i want to get a coker but i don t know what a good crank length. i want to go real fast but I want some good torque going up hills. let me know!
I have 152’s on mine. I didn’t choose them, they came that way. I can’t imagine going shorter w/o brakes. I almost come off now going down hills. The same hill I just make it up. I am sure as I increase my proficiancy, I will be able to handle the hills a little better. I have to say that one the flat it is the most incredible feeling. The wheel starts turning, seemingly w/o effort… almost pulling your legs along. The ride is sooooo smooth. I am unicyclist.com’s newest Coker convert.
I’ve been using 140 mm (5.5 inch) cranks on my Coker. That length seems to be a good compromise between having good power and torque for going uphill, slow speed control, and speed on the flats.
I may try 125 mm (5 inch) cranks. But for now, I’m liking the 140’s. The 140’s let me go fast enough. Any faster and I’d be cruising at faster than my run-out speed, and I’m not to anxious to cross that line yet. The 140’s do well for me in hilly Seattle. I do have a brake that makes it much easier to maintain control on downhills. What’s the fun if you can’t make it down a hill that you just went up?
How big are the hills that you plan on riding? You’re in Colorado so your hills can get pretty big. If you’re thinking of climbing a mountain pass then crank choices are going to change.
At this stage in your Cokering career, it is too early to be thinking about changing the standard cranks.
With me riding, the standard set up with 150 mm cranks is good for speeds of up to 16 mph, and average speeds of between 10 mph and 13 mph, and it’s good for some fairly tough cross country riding, up and down hill.
With me riding, two years ago, it was good for tottering about the car park and falling off the back every now and then.
125s increase the cruising speed but, in my case, made no difference to the top speed. Nerve (or stupidity!) is what dictates your top speed on a Coker. Some people put 110s on, or even shorter. Others put 170s on because they ride lots of steep hills.
Get some miles in on the standard set up. One day, you will know whether you need to make changes.
I am using 150s on mine. They work really well on hills…I can climb almost anything I can climb on my muni. I’m finding, however, that as I get more experience on the 36", they’re becoming a bit limiting for overall riding. I’m going to be purchasing a pair of 140s soon, with a goal of rotating them with the 150s depending on the ride. My favorite current Coker ride is 23 miles of 2% downhill. The 150s are not efficient for this ride. Plus, I want to see how many of my local hills I’ll still be able to climb with the shorter cranks.
My 150s are silver. I will probably buy my 140s in black, just so I don’t have to mount up to remember which set I’m using.
John Childs is also urging me to get the 140s, as this will eliminate my last remaining excuse for why he is faster than me.
I’ve never really understood this 140/150 thing. The difference is only around 7%. In terms of speed, that’s worth about a mile an hour at the top, and rather less on the cruising speed. On the other hand, the extra 10 mm on the 150s will be worth a bit more than that (maybe 10% or so) in terms of torque on a steep hill when the contact patch of the tyre is “uphill” of the hub.
I guess it’s mainly down to comfort. Find a size that suits and ride accordingly.
150mm is a good length to start with- alot is dependent on your riding style and fitness but if you’re starting out then 150 will give you a reasonable amount of leverage for almost all road riding. Anything shorter can feel very scary at first, but once you’ve gotten used to the Coker- I think 125mm are much more comfortable to ride on the road. It seems to spin really nice. 125mm works fine on hills too- it’s surprising what you can crank up if you stand on your pedals. And it’s not that scary going downhill either- there is much less choppy leg movement because of the small pedalling circle. In fact- if I’m going really fast downhill the 150mm scares me silly- feels like my legs are flying everywhere.
So now for me:
150mm for most off-road Cokering. Maybe 170mm if it is really steep/technical
125mm for road including hills
110mm for flat road- I’m still working on this with my TT Coker
I’m not sure what the contact patch has to do with torque. Applied torque is proportional to force on the crank perpendicular to the crank. It has nothing to do with the rider’s balance or forces on the wheel due to the hill. Since torque is F*d, increasing d by 7.1% will result in a torque increase of 7.1%. You may be implying, Mike, that longer cranks also allow one to increase F, or force on the crank perpendicular to the crank. This would have to be something along the lines of better ability to put one’s body weight on the pedal due strictly to the longer cranks. In my opinion, it would take some more details to justify a combined (F,d) increase in applied torque of 10%.
This is the best answer, it seems to me, since there are other factors such as one’s leg dimensions and geometry, pedal/foot match, handle configuration, and even personal weight distribution that affect hill-climbing effectiveness.
It’s a comfort thing for me. It feels better to spin with the 140’s than the 150’s.
I do notice the difference between the 140’s and 150’s on some hills. The difference is only really noticeable on tricky climbs where it’s necessary to do a switchback style turn or some other turning maneuver during the climb. With the 140’s I’ll sometimes stall out and won’t be able to get the pedals going again after trying to turn. With the 150’s I’ll be able to recover and get the pedals going again. The extra leverage of the 150’s does come in handy sometimes.
I’m going to have to try 125’s sometime. But for now I’m happy with the 140’s.
I do agree that it is way to premature for dubmuni2004 to be thinking about changing the crank length from the stock 150’s. One should first get very comfortable with the 150’s before thinking about trying anything shorter. Trying short cranks on a Coker before being comfortable with the 150’s is asking for trouble (and loss of control).
Back to the torque on hills thing…
Perhaps torque was the wrong word. I’m not an engineer.
However, the pedal and crank act as a lever, but the fulcrum is not necessarily the hub. Just because the wheel rotates at the hub, it doesn’t make the question of leverage simple.
Put a unicycle against a vertical cliff. Imagine (for the sake of argument) a tyre with perfect friction, so it will potentially stick to a vertical cliff.
Now, on a 20 inch wheel with a 5 inch crank, and the crank horizontal, the centre of the pedal spindle is half way between the hub and the contact patch between the tyre and the cliff. So, pushing down on the pedal will NOT help the unicycle to climb up the cliff.
Now put the same unicycle on a perfectly horizontal surface. With the cranks perfectly horizontal, the pedal spindle is 5 inches to one side of the contact patch, and downward pressure on the pedal will cause the wheel to move.
Now find a quarter pipe.
Position the unicycle with the cranks horizontal, and place it carefully at various heights on the concave slope of the quarter pipe.
There will be a critical point at which the centre of the pedal spindle is exactly above the contact patch between the tyre and the slope.
If the slope gets any steeper, then the pedal is on the wrong side of the contact patch, and downward pressure on the pedal will not cause the uni to rise.
If the slope gets shallower, then the centre of the pedal spindle is “uphill” of the contact patch, and downward pressure WILLcause the pedal to go down, and the rest of the unicycle to rise.
I recognise it isn’t quite this simple, becase there are a number of other factors, like momentum, a continuous circular pedaling action, and whether or not you pull up on the seat. However, the basic principle seems sound: as you are grunting up a hill, you stomp down on the pedals in the hope that the pedals will go down, and the leverage will act through the contact patch to cause the uni and rider to rise.
So, imagine a situation where the pedal on a 140mm crank is exactly 10 mm “uphill” of the contact patch. Now swap to 150 mm cranks… the pedal is now 20 mm “uphill” which will give approximately twice as much hill climbing ability, call it torque, grunt or whatever.
Now, I accept this is a very simplified explanation, and that the maths will not be simple, because the pedal is only in this position for a tiny part of the pedaling circle, and for a dozen other reasons, but it seems clear to me that as the slope gets steeper, the extra length of a longer crank makes more difference than it would on the flat. This is because we are not simply looking at the “magnification” of the movement of the rim, compared to the pedal, but we are also looking at whether the pedal is moving down relative to the hill, or the unicycle, or both.
This diagram might explain, if I can work out how to post it:
hillclimb.bmp (108 KB)
Hmmm well not to push this thread “over the edge” like so many have gone before…
Imagine you are sitting on a seat behind the unicycle wheel, with a counterbalancing weight in front. Just by pedaling you could still go up the hill, even though your feet are completely behind the contact patch.
Your point of view is difficult to work with. This in physics is called “reference frame”. It is easier to find insight in some reference frames than in others. Many times the skill lies in finding the right way to look at things. For example, in rectangular coordinates, a circle is hard to describe; in circular, easy (r=4.5, say).
Here let’s split the notions. First, the only thing pushing the wheel forward along the ground is pressure along the surface of the ground. The only way to do this is to apply pressure on the pedal perpendicular to the crank (torque). Any pressure parallel to the crank doesn’t help.
Second, the only way the rider is going to stay on the unicycle is for the center of balance of the unicycle to be vertically ahead of the contact patch, otherwise when he applies torque the wheel will move ahead of him and he will fall off (oh, the memories).
By examining these concepts carefully, we can see that, if we had a way to apply torque without shifting our weight forward, all we have to do is keep the balance point of the uni slightly ahead of the contact patch, so that we move forward. Assume that we have infinite friction at the ground so the wheel won’t slip.
However, when the hill gets steep we don’t have enough strength in our legs to apply enough torque, unless we shift our weight forward to put all our weight on the pedal. This is actually more than we need, but because we can’t do that at all positions of the pedals we need to invest a little bit in the momentum of the wheel to get through the vertical pedal position.
However, if we had, say, a motor on the unicycle that could apply torque uniformly around the circle, we wouldn’t have to do that, and could actually creep up the hill as easily as riding slowly on a flat smooth surface.
So although a longer crank would help us move our weight a little farther out ahead of the contact patch, that’s not the real issue, and the contact patch explanation is a little misleading. The real issue is that shorter cranks make the strength/weight limitations on the torque (F*d) more severe, not that they make it harder to reach ahead of the contact patch.
To see this, think of our motor-driven uni with perfect 360 degree torque. With shorter cranks, the motor would have to push harder, but our balancing act, just barely ahead of the contact patch, would be exactly the same. With longer cranks, the motor would not have to push as hard, but our balancing act is unchanged.
< I’m ignoring the diagram because it may just make the explanation more complex. To make the diagram useful would take more elements and vectors. >
I hope that helps, but it may not be clear enough.
I see what you mean, and, yes, I’d forgotten to allow for the fact that the rider shifts his/her mass away from the centre of the uni. However, I do think that at the absolute limit, when it’s a choice between moving and not moving, the rider can only meaningfully apply force on the pedal vertically by putting his/her weight on it, perhaps augmented by a certain amount of pulling on the handle. In this situation, I’m still not convinced my theory is wrong.
Perhaps it’s my riding style. When I’m stomping up a steep hill, I tend to put all my weight on the up hill pedal, rather than sitting own and pedalling smoothly.
In fact, the effect I’m describing may be most noticeable on descents - but again, my riding style relies on keeping control, rather than spinning out in a mad dash for glory. So I find myself using all of my weight to keep the pedal from rising. Now, where is my Centre of Mass relative to the contact patch? If it’s downhill of the CP, then I’m either going to have to accelerate, or I’m going to swallow dive. If it’s up hill of the CP, I’m going to have to decelerate, or fall on my back. So if I’m plodding down a hill, step by step, I guess my centre of Mass is almost exactly over the contact patch, and I have only my weight on the pedal, and a small amount of down force via the handle to stop that wheel from running away. And that is where I think my argument has some validity.
But you’re right, I hadn’t looked athe whole reference frame. That was partly deliverate (see all the caveats in my description) and partly laziness.
And IMHO there’s nothing wrong with a thread wandering off the initial subject, as long as it remains legal, decent, relevant and interesting.
I would go along with what appears to be a rough consensus by Coker experts here, start with the stock 150s. If you’re not used to the larger, heavier wheel, this length gives you enough to work with, and feel comfortable riding the beast. Then, depending on the terrain you take it on, you can go up or down from there.
I use 125s for riding to work, with very little in the way of hills. It works pretty good. A smaller pedaling circle might be nice, but I don’t think I want to give up any more leverage.
I tried 150s for trails, but I thought they were way too short. This is highly dependent on how much up & down your trails have, and how much you like to grunt & groan on the uphills. With 170s I was much more comfortable, except of course on the flats. But I had to admit a Coker with 170s was quite a bit of fun on a trail as long as it wasn’t going uphill!
I think cruising speed is what matters the most. I haven’t messed with top speed on my Coker much. I don’t need to find out how fast I can survive a crash. I’ll get enough of that with a 29" in the coming weeks 
When you lay the cranks side by side, 10mm hardly looks like anything. But if it’s 8 miles to work and 8 miles back, small changes in the size of circle your feet have to make are very noticeable!