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Torque and Gearing #779394 07/22/19 4:16 pm
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DMadigan Offline OP
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Allen and I were discussing gearing and I thought this might be of interest to others. I copied the Torque curve that Alex posted for his A65L with Megacycle X1 cam, 9:1 pistons, ported head, 49.49 HPmax into a spreadsheet. Standard ratios with 19/47 and 20/47 sprockets. Tyre and aerodynamic drag from information on the web were used to calculate the torque required at the rear wheel. I plotted the data two ways, torque available minus required versus RPM and torque available/required versus MPH.
[Linked Image]
Changing the gearing from 19/47 to 20/47 changes the RPM where the Tavail - Treq curve crosses zero (no acceleration) in top gear but the maximum speed difference is almost zero. The CdA values are for sitting upright and leaned forward cafe style. The 19/47 gearing increases the acceleration between 60-90 MPH but little above that. The 19/47 gearing is still too high to reach the 7500 RPM maximum.
Changing the CdA is the only way to increase top speed with this torque curve.

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Re: Torque and Gearing [Re: DMadigan] #779413 07/22/19 7:43 pm
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Allan Gill Offline
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Am I right in thinking Dave that the higher the CdA value the higher the aerodynamic efficiency vs power/speed ?


beerchug
Re: Torque and Gearing [Re: DMadigan] #779417 07/22/19 8:26 pm
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DMadigan Offline OP
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I labeled the curves backward. The blue curve is CdA 3.5 and the yellow is CdA 4.5.
CdA is drag*area. You want this to be low. Motorcycles are very high drag vehicles due to all the bits hanging out in the breeze.

Re: Torque and Gearing [Re: DMadigan] #779423 07/22/19 9:41 pm
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CdA is coefficient of drag x area, strictly speaking, which is why it's super-interesting to me.

you can have two identical frontal areas that have different coefficients of drag because they have different shapes not apparent in the frontal silhouette.

i'm thinking of noisy norton's biplane wing struts, one constructed of a round piece of spruce with a certain frontal area, and another planed into an airfoil, with the same frontal area. the airfoil strut will have less drag than the round strut. or lots of other examples.

i'm interested in this because you can have two motorcycles with the same frontal area but different coefficients of drag because of their shape.

in naked land speed racing, you can decrease drag even if you can't change the frontal area by putting a kamm tail behind the rider, or a fender over the front wheel. probably also by changing from spokes to a cast wheel, or by covering up the busy motor and oil tank with a sidecover. or by folding the rider into an aerodynamic riding position, one which might not be the lowest frontal area.

in my LSR race bike, i've got all the horsepower the motor design and i can live with. everything i'm doing now is to lower the CdA of the overall package, so i can lower the numerical gear ratio and go faster at the same available torque.


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Re: Torque and Gearing [Re: DMadigan] #779455 07/23/19 7:11 am
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Allan Gill Offline
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I remember your discussion on this not all that long ago regarding drag etc. Good merge of subject.

I aught to get my data logger setup again, it’s the same as Mark Parker’s but I really need to spend more time setting it up. It works fine for the most part but sometimes drops out (goes scatty) at higher RPM, most likely a sensitivity adjustment needs to be corrected. Although it is usually fine still for reading speed if I remember rightly. Down side is mine is far from stock so it wouldn’t show a torque curve more representative of a standard model, but then it’s what set of ratios for which market.

Would it be right in thinking that if the CdA was lower then it would be more effective on overall gearing rather than the intermediate ratios? With the higher CdA you’d be wanting those higher gears much closer. I guess I’m trying to say that with the change in CdA the real world torque curve will widen or narrow depending on if it’s the 3.5 or 4.5 (or less like in Kevin’s situation) hopefully the datalogger would show this.


beerchug
Re: Torque and Gearing [Re: DMadigan] #779497 07/23/19 3:54 pm
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DMadigan Offline OP
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My correction is wrong. The plot was labeled correctly. Obviously, higher drag means more torque required and so the top gear torque curve crosses at a lower MPH. Sorry for the confusion.
The torque required is a power function so it has more effect at higher speeds. At 60 MPH the required torque is about 15.8% of the available in 2nd gear with a CdA of 4.5. Changing the CdA to 3.5 makes the required torque 12.5% of the available. At 90MPH in 4th a CdA of 4.5 is 52.7% and a CdA of 3.5 is 41.2%.
Changing ratios moves the torque vs MPH curve up and left with lower ratio. The curve does not change shape.

Re: Torque and Gearing [Re: DMadigan] #779609 07/24/19 5:42 pm
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DMadigan Offline OP
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This may be interesting when choosing ratios for the gears. Plotted are the ratios for the standard 4 speed with lines showing the RPM drop at each shift.
[Linked Image]
The 1st-2nd gear shift moves the torque below the peak. The 2nd-3rd and 3rd-4th shifts move the torque past the peak. The total torque change on the 3rd-4th shift is almost zero.
If 3rd gear were lower, moving the curve up and left, the 2nd-3rd shift would be higher on the torque curve (higher RPM) and the 3rd-4th shift would move to a lower RPM. Since the RPM drop is now larger the slope of the of the 3rd-4th shift change is nearly the same. Changing the ratio has negligible affect the torque since the maximum RPM is reached earlier, just where on the curve the next gear RPM starts.
Adding another gear, shown in the second plot, makes the torque changes with each shift smaller.

Re: Torque and Gearing [Re: DMadigan] #779638 07/24/19 11:31 pm
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johnm Offline
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Originally Posted by DMadigan
This may be interesting when choosing ratios for the gears. Plotted are the ratios for the standard 4 speed with lines showing the RPM drop at each shift.
[Linked Image]
The 1st-2nd gear shift moves the torque below the peak. The 2nd-3rd and 3rd-4th shifts move the torque past the peak. The total torque change on the 3rd-4th shift is almost zero.
If 3rd gear were lower, moving the curve up and left, the 2nd-3rd shift would be higher on the torque curve (higher RPM) and the 3rd-4th shift would move to a lower RPM. Since the RPM drop is now larger the slope of the of the 3rd-4th shift change is nearly the same. Changing the ratio has negligible affect the torque since the maximum RPM is reached earlier, just where on the curve the next gear RPM starts.
Adding another gear, shown in the second plot, makes the torque changes with each shift smaller.


This is how I decided on the internal gear selection for my Dommie race bike.


Moderated by  Allan Gill, Jon W. Whitley 

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