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Exact method for determining fore-aft position?(12 posts)

Exact method for determining fore-aft position?sodade
Apr 23, 2003 4:14 AM
I have two books and they both say different things. One says to hold the plumb bob string on some bony outcropping on the side of the knee (I hate this kind of thing because it is so imprecise and subject to interptretation). The other book says to hold the string on the front of the kneecap, but then it says that it can be either lined up with the pedal axle, or slightly in front of it. Another vagary! Does someone have a definitive answer?

Thanks in advance!
re: Exact method for determining fore-aft position?mainframe
Apr 23, 2003 5:05 AM
See www.cyfacusa.com for an exhaustive discussion. Really helps, if attempting this methodology, to have a trainer stand and an assistant.
it does not need to be exact...C-40
Apr 23, 2003 5:47 AM
This type of measurement is just a starting point to get you in the ballpark. After an intial setting, further adjustment is made on the road of the course of weeks, maybe months of riding.

Once you get an initial setting, mark the saddle rail and make fore/aft adjustments from this mark. Moving the saddle back will enchance torque and moving the saddle forward wil enchance cadence. Since power = torque x cadence, the idea is to find a balanced position that produces the most power without sacrificing endurance.

Equally important is saddle height. The saddle should be moved down by about 1/3 the amount that it is moved back or up by 1/3 the amount that it is moved forward to maintain the same height. If your saddle is set too high it will reduce cadence and power.

Stem length must also be adjusted if significant changes to saddle position are made. Otherwise, the tendency will be to use saddle position to compensate for a reach problem.

I've found it difficult to duplicate my riding position with measurements taken on a trainer. Just changed my saddle this week and took careful measurements of my knee position while riding the trainer, with the old saddle. Despite my best efforts, it was obvious that I missed my KOP position by about 1cm with the new saddle. My reach was longer than before, so I adjusted it by feel on the road.
Good post C-40teoteoteo
Apr 23, 2003 5:59 AM
I do a lot of fit stuff at work (LBS) and C-40 hit the major points. I never get super anal about fore/aft precison but I do use a plumb line from the bony protrusion at the base of kneecap on the front (tibial tuberosity?)

Think where the shin bone ends and the knee begins. On myself I just measured the point an it was about 10 cm from center of kneecap to this bony protrusion.

I am aware of both methods using two points and in a fit clinic it was demonstrated why both work--just be consistant and you'll be fine. Also follow C-40s advice about getting close and then dialing in over the following weeks.

Good luck
there is no definitive answer....Steve_0
Apr 23, 2003 5:50 AM
bike fitting is highly individualized. There is no one right answer. Opinions by individual fitters (most of whom have never performed any proper testing) and rider's physiologies differ.

I think the GENERAL method used for the AVERAGE rider tends to be 'back of kneecap over spindle; forward for 'power', rearward for climbing'. Again, this is a generality that doesnt necessarily reflect optimallity for everyone.

My personal method is 'use whatever works'. Pick a starting point, and move the saddle position once per weak until you find what works. There's no 'wrong' answer.
'back of kneecap'?? Uhh, explain this?Spunout
Apr 23, 2003 5:58 AM
I measured from the bottom/front of my kneecap (in the depression). My bob was 2cm behind the pedal spindle, maybe this is where I like it. Not sure, I should not have read this thread, now I'm going to be bothered by it!!
'back of kneecap'?? Uhh, explain this?Steve_0
Apr 23, 2003 6:26 AM
Grab your kneecap as if you wanted to rip it out of your leg. The area where you're grabbing is the 'back' of the cap.

Agree you should use what works for you....one would be silly to ignore optimality or comfort for mathematical 'correctness'.
You may want to check this link out...msmootsiemartin
Apr 23, 2003 6:08 AM
http://www.peterwhitecycles.com/fitting.htm

There is some interesting information. Good luck!
One of my favorite articles.dzrider
Apr 23, 2003 7:04 AM
Measurements are, to me, less important than feeling centered and balanced on my bike. I used to try to measure the triangle between my bars, seat and bottom bracket. Now I ride a new bike on the stand and make little adjustments until my legs move freely and comfortably whether I'm spinning or pushing a hard gear at 60 rpms. It takes a while, but eventually I find the spot.
Some clarificationKEN2
Apr 23, 2003 6:59 AM
I agree with what other posters have written about this being a good starting point to getting your fit dialed in. It's especially important to get the seat height right before you work on KOPS.

However, there is some rationale for the two different methods, which are a little garbled in your rendition. The one says to hold the string on the bony protuberance below the kneecap, then line up the bob with the pedal axle. The other, espoused by expert Dr. Andrew Pruitt in his book on cyclists' injuries (can't remember exact title), is to hold the string on the kneecap and line the bob up with the tip of the front crankarm. A quick experiment will confirm what he says, which is that one method gives the same result as the other, i.e. in this position the kneecap is "forward" of the bony protuberance below it by the same distance as the tip of the crankarm is in front of the center of the pedal spindle.
OK, am I the only guy who slides forward to spin and...BowWow
Apr 23, 2003 9:03 PM
back for power? KOPS works for me when I sit in the center of the saddle, but I don't stay there...

Maybe my technique needs refinement? dzrider above says he can find "the spot" where he can both spin and jam effectively from the same point on the saddle. Man, I've gotta do some adjusting! When I jam the big gears my saddle is at just the right height, but when I slide forward to spin the saddle is too low. Of course, I've got it set for the rearward position - if I set for the front, it'd be too high for pushing the big gears. Hmmmmm...

Steve (when I find "the spot" I'm gonna need a new saddle... oh, well, it's only money!)
OK, am I the only guy ..NO...Spunout
Apr 24, 2003 3:50 AM
I check my KOPS when sitting in my power position, I am 2 cm behind. It just works here. I ride a Lemond with a Thomson setback seatpost: I really get behind the BB.

One thing about saddle tilt, here is where a high nose helps you out: moving forward also moves you up. I find(as may most) that a Flite Ti fits best with a bit of up-tilt. I do move on the saddle depending on the work.