Frame Sizing/Geometry Questions

[Archibald]

Usually a subject with many varied opinions, but how exactly do you tell/workout what your ideal frame size/geometry is?

Is there a specific formula or is it more trial & error - with different makes suiting different people depending on your build?

 

[Master50]

You probably would find 100 answers to this question on bicycling.com. A magazine for beginners.
Yes there are a number of fit systems, formulae, and rules of thumb which will get better than %90 of the people on the right sized bike. From that point there are many options to get the general fit dialed in for you. There are some people far enough out of the "average or mean" in terms of body shape and morphology that will have special sizing needs and may need a custom build.

Almost any good bike shop will use some sizing system to get you started. Some really experienced bike fitters can almost do it from just looking at you. Trial and error is involved once the fine tuning is all that is left.

So how does a senior member come to post such a simple question? Bored with nothing to talk about?

 

[LugHugger]

I've used the Competitive Cyclist fit model for my bikes over the 4-5 years and then tweaked things from there.

What these models can not take into account are personal factors. These can be a factor of flexibility, core strength, medical history, terrain, type of riding, riding style and many others. I guess this is where a bike fit can make the difference. See Cyclefit in Covent Garden and Mosquito in Islington for SICI qualified fitters.

 

[brianf7]

If you are buying a new frame or bike the bike shop will fit it to you if they dont go to another bike shop that dose.

 

[LugHugger]

If you are buying a new frame or bike the bike shop will fit it to you if they dont go to another bike shop that dose.

That's right, Brian. The problem is, I know of shops who say that they offer a bike fitting but really know very little about the technicalities. They seem to think that a bike fit consists of adjusting the saddle height and swapping stem spacers. If you don't know any better, then you could be mislead into thinking this is all there is to 'fit'.

 

[M Sport]

If you are buying a new frame or bike the bike shop will fit it to you if they dont go to another bike shop that dose.

I wouldn't trust a bike shop giving me the right size, I would study the geometry charts and work out which had the correct TT length for me, there are of course other important things like head tube length and seat angles and again these are things I would consider myself knowing exactly what I need after riding bikes over a number of years.

Anyone new to bike riding or that's been on the wrong size frame before should of course consult a fitter, but don't be shy on asking about their experience and the methodology they use.

 

[brianf7]

That's right, Brian. The problem is, I know of shops who say that they offer a bike fitting but really know very little about the technicalities. They seem to think that a bike fit consists of adjusting the saddle height and swapping stem spacers. If you don't know any better, then you could be mislead into thinking this is all there is to 'fit'.

Sorry I meant go to a bike shop that is also a frame builder who has years of experience but dont put all eggs in one basket.

Not many build frames now but they usualy know their bikes. Dont let them fit you to a bike they must fit the bike to you.

 

[Archibald]

So how does a senior member come to post such a simple question? Bored with nothing to talk about?

being a "senior member" doesn't guarantee that I know how to fit a bike to myself...

I'm simply looking at spending a sizeable amount of wedge on a new bike or frame, and I'd like to get the closest fit to me. I don't trust some of the jokers in the shops. Just doing research and looking for what info/formula/methods/etc... that folks are willing to share...

 

[GreasyMonkey]

Frame geometry thoughts.

A lot depends on the type of riding/racing you do, also the style of your riding.

For myself, I like a frame that has a slack seat-tube (73.5 deg for a 53.5cm C-C height), also a slack head-tube with a 45mm rake fork, to give a trail of 58mm.
This set-up gives a frame that is very nice for long rides, stage-racing, but not too suited to crit's (which I hate, and don't race). It also is better for seated climbing, not so much for OOTS climbing at low speeds, due to the "twitchiness".
The ride characteristic is that it is very fast handling at lower speeds, but becomes more stable as speed goes up - ideal for descending very fast with confidence and stability, with low-speed cornering being very quick. It places your weight more toward the rear of the bike, rather than over the front, like you get with most "crit-style" frames.

If you have a trail measurement of less than 55mm, it will tend to be slower steering at low speeds, becoming more twitchy as the speed increases. For a "neutral" feel, you should be looking at 54 - 56mm. A "crit" frame is usually in the range of 52 - 54mm trail.
As a reference, my road frames have 58mm trail, my TT bike is designed for a 61mm trail - I like them STABLE at high speed, faster/twitchier at low speeds.

Frames with the slack geometry are not easy to find in the smaller sizes, I use custom frames which meet my build & needs.

 

[Bustedknuckle]

being a "senior member" doesn't guarantee that I know how to fit a bike to myself...

I'm simply looking at spending a sizeable amount of wedge on a new bike or frame, and I'd like to get the closest fit to me. I don't trust some of the jokers in the shops. Just doing research and looking for what info/formula/methods/etc... that folks are willing to share...

Formulas are a starting point at best. The most important item, and the most difficult to determine, is femur length. This determines seat tube angle, the rest of the fit develops from there. The only way to determine bike fit is to sit on a fit cycle, with somebody who knows what they are doing, who will listen to your needs, riding style, physical issues, etc.

Go to a shop, no fit cycle? Go elsewhere.

 

[Captain_Cavman]

http://www.peterwhitecycles.com/fitting.htm

I've always thought this guy talks a lot of sense.

 

[ustabe]

http://www.peterwhitecycles.com/fitting.htm

I've always thought this guy talks a lot of sense.

I second that. Especially the parts about fore-and-aft saddle position and hand position. Looking at the top pro road riders, the first thing I see is that they are in balance.

It's funny, there are no coaches with rulers, protractors, plumb bobs, and tables of metrics to tell you how to stand in order to make your skis turn or bump a volleyball, but most of us who work at it awhile learn to do it without falling on our faces. Why is riding a road bike so different?

 

[Archibald]

here's where the fun starts...

[My height is 6ft exactly (182.8cm)]

Geometry of my current Giant OCR1 Compact (Large):

Seat Tube: 55.5cm
Top Tube: 58.5cm
Head Tube: 18.4cm
H/Tube Angle: 73'
Seat Angle: 73'
BB Drop: 70

Geometry for BMC Roadracer SL01 (57cm) - recommended for rider height 178-188cm

Seat Tube: 57cm
Top Tube: 56.5cm
Head Tube: 17.5cm
H/Tube Angle: 73.5'
Seat Angle: 73.5'
BB Drop: 69

Geometry for BMC Roadracer SL01 (60cm) - recommended for rider height >188cm

Seat Tube: 55.5cm
Top Tube: 58.5cm
Head Tube: 21.0cm
H/Tube Angle: 73.5'
Seat Angle: 73.5'
BB Drop: 69

The BMC Geometries also give a "reach" figure that is not present in the OCR Geometry - the horizontal distance from directly below the top of the head tube to the BB. 10cm difference between the 57cm to the 60cm frames...

So, looking at the matching figures in bold, is my current frame too big for me or is the BMC recommendation out regarding rider height?
I'm quite comfortable on my current set up, and don't feel like I'm on something that's too big for me.
So would I be likely to be more comfortable on the BMC (57cm) and get more out of it? Or would the 60cm BMC frame be more suitable considering what I'm already on and the two matching dimensions?

 

[Night Rider]

here's where the fun starts...

[My height is 6ft exactly (182.8cm)]

Geometry of my current Giant OCR1 Compact (Large):

Seat Tube: 55.5cm
Top Tube: 58.5cm
Head Tube: 18.4cm
H/Tube Angle: 73'
Seat Angle: 73'
BB Drop: 70

Geometry for BMC Roadracer SL01 (57cm) - recommended for rider height 178-188cm

Seat Tube: 57cm
Top Tube: 56.5cm
Head Tube: 17.5cm
H/Tube Angle: 73.5'
Seat Angle: 73.5'
BB Drop: 69

Geometry for BMC Roadracer SL01 (60cm) - recommended for rider height >188cm

Seat Tube: 55.5cm
Top Tube: 58.5cm
Head Tube: 21.0cm
H/Tube Angle: 73.5'
Seat Angle: 73.5'
BB Drop: 69

The BMC Geometries also give a "reach" figure that is not present in the OCR Geometry - the horizontal distance from directly below the top of the head tube to the BB. 10cm difference between the 57cm to the 60cm frames...So, looking at the matching figures in bold, is my current frame too big for me or is the BMC recommendation out regarding rider height?
I'm quite comfortable on my current set up, and don't feel like I'm on something that's too big for me.
So would I be likely to be more comfortable on the BMC (57cm) and get more out of it? Or would the 60cm BMC frame be more suitable considering what I'm already on and the two matching dimensions?

That looked odd so I looked at the BMC site for you. It's only 10mm difference not 10cm.

My personal opinion and please don't take it as gospel as all body comparisons are different is that you are on the wrong size Giant and that the 60cm BMC will be too big for you.

I'm a fraction under 5'11" and I ride a medium Giant, and even that is one cm too long for me in the top tube. Giant's sizes are odd, their top tubes are way to long.

Looking at BMC's geometry chart I could really drop down to a 51 on their frames instead of the 54.

 

[TheDude]

Archibald -
It really depends, are you long in the arms or the legs? Look at the head tube length as well, there is a different geometry in play here. I got short legs and a long body - I'm going to ride a different geometry than somebody that is all legs.

 

[ustabe]

I'm a fraction under 5'11" and I ride a medium Giant, and even that is one cm too long for me in the top tube. Giant's sizes are odd, their top tubes are way to long.

I'm a fraction under 6', and I ride a ML Giant TCR with a 13cm stem, and I'm in fit heaven. But my build is short torso, long arms and legs, and my handlebar has a short reach (with a longer reach bar, I was using a 12 cm stem), and I ride with a very flat back. This is pretty much the setup I've used since 1976. If I needed a little more handlebar height I'd be riding the large with an 11 stem, but any way to cut it, a medium Giant would be too small. When shopping for myself (mostly looking), I simply rule out any bike that isn't offered with a 57-58 top tube and a 17-18.5 head tube.

So if you're of more average proportions, I'd guess that your large Giant is a good fit for you.

Regarding the BMC, I'd guess that the 56.5 top tube of the 57 is too short and the the 21.0 head tube is too tall. If it was my money I'd be looking at a different bike.

One funny thing I've noticed about sizing bikes to riders is that once on the saddle, some appear to get taller and some get smaller. I can't count the number of riders I've sized, usually the taller guys, who need a larger frame for the head tube height, but with a shorter stem, because they get humps in their backs the minute they reach for the brake hoods.

 

[ustabe]

I'm quite comfortable on my current set up, and don't feel like I'm on something that's too big for me.
So would I be likely to be more comfortable on the BMC (57cm) and get more out of it? Or would the 60cm BMC frame be more suitable considering what I'm already on and the two matching dimensions?

What's your stem length on your current setup, the Giant? And while we're here, is it flipped up or down, and what does the spacer situation look like? Finally, is there anything about your fit on the Giant that you might want to change?

 

[Notso Swift]

I'm a fraction under 6', and I ride a ML Giant TCR with a 13cm stem, and I'm in fit heaven. But my build is short torso, long arms and legs, and my handlebar has a short reach (with a longer reach bar, I was using a 12 cm stem), and I ride with a very flat back. This is pretty much the setup I've used since 1976. If I needed a little more handlebar height I'd be riding the large with an 11 stem, but any way to cut it, a medium Giant would be too small. When shopping for myself (mostly looking), I simply rule out any bike that isn't offered with a 57-58 top tube and a 17-18.5 head tube.

So if you're of more average proportions, I'd guess that your large Giant is a good fit for you.

Regarding the BMC, I'd guess that the 56.5 top tube of the 57 is too short and the the 21.0 head tube is too tall. If it was my money I'd be looking at a different bike.

One funny thing I've noticed about sizing bikes to riders is that once on the saddle, some appear to get taller and some get smaller. I can't count the number of riders I've sized, usually the taller guys, who need a larger frame for the head tube height, but with a shorter stem, because they get humps in their backs the minute they reach for the brake hoods.

Unfortunately it is virtually impossible to buy a bike with a top tube longer than 60cm, likewise stems normally max out at about 14, and for seat posts 32mm, though there are custom options, obviously

Archibald
The taller BMC... do you have zero stack on you stem, and are you using a -17 degree stem already?
If you are and the position is wrong I would have to say look at a different bike. As much as we all (and I include myself) are branding victims and the BMC SL01 is a sweet ride at a much more reasonable price point that the RM or SLR series bikes (which I think you would find are longer and lower for the equivalent "size") it all means nothing if you are not comfortable, and potentially could lead to an injury, so the choice becomes moot

 

[Archibald]

What's your stem length on your current setup, the Giant? And while we're here, is it flipped up or down, and what does the spacer situation look like? Finally, is there anything about your fit on the Giant that you might want to change?

not sure on the stem length, I'd have to check. I remember the guy that sized it to me reduced it's length, as well as reducing the width of my handlebars (new set). The stem is quite short.
it's angled upwards slightly
one or two spacers (again, from memory)

as for anything that I feel I want to change, the bike "feels" a little low, but only for a little while getting on it. this is probably more due to my ss commuter being an unknown brand 56cm frame with slightly longer cranks despite the pedal to seat distance being the same.
otherwise, not really. again, possibly because i've grown accustomed to it, and it is the first decent road bike i've ever had - thereby no much to compare it with...

Archibald
The taller BMC... do you have zero stack on you stem, and are you using a -17 degree stem already?
If you are and the position is wrong I would have to say look at a different bike. As much as we all (and I include myself) are branding victims and the BMC SL01 is a sweet ride at a much more reasonable price point that the RM or SLR series bikes (which I think you would find are longer and lower for the equivalent "size") it all means nothing if you are not comfortable, and potentially could lead to an injury, so the choice becomes moot

I couldn't tell you the angle of the stem - i'll check the model of Deda that is is in case that may help?
I'm pretty sure there's 2 spacers between the stem and frame if that's what you're referring to as "stack".
I'm looking to switch the frames over, so not much chance of a 14-day return should it be a problem...

 

[ustabe]

as for anything that I feel I want to change, the bike "feels" a little low, but only for a little while getting on it.

How does it feel after you've been riding for a an hour? You need to evaluate where you are going with your riding.

If you want to keep your current position or raise the handlebar, the 60 will give you that opportunity. Using the same stem, you should be able to duplicate your current position by removing spacers and/or flipping the stem. Retaining some spacers would allow you to raise the bar.

I you want to evolve to a more aggressive position, the 57 would work if you use a stem 2 cm longer than what you're using now. Because of the 17.5 cm head tube on the 57, I can see difficulty trying to attain the same handlebar height as on your current setup.

Remember that the BMC that you're considering is a racing type frame, so the size recommendations by the manufacturer are aimed at riders who will set the handlebar some 4-10 cm lower than the saddle. But there is nothing wrong with getting a larger size to raise the handlebar, as long as you can straddle the bike comfortably and you can get the right stem for your preferred reach.

On a short-wheelbase bike, though, I do not recommend a position that puts the handlebar higher than the saddle.

 

[dsut4392]

I've used the Competitive Cyclist fit model for my bikes over the 4-5 years and then tweaked things from there.

What these models can not take into account are personal factors. These can be a factor of flexibility, core strength, medical history, terrain, type of riding, riding style and many others. I guess this is where a bike fit can make the difference. See Cyclefit in Covent Garden and Mosquito in Islington for SICI qualified fitters.

I also used the competitive cyclist fit calculator as a starting point and am quite happy with the result. At the very least (assuming you measure yourself up properly) it gives you a middle of the road fit , from where you can think about how it differs from your current position, and whether you might want to change things a little. (i.e. take the measurements you get out of the fit calculator and compare them to how your Giant is set up).

Does the shop you're buying the BMC from have a built-up frame of the same geometry they can test-fit you on, or even better that you could take for an extended test ride? If you're not entirely sure of what you're actually looking for in geometry, it really helps to try a few bikes at different points in the spectrum.

 

[Archibald]

How does it feel after you've been riding for a an hour? You need to evaluate where you are going with your riding.

If you want to keep your current position or raise the handlebar, the 60 will give you that opportunity. Using the same stem, you should be able to duplicate your current position by removing spacers and/or flipping the stem. Retaining some spacers would allow you to raise the bar.

I you want to evolve to a more aggressive position, the 57 would work if you use a stem 2 cm longer than what you're using now. Because of the 17.5 cm head tube on the 57, I can see difficulty trying to attain the same handlebar height as on your current setup.

Remember that the BMC that you're considering is a racing type frame, so the size recommendations by the manufacturer are aimed at riders who will set the handlebar some 4-10 cm lower than the saddle. But there is nothing wrong with getting a larger size to raise the handlebar, as long as you can straddle the bike comfortably and you can get the right stem for your preferred reach.

On a short-wheelbase bike, though, I do not recommend a position that puts the handlebar higher than the saddle.

interesting.

I did over 5hrs on it on the weekend, and other than getting a little soreness under my kneecaps, I was pretty comfortable for the entire ride (not including some sore/numb ar$e issues at around the 3.5hr mark).

The stem is actually upside down (angled downward), but with the 2 spacers it is actually level (horizontal).
My bars are a couple of centimetres lower than my saddle, and I was most comfortable with my hands on the flats compared to them being on the hoods...

Does the shop you're buying the BMC from have a built-up frame of the same geometry they can test-fit you on, or even better that you could take for an extended test ride? If you're not entirely sure of what you're actually looking for in geometry, it really helps to try a few bikes at different points in the spectrum.

I'd have to check what BMC models they have on the floor - I just know that this isn't one of them. They have a "test ride" offer, but it involves actually purchasing the bike and then being allowed to bring it back before a certain number of days for full refund...
Might see what events I can find with demo bikes too...

 

[Archibald]

Does the shop you're buying the BMC from have a built-up frame of the same geometry they can test-fit you on, or even better that you could take for an extended test ride?

they're shipping in the fully made BMC for me to try out - I didn't think they would, but asked on a whim to day. "No problem" was the reply, they're due to get a couple in in June, but there's already one in another store, so they're shipping it across...

 

[Archibald]

Gone for the 57
test rode it tuesday, and damn, it was a good ride.
the frames are being currently swapped over now...