Horsepower loss in relation to driveshaft angle


#1

I am building a utility truck and need to figure the horsepower loss with respect to the driveshaft angle. We know it should be as straight as possible but we need to figure the loss as the angle increases. I need to try 2 different engines that sit at very different angles on the mounting platform as we have it now.


#2

I can’t even begin to help you with figuring out horsepower loss but generally you do not want the driveshaft to be straight as that can cause a vibration. An angle is preferred.


#3

To do that you’d need to know the amount of horsepower it takes to actually operate the U-joints at different angles. I’m not sure that data is relevant, and there fore not available. The biggest problem with increasing the articulation abgle on a U-joint is not horsepower loss…it’s that U-joints do not transmit power in a linear fashion. The higher the articulation, the less linear the output power becomes. I’ve attached a good link on the subject.

Usually in a truck the problem is compensated for by (1) adding an interim driveshaft with a carrier bearing and splitting the necessary articulation angle in half, asigning half the angle to each U-joint, and (2) out-of-phasing the U-joints such that one will tend to offset the other, That moderates the effective final wobble (my term).

Tilting the engine also helps. Just be sure the oil pump pickup will stay submersed. The pickup tubes are easily reformed to move the pickup location. Also be sure you son;t create a nice air pocket in the top of the cooling system…like by putting the high point in the system higher than the fill hole in the radiator;. The engine couldn;t care less if it’s going uphill or downhill, so this is an easy compensation.


#4

In theory, there is no horse power loss due to angle. In practice, universal joints do not do well at any but the smallest angles, that is why CV joints are used in higher angle applications.

With universal joints, with higher angles, the angular velocity of the output shaft is varying above and below the angular velocity of the input shaft. During the portions of the rotation of the input where the output shaft is going faster, there is a loss of horse power, but during the parts of rotation when the output shaft is going slower, the horse power is actually higher.


#5

Excellent point. During the times of the cycle when the output shaft is going slower, the joint would be almost acting as a gearset with the input shaft being the smaller gear and the output shaft being the larger. I like that.


#6

OK,never heard that before,if the angle gets too severe it will bind,have heard never go over 30 degrees -somewhere around 15 degrees should be fine-Kevin


#7

Another option to consider is shimming your rear axle to correct the pinion angle to keep the driveshaft angle moderate. These shims are available from most any 4X4 accessory company for just about any reasonably common axle/spring combination. We had to do this on my brother’s Jeep when he decided to add 6" of suspension lift to it, which significantly increased the driveshaft angle. The shims were cheap and easy to install. I think he got them from Quadratec.


#8

" the angular velocity of the output shaft is varying above and below the angular velocity of the input shaft"
" the joint would be almost acting as a gearset with the input shaft being the smaller gear and the output shaft being the larger"

What that really means is that the output velocity and torque vary in opposition, cancelling any variation in horsepower.
For the horsepower to vary there would have to be some means of storing energy, since we’re ignoring friction.


#9

A “utility truck”? A driveshaft angle of over 10 degrees on ANY vehicle is asking for trouble. The torque on the shaft starts to be directed at exploding the U-joints instead of transmitting it to the next gear-box…CV joints can accommodate little more angle, maybe 20 degrees max. The greater the angle, the shorter the joint life will be…


#10

Thanks for the thoughts. On one engine it looks like we are within 10 degrees but the higher horsepower engine is bigger and pushes our cvt over a bit too far. Next step may be to redesign chassi to accomodate the bigger one and avoid an angle that negates the higher horsepower.


#11

The problem is not that a higher angle will negate higher horsepower, the problem is that the higher the angle the less continuous the output until at some point the joint will cease to be able to transmit the rotating motion through the center “X” and into the following portion of the driveshaft. The output will be push-pull, push-pull, push-pull higher and higher amounts as the angel becomes greater until it’ll suddenly fail to function. It’ll be unusable.

What kind of vehicle are you designing? Have you considered using a driveshaft with constant veloocity joints?

One more point: on trucks, rear axles are generally tilted to creat as straight a path as possible through the last U-joint. I add the fact as a tip to consider incorporating into your design.


#12

No horsepower loss, just torque loss.


#13

That’s an oversimplification that I’d argue is misleading. The torque actually changes to a wave…until it suddenly drops off a cliff when the angle of articulation becomes too great. Then everything shuts down.

Forget about horsepower and/or torques loss. That won’t be the problem. Focus on articulation angles. That WILL be the problem.

And remember; even with small artiulation angles the torque output will still be a wave. With small angles the wave can get absorbed by the rest of the drivetrain and the tires. as the angles grow, the wave amplitude grows, until the ride becomes a bucking bronco. Or until everything physically fails.


#14

please share the graph of powerloss vs. change in the angle of driveshaft. it would better understood


#15

What’s really important is that the transmission output shaft and the differential input shaft are parallel to each other. If there is a 15 degree angle on the front U-joint, then there must also be a 15 degree angle on the rear U-joint.

When you do it that way, the transmission output’s steady speed gets converted to a cyclic speed variation by the first U-joint due to the angle, but the second U-joint turns that cyclic speed variation of the drive shaft back into a steady speed.