crank help?

[justbob20]

I have new V20 with Sram red gpx compact 50/34 with 165 mm crankarms. I have boom bottomed out and find that I have over flexion/extension. My thought is shorter crank arms, 145/155 ? and adjust the boom accordingly. Is there a way to accomplish this w/o having to change the whole crankset/bb etc. Also, if this change is made shouls I consider bigger gear which of course would mean the works changed ? My first recumbent, and I am not tech savvy enough to know pros, cons idiosyncrasies etc. Currently churning 50/34, 11/36 feeling like I need more. Please advise my recumbent bros and sisses.

 

[LarryOz]

Hi Bob,
Congrats on the new V20! Welcome to the Tribe!
Do a search on this forum for "crank length" . You will find a ton of help.
It sounds like you will need shorter cranks. I did. I ride 140's now and they are very nice!
Knees do not hurt, legs don't flail all over the place at 110 rpm, and you can spin up quicker. I do not see a downside at all.
I also did not really have to change my chain ring when I changed either and manged to ride the "Assault on Mount Mitchell" with a 38 small ring and 32 large cog, but that is really dependent on what kind of terrain you will ride.
One of the threads has a link to a recumbent crank tool. Here it is: http://www.recumbents.com/wisil/misc/crank_angle.asp
This should get you in the right ball-park.
I don't think you will be able to shorten your SRAM red cranks.
I have had best luck with Apex - Rival. You can pick up a set for under $100 easily on amazon or ebay.
They are solid aluminum and you can probably find a local guy a any machine shop to thread them for you pretty easily and cheaply.
Where do you live?
Larry

 

[unc99]

I believe some others on the board have discussed cutting the boom to shorten it so that the cranks can be pulled closer in. One option would be to contact Cruzbike for their thoughts on this issue.

 

[ratz]

I have boom bottomed out and find that I have over flexion/extension

To clarify you are saying that you feel that both of these are true:

1. Your knee is bending too much at the back end of the stroke
2. The reach is too far when the pedal is fully extended

Two Photos of you sitting on the bike would help a lot; one with your leg all the way back and one with it all the way out. T here is a tendency to want to sit too far back or have a shorter boom length when learning; fixing just one can be done without shortening the cranks.

 

[justbob20]

Dang you guys are quick to respond! Ratz that is the case

 

[justbob20]

leg fully extended lying low and up on seat, toes pointed, flexion ~ 100/110

 

[LarryOz]

I have boom bottomed out and find that I have over flexion/extension.

Bob, pictures are worth a thousand words - sometime even more... So get in your best cycling kit and get us a couple pictures!
Also as noted above, many people (of shorter stature) find it necessary to shorten their boom. I did. You can probably wack a good 4" off of it and 95% people would not have an issue.
Just don't cram it down in there too far to try and get the boom shorter - it have been know to become hermetically sealed!

 

[ratz]

Before buying cranks and cutting booms;

Step one is to make sure you have the "A" length of your boom set correctly. PDF attached if you want to print this off. You want a boom length that allows around 5mm of knee clearance at the handle bars. This will tell you if you need to cut your boom to make it shorter; or if you have set it too short. "A" is measured from the center of the BB. I'm 5'9". The distance to the top of my knee (wearing bike shoes) happens to be 63.5 cm I have 165mm cranks. So my boom A length is 63.5 + 16.5 + 5 = 85 cm. That requires me to cut my boom 2 include to get some adjustment on either side of the range

Interestingly my wife is 5'7" but she also has a 63 cm knee height. So our booms are set to roughly to same length; on her bike the handle bars and the entire boom is translated almost 2 inches further back into the cockpit.

Boom length is about the length of your lower leg; and Boom position is a function of your arm length relative to your height. It's important to get those dialed in; then you will have be able to tell if you need shorter cranks.

 

[justbob20]

great info, and I have seen measurement instructions similar on posts here, and by that I would need to cut my boom. But am I missing something in the fact that this would increase flexion degree? I am also 5' 9" BTW

 

[justbob20]

LarryOz, enjoyed race videos, rock on!

 

[ratz]

great info, and I have seen measurement instructions similar on posts here, and by that I would need to cut my boom. But am I missing something in the fact that this would increase flexion degree? I am also 5' 9" BTW

If you are 5'9" and you shorten the boom; then you can move the entire boom as a fixed whole further out. That outward translation will straighten your arms and reduce the flex in your knee. From your avatar if that is your bike, and you are 5'9" then your handle bars do look a bit too far back.

It doesn't make sense until you do it; I was very confused by it 3 years ago.

The length gauge on your boom will wind up being something like 13 or 12 of the hash marks if my guess right; but again lower leg length it the driving factor

You might still want shorter cranks but this is the first step; to getting the length and reach dialed in. Being 5'9" I find I can ride 155-165mm cranks anything shorter is too short and anything longer is too long.

Another thing to consider is that when learning it's easier with the boom back closer to you; and as you get better the desire is to push the boom out more is common. In both cases the boom never changes in length.

 

[justbob20]

got it and will give it a go, thanks a bunch!

 

[LarryOz]

great info, and I have seen measurement instructions similar on posts here, and by that I would need to cut my boom. But am I missing something in the fact that this would increase flexion degree? I am also 5' 9" BTW

It's kind of a balancing act.
It may take several iterations of changes to get what you like and what feels good.
You also have to take into account your pedaling style. If you really point your toes as far forward as possible (like Maria P), then you can probably handle a little longer cranks.
Me - I like my toes pointed to the sky and feel I can make better power that way.
Of course it is a little bit difficult to do if you don't have several different lengths of cranks to play with.
But to get started, once you have the boom short enough that you can at least move it to what you think is a good location and get you toe point and leg flexion (almost straight) on your "push", then all you really have to do is see where you knee is on your other leg which should be at it's maximum "bend" for your "pull" stroke. If you have someone help you, you unclip and "move" you leg and foot forward until you knee is about at a 90 degree bend. Then measure the difference to what the pedal is. Take 50% of that value in mm, and that is what you will need to subtract from you current crank length for it to match up.
You would also theoretically have to push the boom out that other 50% to get your same "toe" point which the shorter cranks.
Hope that makes sense.
Or you could just make a set of these adjustable cranks - and keep changing the pedals until you get the right fit.

 

[justbob20]

I think you guys turned the light on for me. Can I just remove FD cable and chain with boom still attached to front fork, or will I have to remove crankset and boom alone, make sense?

 

[LarryOz]

I think you guys turned the light on for me. Can I just remove FD cable and chain with boom still attached to front fork, or will I have to remove crankset and boom alone, make sense?

Not exactly sure what you are asking:
But if you need to cut the "slider" (the piece that 'slides' into the boom), which is the piece that you want to cut (the smaller diameter piece), then you will probably have to minimally unhook your FD cable, release the skewer, and loosen the clam-shell bolts around the BB. This should let you pivot and slide the "slider" out of the "boom". And that is if you are willing to "cut" it while still attached to the fork. But if you want to cut it totally off from the bike, you are going to need to remove that clamp above the fork and then unscrew the handle bar from the other end. If you have the FD cable laced through then you'll have to pull it out all the way too. Then you can take it to a "safe" location and whack it off.
The chain, BB, and cranks are all connected to the chainstay and boom, so you won't really have to touch them.

 

[ratz]

If you are using a pipe cutter (cheap ones can be had a homedepot) and not a hacksaw .

1. Put bike front wheel in a floor stand
2. Disconnect front derailleur cable
3. Loosen BB clamp bolts
4. Remove Bolt from Boom Fork Stem Clamp
5. Loosen wheel Skewer
6. Put a towel on ground and lower crank to ground
7. Position Boom vertically
8. Loose boom clamp
9. Slide handle bar slider off boom
10. Cut boom

Reverse steps to re-assemble.

As you can see in this photo you can leave the chain on.

 

[justbob20]

YES, perfect, you guys are awesome !,,,

 

[LarryOz]

Reverse steps to re-assemble.

Gosh Ratz - you sure have it all together, with numbers and everything.... as expected - you are the Cruzbike King assembler....
But just one question ..... - if you reverse step #10, don't you just end up with what you started with?
Sorry, I just could not resist

 

[JOSEPHWEISSERT]

If you do end up going with short crank arms, my height is 5 feet 9.5 inches with a 42.5-inch x-seam. I use 150mm cranks and love them. I could probably drop to 145mm or even 140mm, but have not tried those lengths. Also, I had a local guy use his expensive computerized mill to drill out my cranks on my stationary recumbent for winter training (150mm) but then I had to go to a local bike shop to get the holes tapped because the size and thread are uncommon for tool shops apparently.

 

[ratz]

Gosh Ratz - you sure have it all together, with numbers and everything.... as expected - you are the Cruzbike King assembler....
But just one question ..... - if you reverse step #10, don't you just end up with what you started with?
Sorry, I just could not resist

IF step 10 is the chip barrel at Sebring, you are saying you ride around it twice?