Cranks: 140mm 40t or 170mm 39/52t?
 

For my recumbent project, I have two cranks to choose from and can't decide which to use. No doubt I'll probably end up trying both, but I was just looking for experiences from others.

140mm 40 tooth single
170mm double 39/52 tooth, I think

Is 140mm too small? I see people putting 150's and such on, but nothing this small. Which should I try, at least to begin with?

http://www.imagestation.com/picture/...2/ef5d2768.jpg
http://www.imagestation.com/picture/...2/ef5d2768.jpg

I had the boom brazed on before but it was quite crooked so I took it off and plan to re-braze it. To do this, I used a file to file down the fillet. Until I got down to the very last bit, it seemed very sturdy. Will brazing alone be enough to hold the boom on? I'm planning on just brazing it on, and if it falls off take another approach, but I'm just wondering if I'm facing imminent failure.

I linked a picture, hopefully the imagestation picture will show up. I won't be using that fork as the seat, but what I have done is put the handlebars down the seat post. I think they will make an okay backrest for the seat.

Try the 170 first. 140mm seems awfully small. Did you pull it from a BMX or children's bike?

yep, you're right about where it came from. I think it was a children's wannabe bmx, cranks from the front half of the bike.

Thanks for the advice, I'll be sure to let you know how the bike rides...once I get it done... :)

Kevin

I ride with 150mm cranks. My gearing is 22/34/44 front and 11-34 rear. With a 26" rear wheel, I can go down to less than walking speed to climb big hills, and I can comfortably spin up to about 55kph (going downhill, don't think I'm a racer :) ).

When you install smaller cranks without changing anything else, you effectively increase your gearing, i.e. your bike feels like you have larger rings. My rule of thumb is, one less centimeter of crank length equals 4 more "virtual teeth" on your chainrings, so your 40T ring with 140mm cranks will feel like a 52T ring with 170mm cranks. With a 26" wheel, that's a reasonable gearing if you plan on touring or casual riding, but it might not be enough if you plan to race the bike.

As for the question of how small is too small, I read somewhere that some people go down to 110mm, and even 80mm, and are happy with it. I reckon that's a bit extreme though. But imho, riding short cranks on a bent is a good idea, because (1) you save your knees (recumbents can be harder on knees that uprights), (2) you can spin faster, which is an advantage for climbing hills, (3) you can go faster on a large compact chainring yet retain a super-low granny to climb anything. and (4) shorter cranks help improve clearance between your feet and the front wheel, if you have a tire/heel interference problem.

At any rate, I don't think 140mm is too small. Maybe on an upright, although it's debatable, but I think it can be a reasonable choice on a recumbent if you feel more comfortable/efficient with them. It's really a matter of personal preference, but I personally only find advantages to shorter cranks.

If the boom is mitered and brazed to the head tube, I suspect it will be real flexy without some triangular support down to the front axle area or triangular gussets to the head tube. The torque you exert riding will cause the boom to move appreciably side to side, for one as long as pictured. 140mm cranks will give you a lot of spin, so if you can sustain cadences of 90+ it would be satisfactory. Most advocates of shorter cranks look at 150-160mm range.
Steve

Wow, thanks for all the detail! I'm not planning on racing this bike, just cruising around on it, and hopefully learning a bit so I can possibly make another with improvements.

I did a little math comparing the crank length in inches to the radius of the gear in inches. I'm not sure if this is any kind of reasonable comparison, but never the less...

arm length (in) : gear radius (in)
1.62:1 140mm 40t
1.73:1 170mm 52t
3.10:1 170mm 39t

...which is interesting. I guess thats saying that the small crank will have the tallest gearing.

Small cranks make sense to me because you have to lift your knees less high, and you have more clearance between the front wheel and your feet.

It is good to know that I wouldn't be going off the extreme end with 140's and that some people go even smaller. Thanks

I was afraid it would give me lots of flex. I was trying to think of where the flex would come from though. I don't think the joint would actually flex a whole lot. My GUESS is that it would be the boom tube flexing or maybe even flexing the head tube itself, but not directly at the joint. I'm thinking of doing something like this to solve the boom flex: http://www.biketcba.org/TRICORR/proj.../junkbike.html

I have a limited length I can make the gusset go, maybe 2 inches. I want the boom to be adjustable, so I need to leave room for it to slide. I wonder if the Minimum Insertion Distance specified for a seat post would be enough for a boom. I'm just thinking out loud now, I will just have to see what works out.

Thanks for all the ideas and discussion!

Kevin

Your boom is a whole lot longer than the boom in the biketcba site's example, but that is what I had in mind.
Seat post has to bear the weight of the rider plus all the vertical loading induced by bumps, jumps and holes. Whole different scenario from the boom which is absorbing the torque of legs pushing on the pedals. It will move, if it breaks you might fall, but not as bad a scene as broken seatpost where torn scrota, perforate posteriors and falling off the back of the bike can occur.
Steve

This biketcba? I'm not sure what example.

torn scrota and perforated posteriors don't sound like fun, but my fear is parts of my anatomy getting sucked in between the spinning wheel and the frame on a DF when behind the seat.