Does the pinion angle help in the tire "bite" process? My racing buddy has a 32 ford coupe that runs 9.0's and he's been messing with the instant center and the four link geometry to get it to hook more consistently.

Yesterday he changed the I/C to 50" from 48" and he was hooking badly, was only running 9.50's and was all over the track. Well when he was under the car at the track he felt the pinion angle was too positive or up so he nosed the pinion down (two turns on the adjustment), we're not sure what that meant in degrees but the next too runs were great, 9.07's with a 1.32 60' foot. Not perfect but better. ALL he did was change the pinion angle. I never knew that the pinion angle would aid in traction and an aggressive bite.

Would this be true with our cars? Right now I can only get 1 degree down with my ford 9" and hotchkis adjustable uppers, they are adjusted all the way in.

What's your thoughts and experience?

Lee

No One! No ideas or anything?

Lee

Well, I don't have any real ideas, but real interested to hear the answers!!

I have the adjustable rear control arms, But I don't know what the best setting is compared to stock which is where it is set now. I suppose I could do some experimenting whne I get a lot of spare time!

Bill C.

I have heard that increasing or decreasing pinion
angle can help or hurt traction. I have not tried
it for myself yet though.

Check out what www.dickmillerracing.com (http://www.dickmillerracing.com) has to say

Phil,
I checked the website you gave, but could not find any relevant info... could you please give details?

Thanks,
Bill C.

OK, here's what I know from circle track racing...

Might be dated info, but I'll give it a shot.

Pinion angle affects where inder the tire the contact patch acts. If you have too much up angle (like this \ from the side), the tire can't "catch" the patch. It's always chasing it. Poor traction, squirrely coming off a corner (or on a launch, I suppose)
If you have too much down (like this / from the side), the patch is behind the tire, and the tire is always running in front of it. Squirrelly, wheel hop, as the suspension bounces, the patch comes under the tire, then as it catches and loads the suspension, flies out the back again, making more tire smoke.

The trick is to put the acting area (or whatever) towards the forward part of the tire so that as the suspension loads at launch, it drives the patch under the tire. If the patch is driven too far rearward, it will come out hte rear of the tire, unloading the suspension, allowing the patch to get under the tire again, loading the suspension, sending the patch back out under the rear again... you get the idea, wheel hop. This is usually caused by an instant center too close to the rear end, which makes the pinion pivot a lot under loading. Moving the instant center forward can help, but the fine tuning should be with the pinion angle.
If the contact patch starts out correct, but doesn't get under the tire far enough, after launch, as the car rises back up to ride height, it will unload again, and break the tires loose. That's that "broke the tires loose at half track" smoke you get.

I hope that helps a little, and I wish I could explain it better, but now maybe someone will chime in to debate me about it and we can all learn something smile.gif

K

Ok, so that makes a little sense Chevello, but I still have confusion! First off, how do you know where this 'acting part' of the tire is? Is there a way to test for that? Next, the whole idea of being able to re-locate the 'patch' seems difficult for me to grasp. If you have your car sitting on the ground with a fully loaded suspension, the tire contact patch will be the part of the tire at 6 o'clock right? So if I'm understanding correctly, you are sating that by changing the pinion angle, you can change where that patch is? Like moving it to 5 o'clock or 7 o'clock? Seems to me that if you rotate the pinion while the tire is on the ground, the patch will not move... it wil always be at 6 o'clock. Now, if you are saying that the 'action part' of the patch is changeable, then I might be able to buy that! But what is needed? How many degrees and which way?

My scenario is something like this: installed the BMR full rear suspension setup last fall, complete with adjustable uppers. After setting it to the same specs as the stock ones I removed, I drove it for a while. Things felt pretty much the same as before I made the swap. I then checked the pinion angle and found it to be off a few degrees. Using the adj. uppers, I adjusted the uppers until the correct setting, approx. -2 degrees at the rearend. I then noticed while driving that the rear seems more 'rigid' when getting on it hard and the front end does not seem to lift as much. The tires seems to want to spin easier also. So would you say my 'action patch' is in front or behind on the tire? I neve ad an issue with wheel hop tho.

Thanks,
Bill C.

My bad, I was talking about "acting patch" If your contact patch isn't on the ground, you have problems that adjustable uppers won't fix smile.gif


Here's an image to look at:
http://www.metco-inc.com/images/icm.gif

The instant center is going to be where the rearend tries to lift the car. If it is right under the CG (center of gravity) it will try to lift the whole car, if forward, it will try to lift the front, if aft, it will try to lift the rear. Basic stuff.
The image shows a line going from the instant center to the tire contact patch. Pick one or the other, it doesn't matter. If you put a line through the centers of the control arm mounts to this line (and I am pretty sure I am going the right way with this one somebody better step in and correct if I am wrong) this is where your acting patch will be. If you use the OEM points in the image, you can see that it will be towards the front of the tire. Seems to be a good thing, but... If you imagine what happens when the car launches, the instant center raises with the body, and the control arm mounts on the rearend rotate around the axle center sending the intersection of these two lines forward. Acting patch scoots forward and the tire tries to "catch" it. It will catch it, but not probably til after the tires break loose, and the car settles back down. Too late.

Looking at the modified points, you can see that the intersection of these lines is farther under the tire. AHA! smile.gif Again imagine what happens when the car launches. With the modified instant center closer to the CG, the car doesn't try to lift as much, thus lessening change in the "acting patch" and keeping that acting patch under the tire. The car launches harder, and doesn't unload the tires as much when the launch is complete.

Another way to look at it is to look at the angles of the two lines going to the contact patch from the instant center. The OEM one is flatter than the modified one. The modified one should drive the tires harder into the ground because more downward force is applied. Picture yourself trying to push your Chevelle on dirty pavement. You have your back to the car, hands under whatever you can grab and are pushing against the ground as much as you can. If you try pushing too flat along the ground, you can't get enough traction to push it. So, you might try to push down, giving your Hush Puppies more grip. If you give it too much downward push, all you will do is lift the car, not push it. So, there is a middle ground between angle and traction.

I hope that made at least some sense, and I hope somebody chimes in who has played with their pinion angle and has some more real life info about this. Most of mine has been with oval track, and getting off the corner. Plus it'll make me feel better to have agreeing or dissenting opinions. At least then I'll know if I'm a genius or an idiot. :D

K

While not claiming to be an expert or someone who's had any real world expirience with this, I will say that I've read that you want to set the pinion angle so that under power, the driveline angle is zero. This gets into alot of the variables that Chevello is talking about. This is because the pinion tries to "climb" the ring gear. Since it can't, it tries to rotate the rearend. At the same time it's trying to spin the whole axle like a propeller. This is why guys put air bags in the right rear.
I got a lot out of "Circle Track Suspension Handbook" There are all kinds of theories about "anti-squat" and "anti-dive" that make my brain hurt. I've read that the ideal is to actually have the body lift when you get on the power because this means it's pushing down on the tires. I'd like to see more replies from people "in the know" about this because my $.02 isn't worth $.01 when it comes to this.

Jeff

Jeff,
I wholeheartedly agree that the lifting action will help plant the tires in the rear and get a better launch... the question remains, how best to do that?! That is the magic question IMO.

Like I said before, when I installed the new rear control arms and 'fixed' the pinion angle to what is 'supposed' to be the correct -2 or so degrees, that is when things seemed to get worse. The car does not feel like it is trying to 'lift' like it used to, but rather than just guessing and trying different angles, I'd like to know how to figure out the best angle. If anyone has any more ideas on that, please feel free to chime in!!

Chevello, I'm going to have to think about that post a bit more... gets kinda deep!

Thanks,
Bill C.

You could always try adjusting the angle a degree at a time and see how your times change. If they go down, keep going til they go up and back off. Just remember to record your track temps and tire pressures, weather conditions and the like. You might find that a cloudy day likes more of one thing, and a sunny day more of another.

My brain was aching after typing it too. :D

Might try the manufacturer of the parts you have, too. They might be able to give you a place to start adjusting from.

K

I believe there are programs to set this type of thing up but the are pricey and in the end, trial and error is still the only real way to do it. It's one thing to figure where the IC's and CG's are but depending on conditions, things will change. There are some good books and programs here, http://www.ssapubl.com/ at Steve Smith Autobooks.
Good luck
Jeff