# Automotive Extreme driveshaft material?

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1. Oct 17, 2016

### Ibexe

Ok so ive ran into some issues figuring out what i material i should use as a driveshaft im not going to give many specs on the engine except 20,000 peak ft-lbs torque. With the transmission in the lowest gear 5:1 and the transfer case in 6:1 the output to the driveshaft is 600,000 ft-lbs of torque Calculated! You can see where im running into an issue here with the driveshafts ability to hold that much torque without breaking using driveshaft calculators i would need a driveshaft in excess of 8in diameter with 4in of wall thickness and still not have the ability to stay together. Im asking for help here on any alternatives to a driveshaft or insight on what material i should use on the driveshaft because this mud truck would look funky with a 8+inch driveshaft. Also the driveshaft will have a max rpm of 28571 rpm in the custom overdrive ratio If that will help

2. Oct 17, 2016

### Staff: Mentor

What kind of engine are you running that puts out that much torque? Per this wiki page, the highest torque outputs in production vehicles are the following:
Forced induction petrol engine – 1,180 lbf·ft – 2016 Bugatti Chiron, 7,993 cc (487.8 cu in) W16
Naturally aspirated petrol engine – 600 lbf·ft – 2012–present Dodge Viper, 8,390 cc (512.0 cu in) V10
Forced induction diesel engine – 1925 lbf·ft – 2017 Ford Super Duty 6.7 Powerstroke, 6,653 cc (406.0 cu in) V8[13]
Naturally aspirated diesel engine – 360 lbf·ft – 1988–94 Ford F-250/350 IDI diesel, 7,276 cc (444.0 cu in) V8
Electric motor – 1,180 lbf·ft – 2015 Rimac Concept One
Are you really putting out more than 10 times the rated torque as the 2017 Ford Super Duty 6.7 L engine?

3. Oct 17, 2016

### Ibexe

I can not tell you the engine but its not a production engine i have everything figured out paper wise but the driveshaft

4. Oct 17, 2016

### Randy Beikmann

Something isn't registering in my head. You have a drive shaft that will turn almost 30000 RPM? With say, a 3:1 reduction to the axle, that's 10000 axle RPM. Assuming a car tire that turns 800 rev's/mile, that would produce a speed of 350 MPH. With tall truck tires... That's quite a mud truck!

One thing to consider is that even with an engine that can produce the numbers you are quoting, the maximum the shafts will carry is limited by tire traction.

5. Oct 17, 2016

### Staff: Mentor

So you have an engine that puts out more than twice the torque as is described here, with 7400 ft-lb for a top fuel dragster?

Last edited by a moderator: May 8, 2017
6. Oct 17, 2016

### Ibexe

Its a non traditional engine all i need is help with the driveshaft so can i get anymore insight Besides criticizing the engine?

7. Oct 17, 2016

### Randy Beikmann

I don't think it's a criticism, so much as needing more context. Any "mud truck" as you describe it could not utilize the power that a shaft that large could carry (maybe if it weighed 100,000 pounds, and then I don't know how it reaches the speeds you mention). There's more to designing a shaft than how much torque the engine could produce in it. If there isn't enough load on the shaft (from traction), the torque would not get that high, and you'd be over-designing the shaft.

To advise you meaningfully, we'd need tire diameter, final drive reduction between the drive shaft and the axle, and vehicle weight.

8. Oct 17, 2016

### Ibexe

Tire diameter 65" diameter 30"wide
Unsure what you mean, in highest overdrive the overall drive ratio?
Vehicle weight as it sits with tires on it 25Klbs

9. Oct 17, 2016

### Randy Beikmann

OK. From the tire diameter, the radius is 32.5 inches, or 2.71 feet. I'm going to make some assumptions and work through an example of what I mean.

Assuming that one axle carries half the vehicle weight, and the coefficient of friction is 1.0 (considerably too high for mud), the maximum traction on one axle is 12,500 pounds. With a 2.71 foot tire radius, it would require 12500 x 2.71 = 33,875 ft-lb torque at the axle.

The final drive reduction I mentioned is the gear ratio between the drive shaft and the axle. Not knowing any better, I'll assume it's a 3.00:1 ratio. Then the drive shaft torque would be (33,875 ft-lb)/3.00 = 11,300 ft-lb. Much lower than could be produced by the engine and transmission you've mentioned.

Of course you need a safety factor when designing a driveshaft, to account for shock loading and unknowns. So instead of 11,300, maybe you design for 40,000 ft-lb. This method, with proper assumptions, should get you in the ballpark.

10. Oct 17, 2016

### Staff: Mentor

I'm not criticizing the engine. I just find it hard to believe that your engine makes so much power, when the state of the art in gasoline engines seems to be currently at about 11,000 HP, and somewhere around that figure for ft-lb of torque.

11. Oct 17, 2016

### Ibexe

Here are the specs so far 5 ton m923a2. 65 inch tall/ 30 inches wide. 4wd/2wd transfer case High: 1 Low: 6 axle gear ratio 32:1 hopefully going to be a 2 speed differental with 1.25:1 for wheel rpm in the mud. engine max rpm 8000 but limited to 7800rpm trans ratios: Axle ratio (32:1) 5, 2.75, 1.75, 1, .65, .48, .28 Reverse: 5:1 Axle ratio (1.25:1) 5 , 2.75, 1.75, 1 Reverse: 1:1

12. Oct 17, 2016

### Staff: Mentor

13. Oct 17, 2016

### Ibexe

its a completely custom engine never in production but yes thats the right vehicle but mine currently is almost completely not stock anymore

Last edited by a moderator: Oct 18, 2016
14. Oct 17, 2016

### Staff: Mentor

https://en.wikipedia.org/wiki/GE_AC6000CW

Diesel locomotive with 6600 HP. I believe the engine is about the size of your truck and probably about 8-10 times the mass of the truck. Point is: it does not seem feasible to have an engine to have the HP you mentioned and still be viable in a truck of that size.

I give up. @Ranger Mike probably knows more than all of us on this subject.

15. Oct 17, 2016

### Ibexe

All i need is help on the driveshaft

16. Oct 18, 2016

### xxChrisxx

Just one question.
How big were you expecting the driveshafts to be?

You are getting silly answers for a driveshaft, because you are putting silly numbers in.

I'm hesitant to say, the numbers youve provided for the application simply dont add up.

Lets assume you've not made an error. Just to put it into context. Somethong on the order of 11,000 HP is 8MW. thats the same power output as a large wind turbie. Or a high speed marine diesel engine that powers ships.

If the numbers you posted are right. Are you really surprised that your driveshafts need to be physically colossal?

17. Oct 18, 2016

### Randy Beikmann

I put together a table of speeds in the different gears. I hope I understood them right - I'm not familiar with a 2-speed diff with either a 32:1 or 1.2:1 option. If correct, the top speeds in each gear would range from 9.43 MPH to 168 MPH, assuming the 32:1 ratio (you can double check the numbers). As you can see, with the 1.2:1 ratio, the top speeds would range from 251 to 4500 MPH!
1st 2nd 3rd 4th 5th 6th 7th
RPM Tire Axle 5.00 2.75 1.75 1.00 0.65 0.48 0.28
7800 65 32.0 Speed (mph) 9.43 17.14 26.93 47.14 72.52 98.20 168.34
1.2 Speed (mph) 251.39 457.07 718.25 1256.93 1933.75 2618.61 4489.05

Even with the 32:1 ratio, a 168 MPH forward speed, or just spinning the wheels that fast) would most likely explode the tires. The 1.2:1 ratio would be unnecessary, unless there's more to this. I imagine spinning the components in the driveline this fast, with the torque your are talking about, will be a major issue. And you won't have enough power to exercise most of the gears fully - especially in the mud.

18. Oct 18, 2016

### Ibexe

I have used multiple gear calculators and with the 32:1 ratio im still debating on dropping the .28 gear ratio, the 2 speed diff is going to have to be a custom order along with tires due to the extreme nature of this build so axletech and which ever tire company i pick is going to have a challenge. I have linked a photo album of the calculators with the info imputed. http://m.imgur.com/yU5xUab,EesuWAq,1oTLSWj,rLYBMN1 The 1.25 gearing is only for wheel spin in the mud nothing else

19. Oct 18, 2016

### xxChrisxx

I have no idea what you are trying to achieve with this. From an engineering approach, it just feels a bit 'off'.

Suspect engine outputs asside, the slection of gear ratios, transfer box and FDR. Just doesnt feel right. The selection of a super short and super long FDR is just a bit bizzare tbh.

20. Oct 18, 2016

### Ibexe

The truck is going to be a sled pull truck and a mud truck yea i know the specs and stuff sound off ive had that said before. The transfer case is an atlas ll 6.0 and the transmission is most likely going to be from liberty gears and custom made they have made transmissions for record breaking vehicles. The ratios picked are just from personal preference and pull of each gear stuff like that. The reason i picked 32:1 was mainly just pulling, the 1.25:1 is just for competition mudding for the wheel rpm. When i contact Axletech about engineering a 2 speed diff i may see if they can do a 3 speed diff for a axle ratio more streetable (may ask about a hydro or pneumatic extending axle for onroad compact for street legal width and extendable for offroad. Im open to suggestions and ideas about the build drivetrain wise :)

21. Oct 18, 2016

### jack action

I will second what @Randy Beikmann said: The maximum torque your driveshaft will see depends on the maximum traction you will get, not the maximum engine torque.

The maximum friction force at the tire-road contact will be $\mu W$, where $\mu$ is the friction coefficient between the road and the tires and $W$ is the weight resting on the tires. The wheel torque $T$ is then that force multiplied by the tire radius $r$ or $T = \mu W r$.

So a 2.71 ft tire on an earth road ($\mu \approx$ 0.6) with 25% of the vehicle weight (25 000 lb; I'll assume 4 wheels with equal weight distribution for simplicity) will yield a wheel torque of 10 162.5 lb.ft. That is the maximum torque that this axle will see.

Going into the diff, you have the left and right axles, so 20 325 lb.ft. But the 1.25:1 ratio will lower that value to 16 260 lb.ft (= 20325/1.25) for the driveshaft between the diff and the transfer case.

The input driveshaft of the transfer case will see all the axles, so 32 520 lb.ft. With a 1:1 high gear, it stays the same.

If you do the calculations with the low gears it will be even smaller.

What happen if you put more torque than 32 520 lb.ft on the driveshaft (weither you're in 1st or fifth gear)? The extra torque is converted into acceleration of the drivetrain components, which will lead to spinning and it will stop when the engine fails (if no rpm limiter is used).

Of course a safety factor is necessary as stated in post #9.

If you sunk in a mud pit, the friction coefficient might be higher than 0.6, but evaluation is more difficult to guess. A higher safety factor might be necessary if you can find a reasonable source for that value.

22. Oct 18, 2016

### Ibexe

okay thank you this clarifies it up i believe im going to go with a high quality custom carbon fiber driveshaft and see if they can put a thin layer of dom tubing over the carbon fiber shaft to protect it from scratches leading to weakness. From the calculator i would need a driveshaft 6" diameter and a wall thickness of 3" which would give 98902 lb.ft torsional yield for extra safety. Not sure how much extra the dom tubing will do. Thanks alot everyone for the comments i will definitely be coming back here for answers!