Tech help"dumb question"

[dsimon]

hey i have a question. whats the difference between 170 and 172.5 crank besides 2.5? for example i have a csi with 170's and can believe that i have ridden only 170's as far as i can remeber. but i bought a new to me serotta legend both bikes same size so what will the difference be on the legend that has the 172.5? thanks for you help and support now i finally have 2 bikes :D

[sirroada]

Well, if you were custom fitted for a the Serotta Legend and the fit called for 172.5 then you are now riding the proper length crankarm. If your measurements call for a 172.5 and you are riding a 170 then you are losing pedaling efficiency. In other words, your leg is not extending as far as it should to produce the maximum amount of torque your limbs will allow for. So the longer crankarm will allow you to produce more efficient power for a longer period of time without placing undue stress on your knees. Now, if the fit called for a 170 and you are riding a 172.5 then you will have to lower the seat height 2.5mm in order to maximize your pedalling efficiency however now you are positioned to low on the bike relative to the proper fitting top tube length. Now we are talking low back and shoulder fatigue. Or you could just say...it's only 2.5 mm.

[dsimon]

??????????? :crap: wow im not properly fitted i bought a 55csi they way it was set up adjusted the seat hight slightly a violla its perrrrfect so should i change the cranks on the legend when i get it to image my csi or play around with it? :confused:

[Serotta PETE]

If both are working for you,,there is not a problem. I have a bike with 170 and another with 172.5. Seat post adjustment takes care of size difference in crank arm. (which is very little).

As to effieciency,,,,I hear that 172.5 gives more leverage but for me - -my speed is not fast enough to know. Also if you are a "spinner" the 170 might do it "nicer" on knees.

I would not rush out to change either - - -unless you are having problems and then I would let a GOOD fitter look at your position and riding style before doing anything,

PETE

[dbrk]

While it can be said with some veracity that it is the last 2% (n.b., euphemism) of bicycle design and fit that make all the difference (since most road bike designs are long since decided within certain historical boundaries), the difference here in crank length is not a place to be especially concerned about. Those 2.5mm can be easily made up in setback and whole power thing is, well, you decide (I think it's insignificant).

So I wouldn't pay a fitter for this (or much else! another opinion not widely shared...that's okay) because of all the places to look for a difference, this is not one that makes all that much difference.

What difference does this difference make is the issue. Not much.

dbrk

[Ken Robb]

Most of my bikes have 175 cranks. My "Classic" w/Nuovo Record gruppo is 170 and my Allrounder is too. I can tell that much difference in that the shorties are a little easier to spin than the longer ones--but not much--and there is a bit less leverage. The CSI has 172.5 and I can't tell the difference from the 175 bikes and I've never gone right from one of the 170 bikes up to the 172.5 so I don't know if the difference would be noticeable to me.

My Legend came with DA 10 spd. 180mm cranks and I could really notice the extra leverage. Imuscled up hills that required a triple on shorter cranks but------I felt some extra strain in the knees. I don't think it was because of the extra length per se but rather that I was encouraged to really push the too big gears. I went to a compact with 50-34 and my problem was solved.

Bottom line guess from me: if you adjust the new bike so you are comfy I doubt that you will notice 2.5mm one way or the other.

[Fixed]

bro lower seat. cheers :beer:

[Dave]

Well, if you were custom fitted for a the Serotta Legend and the fit called for 172.5 then you are now riding the proper length crankarm. If your measurements call for a 172.5 and you are riding a 170 then you are losing pedaling efficiency. In other words, your leg is not extending as far as it should to produce the maximum amount of torque your limbs will allow for. So the longer crankarm will allow you to produce more efficient power for a longer period of time without placing undue stress on your knees. Now, if the fit called for a 170 and you are riding a 172.5 then you will have to lower the seat height 2.5mm in order to maximize your pedalling efficiency however now you are positioned to low on the bike relative to the proper fitting top tube length. Now we are talking low back and shoulder fatigue. Or you could just say...it's only 2.5 mm.

2.5mm is a very small change. No fitter can optimize the bike geometry, "pedaling effieciency" or the saddle height that accurately. Your statements are baloney, IMO. There's no way for a fitter to know for sure whether a 170 or 172.5mm crank length is best. In reality the 170mm might be better for flatter terrain and the 172.5 better for climbing.

Lowering the seat by a measly 2.5mm won't make any significant difference with regard to the TT length either. If you're that picky about fit, all you have to do is remove 2.5mm of spacer from under the stem when the saddle is lowered 2.5mm.

Just because a saddle a few milimeters does not mean that the leg is not extending as far as it should. It's just as likely to cause a change in the angle of the foot as the maximum leg extension. Riders with the same leg length can have a saddle height that varies by 2-3cm depending on the angle of the foot at the bottom of the stroke. It's not accurate to assume that a rider can't change to his foot angle.

[Fixed]

bro almost all my bike have different crank lengths I go from 165 to 172.5 after a few miles I don't notice . it's just another bike. i.m.h.o. dave is the real expert on these things cheers :beer:

[sirroada]

2.5mm is a very small change. No fitter can optimize the bike geometry, "pedaling effieciency" or the saddle height that accurately. Your statements are baloney, IMO. There's no way for a fitter to know for sure whether a 170 or 172.5mm crank length is best. In reality the 170mm might be better for flatter terrain and the 172.5 better for climbing.

Lowering the seat by a measly 2.5mm won't make any significant difference with regard to the TT length either. If you're that picky about fit, all you have to do is remove 2.5mm of spacer from under the stem when the saddle is lowered 2.5mm.

Just because a saddle a few milimeters does not mean that the leg is not extending as far as it should. It's just as likely to cause a change in the angle of the foot as the maximum leg extension. Riders with the same leg length can have a saddle height that varies by 2-3cm depending on the angle of the foot at the bottom of the stroke. It's not accurate to assume that a rider can't change to his foot angle.

Dave, You are correct about foot angle. Everyone is different and depending on how their foot is positioned at the bottom of the pedal stroke (dorsiflexed, plantarflexed or neutral) can determine geometry and measurements of a bike frame and a good fitter should be able to determine this by observing how their client pedals the MAJORITY of the time. A good fitter SHOULD be able to optimize all the factors you mentioned. Lance Armstrong gets his bike dialed in exactly, in fact, he has earned the nickname of "Mr. Millimeter" because of this.
Lets look at this from an engineering point of view. Our goal is to turn a gear. So we need a lever (crankarm) that is attached to the gear. Then, we need an engine to turn the lever. That engine is our leg, which is a billion times more complicated than a gasoline engine because the leg is not rigid(like a crankshaft, connecting rod, and piston are), it has three bones/joints that will affect the lever (the hip [pelvis/femur], knee [femur, tibia]and ankle [tibia/calcaneus/talus). The femur is one length, the tibia another, and the foot yet another length. In reality all these bones are levers driving the main lever (the crankarm). In addition, the power source that runs the leg is not indefinately renewable (like a gasoline engine). It builds up lactic acid which dereases its overall performance. It can also develop pain in the joints, which will decrease its overall performance. So it is important to position the leg in a manner so as to have a mechanical advantage over the lever. Also, the muscles are a certain length and will only stretch so far and contract so hard for only a certain amount of time before they begin to build up lactic acid. So you don't want to constantly overstretch them (if the lever is to long) or over contract them (if the lever is to short) or you will prematurely tire the engine out.
So if a leg is driving a lever that is too long for it, stress is placed on the joints and on the muscular system in the form of improper biomechanics. Improper biomechanics will eventually produce pain in one of the joints/muscles and cause the engine to tire prematurely (or quit working all together in the case of an injury). If a leg is driving a lever that is to short for it the muscle can't develop optimum strength or efficiency because the leg can't get in a position to develop a mechanical advantage over the lever.
Being that you think my statements are baloney...let me ask you this question. If you were to build a gasoline engine to run the cranks on this bike...do you think 2.5mm would make a difference in how the engine operated? Honda has a tolerance in their engines around 1/1000th of an inch. I think 2.5mm would make a huge differece. Why do professional athletes spend hours studying their postion on the bike? Why do they do it while hooked up to machines that measure VO2 and wattage and areodynamic friction? They do it because every little bit makes a difference.

[dsimon]

hey hey guys be nice i just wanted some not trying to start any fights :argue: