pinion angles for traction

[68Stepbed]

OK, I seem to have a serious issue with traction(actually the lack of it) on my 68. It's lowered 4.5/6.75 and I've never adjusted the pinion angle. Would the correct angle help with traction? if so, what angle do I need to shoot for?

[Keith Seymore]

This might be a controversial statement - but - pinion angle has nothing to do with traction.

Removing tires from the equation, the traction comes from the forces through the suspension attaching points: the relationship between the "instant center" of the suspension and the center of gravity of the vehicle.

The instant center is determined by suspension type (leaf springs react through the forward spring eye; four link reacts to an imaginary point created by extending the control arms forward).

K

[68Stepbed]

OK. That's the reason I asked in the suspension section. What should the pinion angle be compared to the driveline angle?

[Keith Seymore]

For a control arm rear suspension: 1 deg to 3 deg u joint working angle

For leaf spring: 3 deg to 6 deg u joint working angle.

K

[BossHogg69]

My truck is lowered 7.5" in the back (6" spring, 1.5" block) and I used a 6* shim to get mine right. It is 4.5* coming off the trans yoke.

FWIW, I'm also using a PB Fab trans x-member, and the trailing arms are in the 2nd hole from the top. There are 4 adjustment holes on it.

I also had traction problems with my truck, but since I windowed the #1 cylinder, it doesn't spin the tires much at all

[PGSigns]

Pinion angle will not affect traction at all. Suspension geometry, weight split, shocks and tires is where you will find added traction.
Jimmy

[68Stepbed]

Is there a way to optimize the stock trailing arm suspension to gain better geometry, or am I simply just looking at buying parts made for that?

[BossHogg69]

I'm pretty sure I've seen replacement front mounts that will allow you to alter the mounting point of the trailing arms to change anti-squat. I'm using the PB x-member with adjustable mounts. The Hotchkis kit comes with the brackets I'm talking about, but you could probably make some for WAY cheaper.

I really think it's more about weight distribution and spring/shock stiffness than anything. I found that even with the fuel tank and battery mounted behind the rear axle on my truck and 345mm wide (13.5+ inches) rear tires that I could obliterate them in 1st-3rd if I was giving it full throttle, no matter how fast I was going. The truck was really planted around turns and stayed planted as long as I didn't jab at the throttle. Power slides were EASY anytime. I tried softening my shocks (Chassisworks DA) to improve straight line traction, and that didn't help much. I have Hotchkis rear springs, and they might be a bit stiff, which doesn't help at all either.

[69gmcc10]

Changing the instant center and anti-squat of the truck to increase traction on these trailing arm trucks is actually kind of simple. It takes a little work but is simple, it will also help correct some of the pinion angle issues caused by lowering springs.

With every inch you lower the rear with a spring it points the pinion downward more and more. There is "some" power loss with improper pinion angle, but even more concerning is u-joint life, because every degree you get further from the ideal operation angle, fatigue and failure of the joint goes up exponentially, literally. To counter this partially and to raise the Instant Center and anti-squat of the now modified truck suspension you need to raise the front mounting point of the Trailing arms. This will have a positive effect on both traction and u-joint life.

To raise the mounting point of the trailing arms on the crossmember you have some options, there are a few different companies that make trailing arm brackets with adjustment holes on them or you can simply flip the existing mount which will net about 4" of lift for the price of 16 grade 8 bolts. to flip the mounts the arms mount to you simply unbolt the arms from the mounts on the crossmember, drill out the rivets that hold the brackets to the crossmember, flip the mount upside down and bolt it back onto the crossmember with 4 bolts in the holes that are not obscured. Then mark, drill, and bolt the other holes so the mount is real secure. Reinstall trailing arm in the hole and take it for a drive.

When you do this it moves the Instant Center up and relaxes the angle on the u-joints at the same time. I did this just to increase traction because I didn't lower my truck, I think it is in my build thread if you want to look, but now my trucks hooks up and hustles! big improvement!

This post from the Make it Handle thread explains what we are discussing here about Instant Center and Anti-Squat even better and it has an illustration. http://67-72chevytrucks.com/vboard/s...postcount=1032

Do a search for flipping trailing arm mounts for more info. This is all about physics and geometry, after getting the chassis close enough for your liking to correct the issue you can then fine tune with springs, shocks and tires.

[Robznob11]

These guys are absolutely shooting you straight. On the cheap flip your oe mounts. Ece will sell you a set of there brackets if you call and ask. You can get the angled ones or the flat ones that would work on a homemade or aftermarket type x member. The best option if you ar going to stick with the oe member is to get a hotchkis anti squat kit. It is the brackets your looking for adjustable in one inch incredimunts. As far as pinion angle you need to make sure that is correct too. You need the rear @ 1.5* down as compared to the front angle. Example if your tranny is 3* down your rear should be 1.5* up. Some will tell you that it's not as important on our trucks if you have trailing arms because they don't wrap up like leaf springs. Either way your pinion still tries to climb the ring gear under power trying to make your pinion come up and I feel it still has some movement. Either way anything under 2* operating range is a safe place. Never have it straight in line tho. Also many argue over checking angles with ride hight or frame level. I still agree with the ride hight ground level method. Who really cares how it's positioned in the frame as long as the operating range of the joints are correct. All manufactures placed rear motor pinion angles at -3* ish. Hence the reason sbc manifolds are angled where the carb sets. This is for leg room etc. Chassies builders for race cars actually prefer a positive rear engine angle....this is lengthy and very arguable so I plan to say no more on this page but do the research and form your own opinion because 20 people will give you 20 answers of why they are right! Good luck bro!

[68Stepbed]

Thanks for all the advice, guys. I've looked into the anti-squat brackets, but some seem a bit pricey. I'll look into flipping the OE mounts for now, but eventually, I'd like to run a PB tubular crossmember and some aftermarket trailing arms with adjustable end links, or I may just look into the 3-link setup from NoLimit.

[68Stepbed]

OK, I'm strongly considering flipping the trailing arm mounts. When I do, I know it'll change my pinion angle. Does anyone that's done this know what degree shims I'll need to get the correct angle, or am I going to have to figure it out afterward?

[Robznob11]

Originally Posted by 68Stepbed

OK, I'm strongly considering flipping the trailing arm mounts. When I do, I know it'll change my pinion angle. Does anyone that's done this know what degree shims I'll need to get the correct angle, or am I going to have to figure it out afterward?

Measure after, got mine at Calvin's racing

[Robznob11]

I ment to say Carl's racing. They are billet not cast.

[69gmcc10]

Originally Posted by 68Stepbed

OK, I'm strongly considering flipping the trailing arm mounts. When I do, I know it'll change my pinion angle. Does anyone that's done this know what degree shims I'll need to get the correct angle, or am I going to have to figure it out afterward?

Excellent to hear you are going to do this! do you have a 1 piece driveshaft or a 2 piece? How is your truck lowered? Is it a 5" spring with 1.5" block? is it a 6" spring with some shims under the axle? is it on air? what panhard bar do you have (I'm curios, its has little to do with what we are talking about)?

The more info you get us the better we can help, but a simple rule of thumb on a 1 piece drive shaft is that you want the pinion angle to match the trans output shaft and on a 2 piece shaft you want the angle to be same as the first drive shaft. since every truck is a little different it would be wise to flip the mount get it on the ground, measure your angles, and order accordingly so you have the correct shim and not a guess.

[68Stepbed]

It's a 6" spring with 3/4" lowering blocks. 1 piece driveshaft. ECE super track bar.

[68Stepbed]

Originally Posted by Robznob11

I ment to say Carl's racing. They are billet not cast.

Do you have a part number or a link? I can find angle shims, only lowering blocks.