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Old 02-20-07, 09:36 AM   #1
bomber
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frame size,seat height and center of gravity

Please endure a question from a road biker switching to mtb. I see many mtbrs with frames that look somewhat too small, but with the seats extended to almost their maximum range. I also see that many downhill crashes are over the bars type crashes. My question is: does not the high seat height relative to frame size raise the center of gravity of the rider to a point where he is begging to be thrown over? Asking the same question differently: would a rider on a larger framed bike with the seat height somewhat lower be less likely to be thrown over the bars? It would seem that the same pedal thrust could be accomplished in either case. Your expertise is appreciated.
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Old 02-20-07, 09:39 AM   #2
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MTB's are supposed to fit smaller than a roadbike.
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Old 02-20-07, 09:45 AM   #3
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I understand that, in general, mountain bike frames are smaller than road bikes. I guess I may quibble with your use of "fit". I would argue that a bike with a frame so small the seat has to be extended to it's maximum just to pedal it properly doesn't "fit". What are your thoughts on my actual question?
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Old 02-20-07, 09:48 AM   #4
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careful man, he might change you thread title to "you asked the wrong question"
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Old 02-20-07, 09:51 AM   #5
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no worries

just trying to get some real info.
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Old 02-20-07, 10:17 AM   #6
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Going over the handlebars is a technique issue. Mountain bikers can go down vertically and not go over the handlebars. Check out Brian Lopes' Book on technique.

In those conditions where the riders CG is important, your not on the saddle anyhow, you are off of it, switching most of the weight to the pedals (12"+ off the ground) and using body English to maintain control. You'll be doing the same climbing where it's steep and technical.

I use a large size ATB (5' 11" tall) and it's pretty big with a 43" wheelbase. The bottom bracket spindle sits about 3" above the wheel axels where as my road bike, built on a cyclocross frame, the spindle is even with the axels. MTBs are bigger and heavier.

One might have a shorter seat to saddle distance on a MTB to insure you can get behind the saddle on those really steep descents.

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Old 02-20-07, 10:31 AM   #7
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while a good technical rider can ride almost any size bike down any grade slope, riders with less technical skills tend to do better with a smaller frame size. The smaller the frame, the more control you generally have, since it's easier to throw the bike around where you want it.

regarding seat height, it is true that you often see mtb'ers with small frames and long seatposts. this extra seatpost length allows a rider to drop the seat, and lower their center of gravity on steep descents that they aren't comfortable riding.

here's an interesting mtb fit quote from stlbiking.com:
Mountain
Take your inseam in inches and multiply by .59 to get your recommended center-to-top frame size. For example, if your inseam is 32.3 inches, then you would multiply 32.3 by .59, which gives the result, 19.057. This rounds out to 19 inches. Keep in mind that most mountain-bike frames are measured from the center of the cranks to the top of the top tube. A few frames are measured center-to-center, so subtract _-inch from the bike size to correct for the difference in measurement.
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Old 02-20-07, 11:27 AM   #8
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Thanks for the good info. I agree that technique must play an important role, especially in mountain biking. But, where we can use physics to our advantage, we must, I think, consider this. Your responses are very informative, however, there seems to be a misunderstanding of center of gravity(cog). When standing on the pedals of a bike, the center of gravity is not transfered there. That would be like saying the center of gravity of a person is at their feet when they are standing-which is not true. Center of gravity is more a function of the mass of an object. For most people it's somewhere between the knees and the shoulders. (For some, it's the head! LOL) It seems to me that biking, which is all about balancing our cog on two wheels, becomes more stable as we move our cog closer to the center of the frame. Conversly, as we raise our cog, the bike becomes less stable. Add to this the desire for speed and agility and the frame size choice seems to be critical. So, in choosing my first mountain bike, I think a frame size large enough to allow me to keep the seat fairly close to the frame, still with some adjustability, and that will allow me the leg extension need for power is needed. Does any of this make sense?
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Old 02-20-07, 11:36 AM   #9
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On steep descents you should be off the seat and behind it. You've just lowered your center of gravity and moved it towards the back. This will help prevent the endo's you described.

If you intend to ride rails-to-trails type of terrain, you can definitely try to fit your bike like you would a road bike. Basically get a XC rig and fit accordingly. For that matter you can even get a cyclocross bike for this type of terrain.

If you intend to do anything remotely technical, then you should fit your bike the way a mountain bike should be fitted.
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Old 02-20-07, 11:39 AM   #10
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Quote:
Originally Posted by bomber
Does any of this make sense?
Sort of. There are other factors to consider, however. Smaller frames are lighter (less material), stiffer, and perhaps most importanly, have greater standover height. For those reasons, I usually try and get the smallest frame possible while retaining the top tube length and seat height required for efficient pedaling.
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Old 02-20-07, 11:51 AM   #11
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I find that I am much more mobile (meaning on, off, over, behind, etc. the saddle) while on my mountain bike than on my road bike. For that reason, I find a slightly more compact frame to be better.
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Old 02-20-07, 12:33 PM   #12
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Quote:
Originally Posted by bomber
I think a frame size large enough to allow me to keep the seat fairly close to the frame, still with some adjustability, and that will allow me the leg extension need for power is needed. Does any of this make sense?
i understand what you're saying here, but I see it slightly different. what I think you should really be concerned about is less your COG versus the bike's COG, but the combined COG of both you and the bike. On a larger frame, the bike's COG is going to be higher off the ground than a smaller frame. But, if you lower the seat on the larger frame, and raise the seat on the smaller frame, to keep your body's COG at the same place, it does not mean that the combined center of gravity of both you and the bike will be the same. At the same seat height, the combined COG of you and the larger frame will always be higher than the combined of you and the smaller frame. So, on a steep descent or other technical situations, I'd much rather have the smaller frame, to lower the combined COG of both me and the bike.

I don't think that's exactly the answer you were looking for, but hopefully it makes sense and helps!
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Old 02-20-07, 01:18 PM   #13
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MTBs have a higher COG because they have high bottom brackets. The low TT/big seat-post has almost no bearing on the location of COG.
Many modern MTBs have the bars set reletively high compared to classic cross-country riders of old. I find that on steep descents, the lower my bars the more stable I feel.
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Old 02-20-07, 01:36 PM   #14
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Gastro and MichaelW are on the mark. If you look at a road bike with "square" geometry, the top tube and seat tube lengths are the same. Compare this to modern mtn bikes, where a 17.5" frame will have a 21.5"+ top tube.

I ride a 56cm road frame with a flat bar around town. Comparing the fit of that bike to my hardtail and SS, my position relative to the BB is almost identical. I have the seats on my mtn bikes a tad lower to facilitate getting off the back, but it's within 2 cm. The SS looks tiny on me because I have 30 cm of seatpost exposed, compared to the 12 cm on my road bike. Still, the fit is almost identical between the two bikes. This is one occasion where you need to actually get on a bike to know whether it's going to fit. A small frame with a long TT may fit just as well as a "bigger" frame with the same TT length, but the smaller frame should be lighter and stiffer.
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Old 02-20-07, 04:53 PM   #15
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Quote:
Originally Posted by MichaelW
MTBs have a higher COG because they have high bottom brackets. The low TT/big seat-post has almost no bearing on the location of COG.
Many modern MTBs have the bars set reletively high compared to classic cross-country riders of old. I find that on steep descents, the lower my bars the more stable I feel.
What do you consider a "steep" decent?
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Old 02-20-07, 05:13 PM   #16
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I bet my set ups really makes you cringe. Keep in mind these are set up for cross country racing.





Okay, a couple of questions. How would it help the bike any if the top tube were straight across? How would a bigger frame benefit me if the saddle is still going to be the same distance from the center of the bb?

My seat position would still be the same. The only difference would be that I would have less seat post showing and less standover height. As long as your primary points of contact and your angles are correct it really shouldn't matter much how you get there (within reason of course). The benefits to this set up are more standover clearance, less weight.

It's kind of like a compact road frame. Some people like the way they look, some don't. However when you get down to it they perform pretty much the exact same.
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Old 02-21-07, 06:44 AM   #17
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Quote:
Originally Posted by bomber
When standing on the pedals of a bike, the center of gravity is not transfered there. That would be like saying the center of gravity of a person is at their feet when they are standing-which is not true. Center of gravity is more a function of the mass of an object. For most people it's somewhere between the knees and the shoulders. (For some, it's the head! LOL) It seems to me that biking, which is all about balancing our cog on two wheels, becomes more stable as we move our cog closer to the center of the frame. Conversly, as we raise our cog, the bike becomes less stable. Add to this the desire for speed and agility and the frame size choice seems to be critical. So, in choosing my first mountain bike, I think a frame size large enough to allow me to keep the seat fairly close to the frame, still with some adjustability, and that will allow me the leg extension need for power is needed. Does any of this make sense?

You can raise the CG as much as you want if you move that CG to balance the tipping forces.

The rider is not glued to the bike. No one has said that the riders CG is at the pedals, just the weight. Having the force of the riders mass concentrated at the bottom bracket rather than at the saddle, reduces the lever arm between the tires contact with the ground and makes things far more stable when there are upsetting accelerations due to rocks and holes. But, much more importantly, being fixed to the saddle limits the riders range of motion making more of the rider's weight locked onto the bike.

Off the saddle (could be just an inch or two) and on the pedals, the rider can move his weight (called live weight) relative to the bike (dead weight) and thus move the forces due to gravity and turning accelerations forward or backward (and side to side) relative to the bottom bracket to keep the bike from tipping over.

If you've ever done or watched dingy sailing, you know that the crew shifts their weights to keep the boat upright. You sometimes hang out way over the side. It's not CG, it's a weight and balance issue.

Another reason to be off the seat on downhills is letting the bike pivot about the bottom bracket spindle (where the riders weight is concentrated) on the bumps. That gives better steering control and puts less stress on the bike and the rider. Then there's those holes of a size which tend to grab the front wheel. Being off the saddle makes it far easier to lift the front wheel over them.

Al

Last edited by Al.canoe; 02-22-07 at 07:30 AM.
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