Maths help needed!

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Maths help needed!

Postby toolonglegs » Fri Feb 05, 2010 9:31 pm

I just did a stomp test...can someone help me with the maths :oops:
34 ring/19cog 175mm cranks 230.1 lbs weight ---858 powertap reading

Technically, the PowerTap cannot be user-calibrated, but its accuracy can be checked using a simple test that is similar to the SRM calibration check. First, check that the transmission icon is on, and if not, give the rear wheel a spin. Then, enter the torque mode by holding the “Select” button down for 2 seconds or longer (the “WATTS” designation will disappear from the top line.) Apply the rear brake sufficiently to lock up the rear wheel. Now, measure torque as follows: with the cranks exactly horizontal (right crank at 3 o’clock), hang a known weight of at least 50 lbs from the right crank, or simply stand on it – hence the name ‘stomp test’! Measured torque = (weight in lbs) × (crank length in mm) × (1 in/25.4 mm) × (cog teeth/chainring teeth). For a 159 lb rider standing on a 175 mm crank, with the chain on the 39 tooth ring and the 23 tooth cog, 159 lbs × 175 mm × 1 in/25.4 mm × 23/39 = 646 in-lbs. Compare this to the displayed value by calculating % error as (measured torque - displayed torque)/measured torque.


Measured torque = (weight in lbs 230.1) × (crank length in mm 175) × (1 in/25.4 mm) × (cog teeth 19/chainring teeth 34) = ...?
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by BNA » Fri Feb 05, 2010 10:06 pm

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Re: Maths help needed!

Postby toolonglegs » Fri Feb 05, 2010 10:06 pm

OK I found another equation that I find easier...
Hub Torque (lb.ins) = Load(kg) x 2.2 x (Crank length (mm) / 25.4) x (17/34)

Hub Torque (lb.ins) = 104.7(kg) x 2.2 x (Crank length 175(mm) / 25.4) x (17/34) = 793 (I got 776 so 2% out...not knowing how accurate my bathroom scales are means I will give it the benefit of the doubt that it is accurate).
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Re: Maths help needed!

Postby shiv » Fri Feb 05, 2010 10:19 pm

Pardon my ignorance, but what is a stomp test?
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Re: Maths help needed!

Postby toolonglegs » Sat Feb 06, 2010 12:24 am

It is a way off checking the accuracy of your power tap with a known weight (me) standing on one pedal with the rear wheel locked.
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Re: Maths help needed!

Postby Redbull » Sat Feb 06, 2010 12:32 am

I'm not real good at maths - too much contact sport :) - but 1 inch is the same as 25.4mm

so 1"/25.4 = 1 therefore this part doesn't make sense in the equation as written.

Other than that, absolutely no bloody help at all I'm afraid.
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Re: Maths help needed!

Postby toolonglegs » Sat Feb 06, 2010 12:36 am

I didn't get that either...but think it means you divide your crank length by which ever unit you use (ie: imperial or metric)...either way the second equation made more sense to me.
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Re: Maths help needed!

Postby ecru » Sat Feb 06, 2010 7:02 am

I don't know anything about a stomp test or power taps, however both equations are the same, except you were using a different cog (19 tooth in the first, and 17 in the second). 19 tooth - 886; 17 tooth - 793. Hope that helps.

I would be surprised if your bathroom scales are out by 2%. You can check with a known weight - eg place your scales beside a more 'trustworthy' set of scales, and compare your weight.
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Re: Maths help needed!

Postby toolonglegs » Sat Feb 06, 2010 7:09 am

Yes it was the "(1 in/25.4 mm)" that had me confused...now I realise it didn't mean divide one inch by 25.4mm.I change gear ratios so that 17/34 was bit easy (ie: 2).
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Re: Maths help needed!

Postby Ant. » Sat Feb 06, 2010 10:24 am

Just remember, as you're grabbing hold of the brakes, you're putting some of your weight on the bars, so you're not stomping your full body weight on it.

Cable tie the brake lever, and balance on one pedal (no hands grabbing anything) and see how you go.
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Re: Maths help needed!

Postby toolonglegs » Sat Feb 06, 2010 4:35 pm

Bike was on the trainer with rear brake tied off.All my weight was 100% on the pedal...I am happy with the accuracy for now (which means my numbers suck a bit at the moment,although I was very sick so maybe no surprise.I am starting to feel better now and can feel some good feeling finally in my legs...also I have found the right stretch which seems to have cured my hip ache...which is freaking fantastic.Going to do some hurting in the bunch today and see how I go).The things that bugs me is my torque offset is at 470 (should be 512'ish) and seems to slip a bit lower everytime I ride.At least I know what I am doing these days!.
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