Re: Re: Help me, suggestions please
[QUOTE]
Originally posted by Roger Davies
[B]Dan,
You are still thinking bikes, not cycles. You are not comparing the right
factors.
Crank length is the length it is on bikes because of the power can be
applied to it efficiently for that length. Correct. Note power. Unicycles
do not travel at 40mph because they can not because of a series varying
factors, so power is not a critical factor. Unicycles are fixed gear and
not only that they are fixed at 1:1. The force applied to 170mm cranks on a
20" (or even a 26") wheel with your foot moving at 90 cadence would be
massively redundant (Muni is the exception but there is it similar to what
bikes use) and you would move very slowly.
Now forget cadence, it is almost irrelevant in this discussion, we should be
talking about the speed that the foot actually moves and of course it’s
direct link to the force that it can
apply to the system. You can talk cadence on a bikes because of similar
crank lengths and is simple to conceive. Do some maths and look at the
speed of the foot on a 125mm crank on a 24" wheel doing 10mph then on 170mm
on a bike with gears doing 30mph - similar!
Look at the spreadsheet at it gives you some indication of this
http://www.unicycle.uk.com/cranklength.xls
The comments about shorter cranks causing more stress on knees is the
opposite to the evidence that we have on the Minnesota and other long rides.
The knee problems on bikes are normally cause by the stress at the knee from
the forces applied. These forces on a unicyclist knee are tiny in
comparison (look at the energy systems if you don’t believe me) and are not
normally a problem, it is the problem with wheel wobble that causes
problems, this twists the knee at the point where power is applied. If you
have shorter cranks then the movement of the knee is reduced proportionally
by the length of the crank. Learning though is the one time when too much
force it applied to the pedals, this is common and you need to concentrate
on applying your weight to the seat and reducing it on the pedals. This
make your riding smoother and controlled ride. DM’s research on pedal
pressure has shown this. What you loose with shorter cranks is agility, on
a learner this is quite often because additional correction is required due
to over high forces on the pedals. This is why 150mm cranks are sold on 24"
unicycles but are often changed to 125mm when the rider has got more
experienced.
Roger
Roger,
After reading your post, I agree that I must be a bit confused. I gather that what you are saying is that most riders are spinning small wheels in situations where torque or power is not required. If most unicyclists are taking part in activities that inherently do not require torque, than by all means you are correct. Using 170MM cranks with small wheels in activities that do not require torque or power would be inefficient, and foolish. You are in the business and have a far better feel for the needs of others than I.
But as you pointed out, there are some exceptions where either power is, or should be a bigger factor.
You noted that muni’s are an exception. My guess is the reason for the longer 170MM cranks is because leverage is needed to assist with ascending and descending steep hills. In this case, speed is secondary to torque or power. If smaller cranks were used, the vehicle would be faster and more efficient (less foot speed) as your chart suggests. But since torque is required, larger cranks are selected. So, a vehicle that enhances one’s ability to exude torque or power is essential in the sport of mountain unicycling
As we know it, speed and distance unicyling is a sport where, unlike muniing, power and torque is a small factor. As your chart suggests, a Coker spinning 140 RPM’s with short cranks is more efficient (measured in foot speed) than 120 RPM’s with longer cranks. After using shorter cranks myself, I confirm that short crank- high cadence traveling on Coker’s is much faster and more efficient than settling for slower cadences that result with longer cranks. Again, the chart reflects this in foot speed. Contrary to muniing where speed is secondary to torque, in speed and distance unicycling, torque is secondary to speed-hence, short cranks. After all, as you pointed out, sense we are limited to one gear, and most are 1:1 ratio, one has to make a choice between the two.
What is critical to keep in mind is that in the sport of speed and distance unicycling, power is a small factor ONLY because of a limited choice in pneumatic tires. For the most part, our choice is the Coker. If fast speeds are desired our only choice is to use short cranks. For most people power or torque is minimal with a 36-inch wheel, therefore short cranks are desirable. However, a much more effective method to increase speed is to move to a larger wheel, bringing pedal dynamics equal to the bicycle, giving power its much needed place in the sport. In the sport of speed and distance unicyling, the biggest reason we are a " different breed" compared to the bicycle, is that a bigger pneumatic tire in not available. Roger is exactly right-- power is a small factor
Making power a bigger variable by increasing wheel size shouldn’t be a difficult concept. In cycling large gear, large crank combinations at slower cadences (95) compared to small wheel (36 inch Coker) small crank (110MM) fast cadences (130+) are always chosen. Years of research and practice prove the later much less efficient than the first. Simply put, the sport of speed and distance unicycling needs a different vehicle. As I have stated many times, more efficient faster rides will be realized with Harper’s hub. David Stone’s last review of UNI5 adds credibility to this analogy when he reported the same speeds at the same effort as his Coker. This hub effectively gives us our larger wheel we need. Harper is a genius and probably isn’t even aware of it.
As always the proof is in the pudding. We are not too far from someone coupling Harper’s hub with a 700C wheel. I love “I told you so situations”.
As for the gentleman with knee problems-my suggestion is to look at your riding style. If you participate in a sport where little or no torque is required, than the smaller cranks are correct as Roger suggests. However, if torque is required than I am confident your knees will benefit from longer cranks.
The same hills that I used to succeed at climbing on my Coker with 6-inch cranks, I am unable to climb with 5-inch. My knees feel the difference. With that said, I think it probably boils down to the individual.
dan
