cyclotoine

Thanks, everyone, I'm going to stop worrying about it, let the cranks come in and throw them on, I think I could possibly use a 10mm shorter stem so these adjustments probably won't affect that very much. I'll just ride it and post my impressions here. Expect to hear from me in a month. I was thinking about buying another seatpost anyway so I might get one with zero setback.

P.S. all my frames are 80s except my 1993 lugged steel Marinoni and they are all running 175mm cranks right now except the fixie with 170 which I just use to boot around the neighbourhood for fun.

53-11_alltheway

Its not that simple. But essentially by correcting for 3 p'clock and 6 oclock position if the pedal spindle that is what you would come up with. The knees will bend more definitley if that is all you did.

MichaelW

Perhaps your knees need to bend more. If you are a tall guy riding med sized cranks, your knees will not be bending as much as a med size rider.
Using proportional cranks lets you use the same leg angles as "normal" riders.

Big frames are adjusted to compensate for a tall guy riding M cranks. If you use big cranks you dont need that compensation and can just set your saddle the same way everyone else does, using the 6:00 and 3:00 position.

There may be an issue with your weight further forward within the wheelbase BUT most big bikes place the rider very far back over the rear axle compared to small bike/riders.

53-11_alltheway

You have a point about in some cases the knee needs to bend more. For example I don't like too open of a hip angle so I lower my hanflebars to close it up (thereby bring my knee closer to my trunk)

cyclotoine

I came across this link in another thread: https://www.cranklength.info/index.htm

After spending a lot of time there I came to this: https://sheldonbrown.com/kops.html in which Kieth Bontrager dismissessed the knee over pedal spindle fitting method, this means moving your seat forward and backward the amount of you crank length change is bogus. I also feel that when I am climbing out of the saddle I have to use a lot of upper body strength and the pedals are too far behind me in the forward position. I will have to take note of how this feeling differs with longer cranks. In the original link posted above the basic calculator put me at 190. I have calculated a range of 182.5mm to 202mm for my optimal crank length. I feel 185 is sufficiently conservative as a starting point and more importantly I do not have to purchase a new frame. I will be buying a zero (or as near zero as I can find in my price range) set back post and positioning my saddle in the middle of the rails. I will then proceed to adjust it for and aft using longer cranks based on feeling. After spending well over and hour I have bookmarked the above page (I have many pages on bicycle fit and crank length bookmarked and have spent many hours on the net investigating). My feeling right now 53-11 all the way, is that all this stuff about compromising balance and needing an adjusted frame is completely bogus, the actual changes in a 1cm crank length increase are small and can easily be accomodate by small adjustments in seat position (and should only require small adjustments) the point of longer cranks is to change the angles etc.. so trying to maintain you knee over spinle would be silly as your foot would have to go even further back than before (not that I am saying anyone was advocating KOPS, I'd have to go back and read some posts). It seems bicycle fitting techniques are not hard and fast and I feel a lot better about trying longer cranks now than I did a few days ago (because this thread raised some concerns about incompatability). Sorry for the lack of coherence.

cyclotoine

This is also very misleading and in light of my recent reading I would warn against making such drastic changes for changes in crank length.

cyclotoine

This provoked me to do some calculations...

In order to move from an effective seattube angle of 72.5 to 73.5 maintaining a 75cm BB to seat rail distance (i.e. moving the seat slightly up) you would have to move the seat forward 1.25cm. Of course this woudl not translate directly into a measurment on the rails as moving the seat up moves the seat back slightly. so this justified the move to a steeper seattube angled frame.

If you want bigger cranks but are comfortable in your current position:

Since you would might move the seat down it would move the seat forward anyway. Let say you moved it down from 75cm to 74cm (talking about tall guys here) because you were increasing your crank length by 1 cm. If your frame has a 72.5 degree effective seat tube angle this will translate into a movement of the seat 0.25 cm forward leaving you with only .75 cm unaccounted for (not a big deal in light of the dismissal of the KOPS technique) However if you move your seat forward 1cm instead of lowering it this changes you effective distand from BB to saddle rails to 71.7cm in which case you have to raise your seat back up again, moving it back again etc. etc..

So what point am I trying to make? Well I guess that moving your seat down less than 1cm and your seat forward less than 1cm will maintain you old knee positions at the 3 and 6 o'clock positions as close as possible and hardly change your balance. Moving the saddle forward in any case s what you may want if you are a tall rider and your frame has slacker angles to begin with. If you are pretty comfortable currently than this hardly means you need an adjusted frame (unless BB height is a consideration). To finish all this above assumes that the rider is centered of the seattube on the bike with the stated angles.. a zero set back post slide forward 1cm is NOT a big deal!

P.S. this post was as much for satisfying my own curiousity as trying to add valuable input to this thread.

galen_52657

The human body is quite adaptable and given an adjustment period, can conform to a wide variation in knee/hip flexion angles. Just look at any group of riders - pro or tourists. Knee flexion angles at the bottom of the stroke vary from less than 5* to more than 25*. Personally, I don't give KOPS much credence. I think it is something that just developed over time along with the diamond frame bicycle. It's a reference point-of-beginning for positioning, but nothing more. Greg LeMond positioned himself behind the pedals. However, to most seasoned cyclists considering a change in crank length of 1 CM or more, I would think the knee angle at the bottom of the stroke (in line with the seat tube) would be the one thing they might want to preserve, at least at the start of using longer crankarms. Radical change in knee flexion after the power portion of the pedalstroke might cause knee ligament pain, more so I would think than flexion changes at the top of the pedalstroke.

cyclotoine

Agreed and the moving the seat forward 10mm would decrease effective seat tube length much more than say lowering the seat by 7.5mm, which also moves the seat forward and inherently moves the seat down somewhat as well (as described above). Thus when increasing crank length lowering the seat by less than the change in crank size would maintain your former knee flexion at the bottom of the stroke as close as possible..