Frame Size, Center of Gravity, and Bike Handling

Assuming you have a seat post and stems that are long enough to put you into the correct riding position, would you go with a larger or a smaller frame? How does the frame size affect your center of gravity? Would the larger or the samller frame be more stable?


I'm asking because I recently rented a bicycle that was 8cm smaller than my current frame. The operator chose the frame size based on 3 measurements that I sent--the height of the saddle relative to the bottom bracket, the distance from saddle to center of handle bars, and the distance from the handlebars to the center of the front axle. Those measurements matched between my home-bike and the rental bike so position-wise, the bikes were identical (albiet the rental had a lot of seat post showing and a high stem). The mechanic that chose the frame size swears that, all things being equal in bike position, one should always choose the smallest frame.

Initially, I found the bike very squirrelly when standing. But, after a few tries, I adjusted. Over the week that I rode it I found that it felt that the bike handled better when descending and on steep grades (20+%) I never had a problem with the front wheel popping up, a problem I have here at home (although the number of steep grades is much smaller)

Now that we're back, I'm wondering if I should seriously be shopping for a bike with a smaller frame. Does the size of the frame, given the same riding position, have that significant a difference in bike control? That is, should I listen to the bike shops and stick with the larger frame or listen to the tour mechanic who swears that one should be on the smallest frame possible?

Some other parameters--I'm big. I stand 6' tall, but I'm almost "normal"-sized in torso and arms. Shops put me on 60cm frames; the tour operator put me on a 52. My home-bike is a Cannondale aluminum with the usual mid-range Shimano parts; the rental was a steel-frame Torrelli with Campy components.

Thanks for any light you can shed on my clouds of confusion.

Frame size alone won't tell you how well the bike will fit - a lot depends on the type of geometry the manufacturer uses. I just bought my new bike, tried a number of female or relaxed geometry frames, and am riding a larger frame number than I would on a standard / mens / aggressive geo. My husband is taller than me, and even though we both ride Felt but they are the same frame size number, my bike is too small (top tube) for him. (His is an F series, mine a Z)

I'd ask more questions about why a smaller frame is always better - better for what?

The build also will contribute a lot to the handling characteristics. Please don't take this as fact, but if I recall correctly - I read an article that stated a bike that feels squirly when you're standing next to it, pushing it back & forth will handle smoother at speed, and a bike that feels more stable while standing would be more squirly in use. I don't know which specific attributes make one bike feel more zippy than another - but I do know that I test rode a bike that Wanted to Sprint, and then the one that I bought simiply called out "ride me! ride loooong miles! let's go!"

I find you question very interesting. I am 5'2 (inseam I think, 29) and in a Specialized WDS I ride a 51cm. I felt kind of cramped on a 48. By most of the posts here of ladies about the same height as me, most of them ride a 48.

I felt this kind of odd because I do not consider myself to have long legs. I still have a good amount of seat post showing and wouldn't want any more showing. My handlebars are pretty much inline with my seat. It just always make me wonder about my fit compared to others of my height.

~ JoAnn

I'd definitely say that a smaller bike can help with climbing. I used to ride a bike that was too long for me and on very steep hills I would have the front wheel lift up. I'm much shorter than you, but I'm also longer in the leg and quite short in the arms and torso. What it meant was that I couldn't get my weight over the front of the bike to keep that front wheel down - now it only happened on very steep hills - we're talking up in the 20%'s here, but I also now have much, much more power now that I am not stretched out. Even at my shortness shops tend to want to put me on a bike that is too big - I'm 5' with about a 29ish inseam - they always want to put me on a 48, but my torso needs a 44. I would suggest to anyone who thinks that they are bad at climbing hills and/or has knee pain climbing that they really take a critical look at the size of bike they are riding - a too long bike with too long cranks can kill your abililty to get up hills.

I've also hear that the pros tend to ride somewhat smaller frames than are recommended to the general riding public. A lot of it is to get a super aero postition on the bike that would be hard to achieve on a bigger bike. I'm sure they wouldn't do it if it affected the handling since they are epxected to fly down mountains at crazy speeds. They can, I'm sure, get custom geometries that just are readily available to us though - super long stems are said to make a bike squirrely.

This is a very interesting topic. I have to laugh...you guys don't think you have long legs, but your 29" inseams go with 5' and 5'2" heights, etc. I am 5'6", and my inseam is barely 29.5".

Anyhow, awhile back, I emailed some questions to Georgina Terry. I was wondering about whether or not a WSD frame could work for me even though I don't have long legs or short torso. I was wondering if a stem adjustment could work. Despite not having shorter arms or torso, I have wondered if my bike is too long for me. One of my questions, and her reply, follows. I'd be curious what you all think:

My question: I'm curious about what you said re: stems. Does handling, balance, center of gravity, connection with the road change when you put a longer stem on a bike to accomodate fit? I am inexperienced and curious about how geometry & fit affect handling. For instance, I like to sit back on climbs, and have been thinking I would like a bike with the seat positioned well back ... so I wonder if putting a longer stem out front puts more weight forward rather than back?

Her answer: You're right that a stem that's too long can throw off the handling of the bike - the relationship between the handlebars and the wheel axle just won't cut it. Especially on the bike you buy, because it won't be a tall bike even though it will be a long bike. Try to keep the stem 100mm or shorter.

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Originally Posted by Jo-n-NY

I find you question very interesting. I am 5'2 (inseam I think, 29) and in a Specialized WDS I ride a 51cm. I felt kind of cramped on a 48. By most of the posts here of ladies about the same height as me, most of them ride a 48.

I felt this kind of odd because I do not consider myself to have long legs. I still have a good amount of seat post showing and wouldn't want any more showing. My handlebars are pretty much inline with my seat. It just always make me wonder about my fit compared to others of my height.

~ JoAnn

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And I am 5'2.5" with a 28.5" inseam and ride a 44-46! So we're all built differently, that's for sure! One thing I know I've read about a smaller vs. larger frame if you're between sizes is that a smaller frame weighs less. And, you can always get a taller stem or seatpost, but you can only put a stem or seatpost down so far. My older road bike that I sold was a Terry in the 17.5" size (which I believe is about a 44 cm) actually fit me a bit better than my 46 cm Aegis, on which I have my stem as low as it can go, and my seatpost pretty low too. The standover is *just* adequate. But part of that is the frame geometry and how the seat tube is measured -- I test rode a Trek 5200 in the 47 cm size and could stand over it just fine, but felt a bit too stretched out up top and didn't want to put too short a stem on it for handling reasons, so I didn't buy it.

I can't imagine riding a 51 cm with my build!

Emily

Thanks for the replies! Y'all have given me a lot to stew upon. Based on your comments and other reading, I suspect that the correct size for me is somewhere between the two. The 60cm frame is too long and is not as stable as I'd like on hills; the 52cm is probably a little short which is why I ended up with a long stem (see image) and may have caused my initial discomfort when standing.

Last fall I was seriously considering a custom bike, but a bad experience at the bike shop put it on hold. Then this opportunity to ride a steel frame bike with Campy parts came up and I decided Fate was controlling the timing of the purchase. I expected that the lessons I would learn from the week would be about steel and Campy. I did learn about that (oohh...how comfy are those Campy brake hoods), but now I've also seen that frame size matters as much as geometry and material.

Thanks so much!

...........but I would say a 52cm is way too small for your height. I am 5'7" with a 32+ (almost 33) inseam and I ride a 52cm very comfortably as far as seat height. My error was getting a men's frame, so while I'm fairly long legged I am short in the torso. Therefore the overall reach across the top tube is too much for me. According to my LBS changing the stem height, etc. will have a significant effect on my control over the bike. The higher the stem the less control you have because you give up some torque. the way he explained it made complete sense. So I am in the market for a better fitting bike.
Good luck, I think a lot is just trial and error.

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Originally Posted by jusdooit

I think a lot is just trial and error.

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Ain't that the truth!!!

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According to my LBS changing the stem height, etc. will have a significant effect on my control over the bike. The higher the stem the less control you have because you give up some torque. .

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And, ain't it the truth there there is art in the science of bike fit! The mechanic who put me on the 52 thought through the 60cm and concluded that the stem would be so short the effective "lever" wouldn't be there and I'd have no control over the bike.

Arrgggghhh....it is going to be a long, slow (albiet fun) shopping experience!

Hi there... frame size can be n the smallish side, but how you position yourself on the bike will have a big impact.

Below is my TT bike - it is a medium frame size, I am 5'10' (My road race bike is a large)

The seat post is out to its absolute safe max but because I am low on the bike when on the aeros, my centre of gravity is more "central"

If I rode this bike in a more upright position - like my mother's bike which has the bars high (mum cannot lean forward and put weight on her shoulders... her back is near vertical to the ground) then my centre of gravity would be unbalanced on the bike and it would, I imagine feel quite squirrelly.