|Zinn's 210mm long crankset ?||t-moore|
Aug 14, 2003 3:46 PM
|I have posted in the past about replacing my 61cm LeMond with a larger frame. My current cranks are 175mm long. If these 210mm cranks fit my bike, haven't I effectively increased my seat tube my 35mm? Is my assumption flawed?
At $350, they aren't cheap, but cheaper than a special order or a custom frame....
|Top Tube length?||Jervis|
Aug 14, 2003 7:19 PM
|Before you get them you should look at a couple of things. If you do get the longer cranks, it's going to change your gearing and raise the amount of torque you're applying during each stroke. This will cause your highest gear to decrease resulting in a lower top speed (if my logic is slightly askew someone please correct it). On the other hand you will have one hell of a climbing gear. If your problem is with seat tube length (leg extension) this would solve your problem yeah, but you'd still have to look at top tube length. You shouldn't be too bunched up on your reach, this could cause problems with breathing and other muscular pains. If your top tube AND your seat tube are the wrong length you might want to look into custom designs. Another issue could be ground clearance. 35mm can be a lot when all the ground clearance you have in the first place is 50 or 60mm on the down stroke. Could be a problem on turns. Also (disclaimer- I've never ridden anything longer than 175 or shorter than 170 so this is purely speculation) having cranks that much longer than "normal" cranks might cause problems with your spin, or if you have more than one bike it will probably feel much different than your others. It seems you don't want to shell out the cash for a custom frame (me either) so I can understand wanting to make the one you have fit. I don't know who makes anything bigger than a 61. Jeez what a rant. I gave ya probably a whole lot more crap than you needed and/or wanted so disregard what you please. Maybe you could look here for used custom frames in the classifieds. Good luck
|Thanks for the insight...||t-moore|
Aug 14, 2003 9:53 PM
|If I understand you Jervis, and I know you are speculating like myself. My current cassette is a 12-25, according to your logic, are you saying the effective gear will change from 12 to say 13 because of the increase in stroke?
Aug 15, 2003 3:45 AM
|Ther is no change in the gearing no matter how long or short the crankarms are. There is a change in the apparent amount of pressure that must be applied to the pedals in order to produce a given amount of torque to the drivetrain. Look at it this way: the crankarm is a lever, and the longer the lever the less apparent pressure is needed to produce a given force at the opposite end, as long as the fulcrum and lift point remain the same. In the case of the cranks, the bottom bracket spindle is the fulcrum and the chainring is the lift point. Be advised that very long crankarms have their disadvantages: sharply reduced cornering clearance and difficulty in maintaining a reasonable cadence. You are increasing the circumference of the circle that you spin the pedals around, so as this increases the speed that your foot must move at in order to maintain a given cadence also increases.
I'm 6'7" tall, and have tried cranks as long aw 200 mm. Admittedly, I'm a natural spinner, not a gear masher, but I found the disadvantages of long crankarms greatly outweighed the advantages and went back to 175 mm.
Aug 15, 2003 9:27 AM
|I appreciate your insight, what size frame do you ride, and what is your inseam. I am 6'4" with a inseam of 38.5?|
Aug 15, 2003 10:34 AM
|I'm kind of short legged for my height, with an inseam of about 38.5" My bikes are 63cm, 63.5 cm and 64cm. The 63 and 64 have a 61cm top tube, and the 63.5 has a 60cm top tube. I like lots of drop from the saddle to the bars, but that's because I have short legs and arms like an orangutan. Certainly not because I'm young and flexible!|
|The frames you mentioned - measured c-c or c-t.....Thanks||t-moore|
Aug 15, 2003 11:46 AM
|Actually it DECREASES the effective ST length.||brider|
Aug 15, 2003 10:16 AM
|The effective ST length is determined by the BOTTOM of the pedal stroke, so you'd have to lower your saddle.|
|That's the point, here is a pic of my seat post...||t-moore|
Aug 15, 2003 12:02 PM
|So you bought the wrong frame?||brider|
Aug 15, 2003 12:44 PM
|Right, the longer cranks would force you to lower your seat by the difference in the crank lengths, but it's also going to close your knee angle at the top of the stroke, and likely also make you want to come forward over the cranks (to avoid feeling like you're goose-stepping at the 3 o'clock crank position). Aside form that, there's the ground clearance issues...
It might be an interesting exercise to give it a try, but I'm not betting on positive results.