The pro carbon plays the steep seat tube trick, so I my saddle is a fair way back, the head angle is 72 degrees and despite this there is still a lot of toe overlap (so I suspect it is not using a longer offset fork). For me there is no toe overlap on the r5 and the saddle is nicely in the middle of the rails (for the same position on both bikes). The r5 is a much nicer bike to ride and handles steep downhills very well (A lot of my riding is around the South Pennines), much better than the pro carbon which at high speed can at times be downright scary.

Having said that I've ridden many thousands of miles on both of them, with no real issues. My other bikes include a touring bike based on a no suspension 29er mountain bike frame, a steep angled tt bike and a very cheap tandem all of which are different again! Maybe all this proves is that people can adapt pretty well to different bikes!


Planet X pro carbon.....http://www.planetx.co.uk/i/q/FRPXPCRFF/planet-x-pro-carbon-road-frameset

The first thing i picked up on was "why is this bloke riding a 51 Cervelo and a 44 PX...?"
And the answer is the standover is the same within 3mm.....so much for "size"

But I have got a real problem.
Planet X don't tell us the fork offset, I have to assume its an industry-standard 45mm, and they won't change the offset for a range of head angles between 72 and 73.5 deg.....again, nothing strange there....if they were changing the offset with bike size, I think they would tell us.
BUT.....72 deg 45mm is a very well tried and tested steering setup....its on Surly's LHT, its on Spa's Audax....it is NOT a twitchy set up in any sense of the word...
Cervelo's 51 has steeper head at 72.2 deg, and more offset at 53mm, it ought to be twitchier than the PX.

.....i don't believe the geo.,tables....... :shock:
User avatar
531colin
Posts: 16083
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: How important is frame size.

Post by 531colin »

Brucey wrote:
531colin wrote:And just to pick up on Brandt, when you are braking hard enough to lift the rear wheel, which is better....
a) to have long chainstays, ie the back wheel further behind your bum..or
b) to have long front centre, ie the pivot point further in front of your centre of mass?


Too much of b) and you will push the front into a skid before the rear wheel lifts. Most race bikes are far away from that; every single one I've ridden will lift the rear wheel if you brake hard enough.................
Short chainstays are popular on race bikes for one very simple reason; stiffness to weight ratio. If the stays are just 20% longer they might flex laterally by over 70% more if they are built in the same gauge tubes. If this sounds like they might need to be much heavier when longer if the flex is to be kept constant this would be right. Whilst a whippy main triangle is no bad thing, all the signs are that a laterally flexy rear triangle confers no benefits, only losses.

cheers


I think the only bikes I have ridden where I haven't lifted the back wheel have been tandems and bikes where the brakes are crap. Never locked a front wheel except on stuff like ice, mud and gravel. How long FC would you need to overcome the lunge of WUM?
The chainstay bridge means that the triangle of chainstay next to the BB is really very stiff....the chainstay length from bridge to dropout can be constant.
With the main triangle, are you referring to "planing"? because i don't see how the BB can flex laterally without some flexing in the chainstays.
User avatar
531colin
Posts: 16083
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: How important is frame size.

Post by 531colin »

bigjim wrote:
531colin wrote:And just to pick up on Brandt, when you are braking hard enough to lift the rear wheel, which is better....
a) to have long chainstays, ie the back wheel further behind your bum..or
b) to have long front centre, ie the pivot point further in front of your centre of mass?

Why would you be braking hard enough to lift the rear wheel? Surely thats incorrect handling? Can't remember that ever happening. But there again I'm heavy. :)


In an "emergency stop"....the sort of thing you do in an emergency, not every day.
You were quoting Brandt on weight transference on braking, what can this mean apart from keeping the back wheel down?
User avatar
bigjim
Posts: 3244
Joined: 2 Feb 2008, 5:08pm
Location: Manchester

Re: How important is frame size.

Post by bigjim »

531colin wrote:
bigjim wrote:
531colin wrote:And just to pick up on Brandt, when you are braking hard enough to lift the rear wheel, which is better....
a) to have long chainstays, ie the back wheel further behind your bum..or
b) to have long front centre, ie the pivot point further in front of your centre of mass?

Why would you be braking hard enough to lift the rear wheel? Surely thats incorrect handling? Can't remember that ever happening. But there again I'm heavy. :)


In an "emergency stop"....the sort of thing you do in an emergency, not every day.
You were quoting Brandt on weight transference on braking, what can this mean apart from keeping the back wheel down?

Colin. I was quoting Brandt on his thoughts on short frames. I didn't examine it in relation to emergency braking. He does not mention rear wheel lift. My experience on emergency braking is of grabbing both brakes and not lifting the rear wheel. There again, I ride bikes with long chainstays.
I've never lifted a rear wheel on my motorcycle either but it seems I'm in the minority. :(
TimP
Posts: 106
Joined: 25 May 2015, 6:15pm

Re: How important is frame size.

Post by TimP »

What matters is riding position - distance seat to hand position, seat to pedals and pedals to hands. How you get this right is purely personal preference.

I am a little over 6 feet tall. I ended up with a 24" Hobbs of Barbican frame (second hand - its older than me) because it was the right size for me. I hardly ride it as I prefer my other 2 bikes - especially the one I got on my 11th birthday - a 19.5" frame. I've changed everything on it over the years. It has a 4" handlebar stem and the longest seat post I could find and it just fits. It is comfy. It works and that is all that matters.
Post Reply