[Kotta390]

I have looked at many sites and cannot confirm what the speed of the driveshaft rpm would be based on length, O.D, Trans gear, Rear gear?

CCI has a driveshaft "Safe Operating" rpm of 4500 and a "Critical Fail" rpm of around 8000. I am trying to figure out based on what speed the Hellcat Challenger A8 w/2.62s and 28" tire would be with this info?

 

[Speedy]

You wouldn't get to 8000 RPM until about 6700 RPM in 7th gear I'd think. This is based on the output shaft RPM in the transmission tuning tables.

 

[Kotta390]

You wouldn't get to 8000 RPM until about 6700 RPM in 7th gear I'd think. This is based on the output shaft RPM in the transmission tuning tables.

Do you know what safe rpm means? What if you go over 4500?

 

[zhc]

To calculate is pretty simple.

[Engine RPM] / [Trans Gear Ratio] = [Driveshaft RPM]

A8 Ratios
1st 4.71
2nd 3.14
3rd 2.11
4th 1.67
5th 1.26
6th 1.00
7th 0.84
8th 0.67

Diff and tire size does not affect it. It only affects your resulting speed at a specified Engine RPM.

 

[Kotta390]

To calculate is pretty simple.

[Trans Gear Ratio] x [Engine RPM] = [Driveshaft RPM]

A8 Ratios
1st 4.71
2nd 3.14
3rd 2.11
4th 1.67
5th 1.26
6th 1.00
7th 0.84
8th 0.67

Diff and tire size does not affect it. It only affects your resulting speed at a specified Engine RPM.

The differential and tire size does affect the speed at which the critical speed is met. That’s why I want to know. If I had the 3.09 diff then the critical speed is the same I agree but my actual speed is lower compared to a 2.62. I don’t see how that calculation is correct because if it was the critical speed of the driveshaft would be exceeded long before I hit second gear. Or am I reading this wrong?

 

[BULL]

The differential and tire size does affect the speed at which the critical speed is met. That’s why I want to know. If I had the 3.09 diff then the critical speed is the same I agree but my actual speed is lower compared to a 2.62. I don’t see how that calculation is correct because if it was the critical speed of the driveshaft would be exceeded long before I hit second gear. Or am I reading this wrong?

Your question is what is the rpm of the driveshaft, correct?


As stated above, it really is as simple as engine rpm * trans ratio. Nothing else has an affect on the driveshaft. The driveshaft has an affect on the other components.

 

[BULL]

Ah!

Maybe your confusion is this... While we say "engine rpm * trans ratio", what this really means is engine rpm divided by the number.


6200 / 4.71 = 1316

Does that help?

 

[zhc]

Ah!

Maybe your confusion is this... While we say "engine rpm * trans ratio", what this really means is engine rpm divided by the number.


6200 / 4.71 = 1316

Does that help?

Oh yea, thanks for the correction. I fixed my post.

 

[BULL]

Oh yea, thanks for the correction. I fixed my post.

It worked, it's just knowing to use the correct interpretation of the math and the ratios as they apply to the formula.

 

[Slidd]

These figures are way wrong. Driveshaft rpm has nothing to do with transmission ratios, it's only dependent on vehicle speed, tire diameter and rear axle ratio.

At 199 mph with stock tires, A8 and 2.62, your driveshaft will be turning in the range of 6,123 rpm.

Everything the same but swap to 3.09 and now driveshaft speed around 7,222 rpm.



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[BULL]

These figures are way wrong. Driveshaft rpm has nothing to do with transmission ratios, it's only dependent on vehicle speed, tire diameter and rear axle ratio.

Only if you're in neutral....

 

[BULL]

The complete answer depends on your perspective, however, with the OP's question being how to calculate DS rpm, I'll stake my mechanical engineering degree on my answer being correct, but qualifying it to say that VEHICLE speed is further dependant upon the differential ratio and tire circumference.

 

[Slidd]

Don't understand your neutral comment BULL, driveshaft speed not dependent on transmission.


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[TMAC]

Engine RPM / trans ratio = driveshaft rpm

Engine RPM / (trans ratio X rearend ratio) = axle rpm

Axle rpm X rolling radius of tire (dia in inches X pi (3.14159)) / 63360 = mph

63360 is a constant.

12" in a foot X 5280 ft in a mile = 63360

 

[Slidd]

Engine RPM / trans ratio = driveshaft rpm

Negative

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[Kotta390]

Ah!

Maybe your confusion is this... While we say "engine rpm * trans ratio", what this really means is engine rpm divided by the number.


6200 / 4.71 = 1316

Does that help?

Yes that makes more sense!

 

[Slidd]

Yes that makes more sense!

So did you get your numbers?

At what vehicle speed will your driveshaft reach 4500 and 8000 rpm?


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