RMMJ

If the saddle height is adjusted correctly, does a larger frame (61cm vs. 58cm) effect the position of the knee over the pedal? Is the knee further back on a larger frame?

Keepfiring

If both frames have the same seat tube angle and you have the seatpost height adjusted the same (check!) I don't see how it would. Only thing I can maybe think of is if your reach to the bars is further on the larger frame, you might not actually sit the same way on the saddle as with a shorter reach setup

dalava

What?

Unless you also adjust the fore-aft position of the saddle, the position of knee over pedal WILL be different.

mrvile

Nope. Say you have two bikes, one a 53 and one a 56, and the seat tube angles are both 74 degrees. If you have the seatpost set to the same height (say 73cm) on each bike, the entire saddle-to-bb relationship will be the same on both bikes. The bigger bike will have 3cm less seatpost showing, that's all.

laterzre

If the frame is smaller, the saddle should be moved further backwards to maintain the same position.

andrewluke

It should not provided the seat tube angle is the same as stated. It's a little hard to describe without having a bike to show you in person. The distance between bottom bracket to saddle is determined by a factor unrelated to the bike: your legs. If you have a larger frame with the same seat tube angle, your setback from a point on the saddle to the bottom bracket will not change no matter what the frame size. The difference is that on larger frames, the tubes are all longer. All this changes is the ratio of seat tube to seatpost - on a larger frame, you will have less seatpost exposed.

The major factors influencing knee position will be the seatpost setback, saddle fore/aft position, and crank arm length. Unfortunately, some people define position as the entire riding position but in reality, your legs should not change between sizes - only the reach which should be adjusted with the stem, not seat position

EDIT
It seems mrville was a faster on the keyboard on this one and beat me to the punch.

dalava

You are right, didn't think about the seat tube angle being the same.

jrobe

Does frame size effect knee over pedal position?


No, knee over pedal spindle (KOPS) is irrevalent no matter of the frame size.

collegeskier

I contour that it is the same but do note that sometimes in the 58 to 60 jump on a factory built bike the crank arm lengths will change, which will effect the knee over spindle position.

Road Fan

If you go to Peter White's writeup on fitting, he says knee position should be where it needs to be based on your balance on the bike, regardless of the KOPS guideline. All rider should be in balance such that with a level saddle it's not too hard to hold the bars - no excessive arm/hand pressure. For riders of different styles (low position, touring position, upright position, just to give a few common ones) the saddle and hence the knees will be in different positions. If your riding position uses a lower handlebar, your butt needs to move back from the BB to maintain torso balance. Hence the knees will move back as well. For a tourer with a little more upright bars at saddle height position, the saddle will not be quite so far back to create good torso balance, so the knees will also be a bit farther forward.

If one of these "fits" corresponds to a larger frame, then yes, knee position (KOPS or not) would be a result of frame size - but not directly.

OTOH, if you always set your saddle to achieve KOPS, then your knee position can only be KOPS for any frame of any size. AND this is only true if you have the same crank arm length.

dykim90

you just need to get a bike that fits you.

would you buy a pair of big shoes even though you could lace them up tightly?

madpogue 

With many models of bike, seat tube angle changes with frame size. Visit the mfr's web site, and look for a "frame geometry" (or some such) link for the model you're interested in.

Sir Real

affect

DaveSSS 

Gee guys, this is simple. Frame size has nothing to with KOP. If the STA is the same, you could swap a post and saddle over to a larger frame, set it to the same height and the setback would be the same.

I can even get the same KOP with greatly different STAs. All you need to do is move the saddle fore or aft. If you run out of travel, get a seatpost with a different setback. The idea that varying STAs force the rider into a different position is nonsense.

I've had the same saddle setback with identically sized frames (51cm c-c), but one had a 72.5 degree STA and the other a 74.5 degree STA. The difference in the position of the saddle rail clamp is a about 2.3cm at my saddle rail height. cos72.5-cos74.5 x 69 = 2.3 You could use a nonsetback post with the 72.5 degree STA and a common 25mm with the other frame.

scirocco

That wasn't the question the OP asked. He never said he was trying to achieve KOPS. He wanted to know the effect of a larger frame on the knee postion RELATIVE to KOPS (used as a reference point) which most certainly is relevant. And DaveSSS has now explained that.