Pinion Angle 1

[MarkZE]

What is the significance of the pinion angle in a four link live axle setup?

I understand that floor clearances may be an issue in some road cars but I suspect there is a direct effect on the vehicles handling.

The particular example I am working on which has bought this question to my attention is a Westfield style Clubman being built by a freind for a street car. It is four link and uses a panhard bar. I don't know if is is equal or unequal length arms but they are all parallel when the car is viewed in plan.

He hasn't decided on anti squat / anti dive yet either but he is working with a horizontal lower arm at ride height.

I have had experience with one drag car guy who tells me 4° nose down is the go and another person who does street cars stuff who says 1° nose is all you need but neither can back up their claims with any reasoning. They jsut know this!

 

[MarkZE]

I am led to believe it helps traction.

I remember hearing some Dirt track Speedway, "Super Sedan" racers talking about it once. They were going on about mid corner drive and maybe they should do something with pinion angle.

 

[GregLocock]

On a road car the diff is slightly nose down so that under torque the driveline straightens up.

If your rear UJ angle exceeds 4 degrees then you may run into severe rumble type issues, and a typical RWD has 2 or 3 degrees of axle windup at max torque first gear, or reverse, so you don't have much room to play with.

On a race car I would try and keep the diff axis parallel to the gearbox axis at full throttle in (top gear-2), and not worry too much, as a first guess.

Again on a road car it is often important to allow the pinion nose to dip as the suspension rises to avoid hitting the floor.



Cheers

Greg Locock

 

[MarkZE]

Thanks Greg!

I am still interested to know whether there is an effect on traction caused by pinion angle?

Is this just a Myth?

 

[GregLocock]

I'm not saying it can't happen, but yes, I'd guess it was a myth.

There is no more torque being generated, there is no effect on wheel loads, or compliances, so I really can't see where a traction benefit would come from. Cheers

Greg Locock

 

[patdaly]

Mark, as Greg indicates, there is no traction advantage associated with pinion angle other than the losses when they end up not being 0 under dynamic conditions. As to your friends, I would suspect both are right, with different cars/suspensions come varying pinion angle requirements! I would bet your drag racing friend has a leaf sprung rear, complete with slapper bars? If so, he must dial in more pinion angle as the rear rotates far more than a comparable 4 link or ladder bar setup would. I would next bet your street car friend runs either minimal power or has some form of a link suspension. I personally run just a bit over 1 degree on my ladder bar car, and it works quite well.

 

[c2s]

A differential in driveline angles sets up a longitudinal force transmitted down the driveshaft. This force is cyclic and can have the effect of setting up an oscillatory motion in the pinion. The reaction to this oscillatory motion will have an effect on traction. However, since the force is a function of the cos of the angle difference, it's generally peanuts.
Kevin

 

[c2s]

Correction ... Sin, not Cos
Kevin

 

[Artsi]

have a look here:

http://buickperformance.com/Pinion.htm

 

[rocketdoc]

In my opinion, GregLocock and c2c nailed it down.

The cyclical accelerations are a function of the sin of combined angles of the longitudinal axes of the trans (or output shaft), drive shaft (or halfshaft) and the pinion axis (or carrier bearing), and it is opccilatory and the torque on a 4 link rear suspension (or a simple leaf spring suspension) will torque the axes near normal.

As for the occilatory frequency contributing to the cornering forces... I'm just not sure that it's of a magnitude that it can help or hinder.

And remember, anti-dive and anti-squat are nothing more than the suspension going into a binding condition (read, non-linear and mostly indeterminant).

Peter V

 

[BillyShope]

Sorry, Mark, but I didn't see your thread before I started mine on pinion angle. If you'll check it out, you'll see that there is, indeed, an effect on traction, though I'm not convinced many, if any, drag racers realize it.

With horizontal trailing links, your friend will experience a considerable amount of squat. If the intersection of lines through the links, as viewed from the side, falls on a line passing through the rear tire patch and intersecting two other lines, one a horizontal line through the center of gravity and the other a vertical line through the front tire patch, the car will neither squat nor rise.

All of the above applies to a car with a "beam" rear axle. With an independent rear suspension, the no squat/no rise line is parallel to that which I've described, but passes through the center of the rear wheel.

 

[Joest]

A well-designed 4-link suspension used with a beam axle should not allow much pinion angle change under acceleration (maybe 1 degree at most). Therefore, I have always set the pinion angle 1 degree less than the axis of the crankshaft/transmission output shaft. Theoretically, the ideal angle is +/- 1/2 degree when subtracting one from the other, and no more that 2 degrees total angle on the driveline itself. If these conditions are met and the driveline is well built and balanced, operation should be vibration free with minimal losses. If you would like a schematic, feel free to email me. People that suggested traction would not be increased with the pinion angled toward the ground are correct, although I know drag racers using leaf springs (no linkage) run between –5 to -7 degrees (down) pinion angle. This is to compensate for “wrap up” so that pinion angle is closer to optimal off the line when torque is highest (1st gear).

 

[BillyShope]

First, I would point out that the squat/rise characteristics have absolutely nothing to do with a "binding condition" in the suspension. If the force vector at the rear tire patch (of a RWD beam axle car), when viewed from the side, is at an angle with the tangent equal to the C.G. height divided by the wheelbase, the car will neither squat nor rise. If the angle is greater, it will rise, if lesser, squat.

As for my reference (in my thread on equal rear tire loading) to "some" racers realizing the traction effects of pinion angle, I would now modify that to "one possible" racer. I have been able to find only one racer, on another board, who knows a guy who says that he adjusts pinion angle for 60 foot performance. And, I'm rather doubtful of even that one example. But, my analysis does have some value, for I now realize that the first Ramchargers car, a C/A with a steeply inclined drivetrain, would have had a sufficiently large pinion angle to have an effect. There was no angularity in the drivetrain, understand. It was just that the engine valve covers were right up against the hood, resulting in a steeply angled drivetrain. We used an asymmetric 3link with the single upper link offset to the right. But, I did not take pinion angle into account in the setup equations. That upper link should have been offset a bit more for equal rear tire loading.

 

[BEATH]

I can't see how pinion angle can have much effect. There is a simple way to equalise tyre loading with a live rear axle using a torque tube attached to the body at a point offset to the right. To find the location of that point draw a line in plan view through the diff centre towards the front of the car and at an angle to the centerline with a tan equal to the diff ratio; ie if the ratio is 3:1 the line would pass through a point three units forward and one unit to the right of the car centerline, attaching the torque tube anywhere on this line will give equal loading on the rear tyres ( draw adiagram and think about it). It all turns to custard when you hit the brakes unless a transmission brake is fitted. The same effect can be achieved with a 4 link setup if the links are focussed on a point anywhere along this line.

 

[BillyShope]

Yes, Beath, you're right on both counts. Unless the instant center is near the rear of the car and very high (a lot of rise on launch) and, also, most of the roll stiffness is at the rear, adjustment of the pinion angle, within acceptable limits, has very little effect on tire loading. But, there does exist a pinion angle, somewhere between horizontal and vertical, at which equal tire loading would be achieved. Unfortunately, such information is of little use in almost all practical cases.

Your description of an offset, or asymmetrical, trailing link is the best solution for equalization of rear tire loads. This was used by Jaguar in their early C-Types. (Late C-Types had IRS.) The Jaguar design, however, used 3 links. Two were symmetrically placed below and the third was above, offset to the right. (Your suggestion will work, of course. It's just that the 3link is more easily implemented.) This was copied, by the Ramchargers, in their first car, a C/A. Unfortunately, I had not then correctly solved the equation set and the suspension never exhibited its true potential during the competition life of the car. The proper setup equations, offering both equal tire loading and no squat or rise, are included in my contribution to the Student Workbook which accompanies the Millikens' "Race Car Vehicle Dynamics," available through the SAE or Amazon.com.

Braking problems exist only if the fronts are "locked." Of course, with one tire more lightly loaded, dynamically, such a situation is more likely. But, Jaguar seemed to be satisfied with the braking peformance of the C-Type.

 

[patprimmer]

BillyShope

I am more an engine man than a suspension expert, but as we got the car ready and raced at out Supernats last weekend, I have since found some time to contemplate your comments re pinion angle and torque reaction on tyre loadings and traction. I have also seen some discussion on this matter in another forum.

After this disacussion and contemplation, my thoughts go like this:-
For every action there is an equal and opposite reaction.
The original action is the torque generated by the motor.
On a race car, this is transmitted directly to the chassis rails via the solid engine mounts.
The torque of the motor is also transmitted via the longitudinal drive train to the pinion gear.
The pinion gear acts on the crown wheel in several ways.
One action is to turn the crown wheel, axles, wheel and tyres until there is a reaction at the contact patch This moves the car forward. The reaction to the turning of the crown wheel, lifts the pinion and front of the car.
The other action is for the pinion to try to twist the axle houseing in the opposite direction to the pinion rotation.
On a rigidly mounted rear axle like a hard tail dragster, this reaction is applied to the relativly rigid chassis, and is equal and opposite to the reaction at the engine mounts, so they cancel each other out, except for any twist generated in the chassis rails between these points. This twist should be minimal, and easily accomodated by the very flexible tyres, with negligable difference in loading
On a car with IRS, the differential houseing is rigidly mounted, so same situation as a hard tail dragster.
On a beam back axle with suspension, the reaction is transmitted to the chassis via the springs, allowing considerable movement, thus unloading one wheel.
This unloading can be roughly counteracted by preloading the wheel that lifts to try to obtain equal weight under near maximum torque, to get best average weight dureing the phase when traction is most critical.
I wonder if a better solution might be an extremely rigid anti roll bar, like say, one made from 2 or 3' chrome molly tube, with as short as possible longitudinal links.
This would have the effect of makeing the car effectivly rigid in roll, but still retain longitudinal compliance. Is that yaw or pitch? I did say I am not a suspension man.

Sorry for the very long post, but you did get me thinking.

Am I wrong in my analysis, as no one seems to do it this way

Regards
pat

 

[BillyShope]

Pat, you needn't apologize for being an "engine man." You have a very good understanding of drag suspensions. In an attempt to bring together what you've said, I would point out that the torques at the ends of the driveshaft are equal in magnitude and opposite in sense. The one tends to unload the right rear tire and the other is absorbed by the chassis through the engine and transmission mounts. As you said, if the chassis torque can find its way back to the rear axle, there is a torque cancellation and rear tire loads are unaffected. Unfortunately, the chassis torque is distributed, front-to-rear, in proportion to front and rear suspension roll stiffness. Again, as you said, a hefty rear anti-sway bar helps in torque cancellation and many drag racers appreciate this effect. Unfortunately, you can't totally eliminate the roll stiffness at the front, so perfect cancellation is not achievable by this method. You mention static preloading. This, in conjunction with efforts to increase rear roll stiffness, is a good solution. Unfortunately, the static preload means a lightly loaded front tire during braking, with possible wheel lockup and a "pulling" of the car to one side. The best solution is a trailing link suspension that is asymmetric to the car's centerline (in plan view). One small error in your analysis is the assumption that torque loads must be carried through the suspension springs. With asymmetric suspension links, full torque cancellation can be achieved dynamicially with no deflection of the rear springs (if the car is set up to neither squat nor rise). For a more complete explanation, you might want to check out my article at racingarticles.com or read my contributions to the Student Workbook which accompanies the Millikens' "Race Car Vehicle Dynamics."

 

[jloudon]

Is there an relatively easy way to determine where the C of G lies on a car? I have access to corner weight scales to determine where the 50/50 weight split line will be in relation to front and back, but how do I determine how high?

I would like to implement no squat or rise on my clubmans style car with a beam axle and 4 link rear.

John

 

[GregLocock]

It is conceptually easy, but rather difficult to get accurate results.

Basically you jack one axle of the car up, and measure the increase in wheel load on the other end.

You need to pack the springs, and empty or completely fill the fluid tanks.

http://www.jeepaholics.com/tech/cog/#_Toc535118725

doesn't include either of those tips, but shows you what to do.

Cheers

Greg Locock

 

[BillyShope]

Yes, this is a proper procedure, but, if you differentiate the equations involved, you'll find that a very small measurement error can result in a significant difference in calculated CG height.

I prefer what I call the "tabular" method. You tabulate the height, weight, and height*weight products for as many components as you can and then select a reasonable height value for that which remains and include that product. Dividing by the total weight, you then have a fairly accurate measure of CG height.