Using PowerCalc to predict performance due changes in crank arm length.

So I got a hold of a set of 175’s in really good shape for 20$. The difference in the actual cranks and my older cranks is that the new ones have the five arm configuration, so changing rings for fine tuning my gear inch development would take 15 minutes versus impossible.

These are my findings using PoweCalc

Problem: 7% hill on 74 g.i. is a bit tough –

170 = 51.14 pounds @ pedals @ 52 rpm = 08.00 mph. – 291 watts @ pedals

175 = 51.14 pounds @ pedals @ 36 rpm = 11.22 mph. - 425 watts @ pedals!

I do not get it … I will use less pressure on the pedals, yet wattage increases.


Problem: 10% hill @ 5 mph on both cases.

170 = 71.28 pounds @ pedals. 247 watts.

175 = 68.87 pounds @ pedals. 247 watts.

Is Wattage a function of pedal pressure?

In my opinion, it should be …. But PowerCalc seems to disagree.

Who knows this stuff well?

Not just.

Power is the force multiplied by the arm length multiplied by the rate you are spinning at. In order to get Watts, use Newtons, meters, and radians per second (2*pi/60 * RPM's). If you increase the arm length, the force, or the speed you spin the pedals, you increase the power.

You're either using different gear ratios between examples one and two or you mixed up the RPM's between them. It looks like the latter, in which case those numbers make sense according to what I stated above. I didn't bother pulling out my calculator, but they look approximately right.

Note that this assumes a constant force throughout the stroke.

I'm not familiar with the term gear-inch. Can you explain that briefly?

I love this picture

... and that is relevant because.

By the way ... I look pretty much like that guy rather than the "nerd" you are trying to imply.

asshat

In example 2 (175) you are turning less rpm but going 3 mph faster! No matter what you say, the two examples are not the same size gear. Power is work over time, and going faster requires more power.

relax

In any experiment, ou have to figure out what factors to fix and what are variable.
The variables here are your crank length, what happens when you alter it.
Do you want to fix gear ratios? Not neccessarily since the same rider on different cranks would select a different ratio. See Sheldon Browns Gain Ratio measure.
The pedalling force should remain constant but the circumference of a rotation will increase with crank length so you would expect cadence to drop and gear ratio to increase a notch.

Is power output a variable: No, your body has a fixed upper limit and a set crusing level. Bike speed is the variable here.

Fix your heartrate and pedalling force. Select gear ratio and cadence to suit and measure the resultant speed. Do this on a ~5 mile time trial circuit in both directions.

Oh ... the problems with powercalc were to help me understand the forces

Anyhow ... the main problem is that I like to ride in only 1 gear, either fixed or free. Pedaling uphill gets tricky after the second mile of upward slope. The main objective is to optimize the actual bike, and crank length is one of the variables so to add torque.

... sorry for jumping on the gun like that. I wish I was a nerd and do all this math in my head.

Now ... will you contribute with the problem or not?

WTF happened to my post?

I had commented that in the MPH lines, you're gearing was different by about a factor of 2. I.E. 48/18 in the first case and 48/9 in the second.

Gear inches: https://www.sheldonbrown.com/gloss_g.html#gearinch

Although they're more popular than sheldon would have you believe .

The main thing crank length is going to give you is a change in the way your legs work the pedals. Mathematically it's no different than changing the gearing, but practically it is a question of how big a circle do your legs "like" to move in? Finding that will result in your body producing the optimal amount of torque applied to the crank.

I understand about gain ratio, development, etc. The point is, as soon as you introduce a second variable, you've invalidated pointing to the first variable as the cause. Put the same rider up the same hill using the same gear, and only the crank length is changed (s in problem #2,) and you'll find the power requirement is the same for the same speed. Longer cranks have the effect of lower gears in that they break up the total work required into smaller chunks, i.e. the weight is moved a shorter distance per inch of foot travel. But the total work required is still the same because the feet have to move further under that load. I'm not arguing for or against longer cranks, I'm just saying that PowerCalc gave the results I would expect to see.

Anyhow.

The Powercalc prediction is the following ... revisited.

One of the hills @ 5% - Nice suffering for two miles in a 32 mile ride. Pedals go @ 58 rpm. Quads and hams are burning.

170mm crank @ 39.80 pounds of pressure at pedals = 12.87 mph. // cadence = 58 // wattage = 365.9
175mm crank @ 39.80 pounds of pressure at pedals = 14.79 mph. // cadence = 67 // wattage = 434.9

I repeat, this is the powercalc prediction.

Being the pounds of pressure at pedals the same, I would not only go 1.92 miles an hour faster but I would gain some efficiency since my cadence will be closer to 90.

With about the same percieved effort, I can produce 69 more watts.

My experience today in the 32 mile ride was the following: I even could drop some people from the B group on the uphills, keep with them in the downhills, and even take a few turns pulling the paceline.

Last time I simply could not ....

Go longer cranks!

pd: I will try them a few months, then I will decide it it would be reasonable to go up to 177's

Gear Inch is the OLD way of specifying gear ratio's and it makes sense to me because I'm OLD.

From the day's of 27" wheels its a simple calculation of front chainring (42t) divided by rear sprocket (13t) x wheel diameter (27") = 87 gear inches.

Its not realy suitable for accurate calculations but its a method suitable for considering one gear ratio in relation to another thats easy to calculate.

Regards, Anthony

For example = bike works of New York has a chart with all the gear inch combinations.

Being the 700c x 25 wheel +- 2105 mm in circumference, we translate it in inches = 82.874 inches

Now we divide that diameter by 3.14 so we get the diameter. 26.39 inches.

Being the wheel diameter 26.4 inches, multiplied by fron/rear cog (42/15) my gear inch ratio is

Old gear ratio 26.4*(42/13) = 85.2923077 gear inches.
New gear ratio 26.4*(42/15) = 73.9 gear inches.

I was about to change it a bit, but maybe I'll do it if I change my tires first.

Handy dandy formula, eh?

Woah woah woah, since when did you wimp out? What's this 74 gear inch nonsense? What happened to 85? A few dinky little hills and you collapse like a card house under a bulldozer?

The 85 is nice for 15 or 20 miles, but I think that 78 would be more ideal. I need to get stronger and lighter to ride 85 for 50 or 60 miles.

Sorry Jeff, 52 miles of hills last week made me wimp out. <sobbing in disgrace>