4-link setup

[tduffy]

Hi,
I'm trying to setup my '67 Nova's 4-link with no precious experience.

My car's info is here --> http://racetec.cc/1967Nova/index.html

I'm getting no bite at the track. It is poor even for the treaded Sportsmen's tires I'm using right now. Best 60 ft time is 2.04.

Attached is my current setup from the 4-Link Calculator program, with Dave Morgans suggestions, for a 3200 lb car, marked in green.

The car weighs 3400 lbs and should run ~ 11.5 with the ZZ502 engine and 4.10 gears.

Notice there are no IC points, in the suggestions box, below the neutral line. Should I select from above the neutral line, or select an IC farther forward?

Any other sugestions?

Thanks,
Tim

[John_Heard]

Just throwing this out there, from what I've seen with those front ends they have little if any travel the way they're designed. How much travel does yours have? M/T Sportsman tires are not very good racing tires, I'm not saying you can't get them to work but it's going to be more difficult than a slick or a good drag radial.

[tduffy]

Hi MAKO Real Street #6,

DOT slicks are on the menu. The MT Sportsmen tires were on the car when I bought it. I hope to learn about seting up the chassis with these tires in the meantime. Do you have a sugestion - I have 16" wide rims.

Yes - the front travel is lousy. I was dissapointed in the QA-1 Mustang II type adjustable type shocks as their travel was one inch less than the cheapy Car Quest shocks I took off. Any sugesstions for shocks?

Thanks,
Tim

[John_Heard]

Why do you want DOT tires? Are you intending to use the same tires at the track you're using for cruising around? If you had some narrower rims I'd suggust the P315/60R15 Mickey Thompson ET Street Radial as a pretty good compromise, but I think 16" rim might be a bit too wide for their biggest radial.

M & H does make a huge 390/45R15 Drag Radial that should work on 16" wide rims, it has a 14.8 section width. I don't know anyone that has tried them yet though. http://www.mandhtires.com/store.php?crn ... how_detail

On those front shocks, You might want to consider lowering it as much as possible to get some more travel. If you really want to increase it I would probably think about modifying the front end so the shock mounts on the upper cage tube above the a-arm, then use a regular coil over shock setup on it. That wouldn't be too hard of a modification.

[tduffy]

MAKO Real Street #6,

Yes - I drive to the track - about 40 miles each way to either Irwindale or Fontana here in So. Cal. No other choice.

Did I misuse the phrase "DOT tires"? I need street legal tires - I thought they had to have DOT approval?

Thise 390/45R15 MT's look good. I see they are DOT approved.

Here's what I have now:
Rear Tires:
S375/60D15
Tread width is 14 inches
Max tire width is 17 inches
Rim size - 16 1/4 at widest point, 15" diameter

Thanks,
Tim

[John_Heard]

DOT = Street legal, you wouldn't want to drive in the rain with most of them though. Those big M & H's with the suspension sorted out should massive bite. The simple stuff like the tire and putting some front travel in it will make a HUGE difference.

On that 4 link setup I don't know what would work best for your car as a baseline setup.

[David Lemmond]

I think you will need to back the IC to the 48" and 7 to start but those MT sportsmans need to be replaced asap.

[BADGMC]

I see you have Pro Trac tires and not sportsmans. They are only good for street driving as they are hard as a rock but never seem to wear out. The only way to make them hook is to put a 6cyl in the car. A friend of mine uses those 390/45/15 m&h drag radials and they have no issue hooking at the track and they seem to wear really well on the street.

[Dave Morgan]

Howdy,
What happens when you soften the rear shocks? The advice the other cats gave you about tires and front travel is all valid, but let me ask, what type of spin are you getting? Is it immediate or does it plant, then spin? Also, how much travel are you getting at the rear suspension? Ask someone to watch the car and look at the wheel-to-fender clearance. If you race alone, make a poor-man's travel indicator by tying some dental floss to the top shock mount, then span the floss alongside the shock. Tie a knot in the floss and at this point, tape the floss to the body of the shock with the knot at the upper edge of the tape. This is all done with the car at ride height. Once the car launches and the shock extends, the knot on the floss will be drawn upwards and away from the edge of the tape. When you return to the pits, measure how much the knot traveled away from the tape. This will give you a very close idea as to how much the rear suspension separates. One word of caution, drive slowly on the return road, otherwise you may be tuning the car for the dips in the pits and not the launch on the starting line.
Stay Tuned,
Dave Morgan

[tduffy]

Hi Dave,

Thanks for your reply - and thanks to all who offered suggestions.

It's been a while since I posted the original question and there have been many changes. I've read your book at least 1 1/2 times. I have Hoosier Quick Time Pro 30 x 16.5s on the car now. I have experimented with the 4-link setup and now the car is dead hooking, but with a little more squat and twist than I am happy with.

I have two follow up questions if you don't mind:

1. My best ET now is 11.85. I see much faster cars at the track and on TV that get from A to B with very little chassis comotion (twist and rise). Shouldn't I be able to achieve better performance by tuning out this chassis comotion?

2. I can't understand the mathimatical derivation for the neutral line. Not that the rise or squat is minimal for ICs locations along that line - but why? How do the force vector's magnidute and direction add up along that line that makes it neutral?

3. With 4-Link wizard I can see many bar/hole location combinations that result in identical, or very close, IC locations. These must result in significantly different bar forces. How can I evaluate/compare these similar locations?

OK - I lied that's three questions.

It is better now with a 1/2 turn top right bar preload and front rebound damping at 2 - 0 (L - R) - but still lifting the left front off the ground - and squatting the RR.

Launch video is here if interested --> http://racetec.cc/1967Nova/1967%20Nova% ... 8-2007.wmv

And thanks for the great tip with the dental floss.


Cheers,
Tim

[Dave Morgan]

Tim,
First, let me say thanks for sending a video that a computer-challenged fellow like me can watch. I've never been able to download a video image off the internet from a drag racer before.
I'll answer your second question first, concerning the neutral line. I'm not smart enough to explain, in mathematical terms, how the neutral line works. It's a concept I read about in an older edition of the Chevy Powerbook and I kept it around when trying to tune a car. I found that generally, it is applicable to drag racing so I included it in my book.
In regards to your first question, you do seem to have a lot of squat in the back of the car, but I'm not sure it's entirely related to the four-link. By watching the front end, it seems like the prings may be a little stiff, causing the rear to do more squating. It's hard to see because the camera operator jerked the camera ahead of your car once you launched. If the springs are correct for the car, then the reaction I saw (squat) would tend to suggest the IC is a bit long for what you need. You also wrote about having a lot of "twist," (roll rotation). This rotation gets worse as the IC gets longer so we have two clues that your IC is too long. A shorter IC will reduce the squat and the roll.
Concerning the similar IC lengths... yep there is a difference, this is called kinematics (the study of links.) Assuming you'd like to plant the slick harder, which is what appears to be the problem (when you look at the wheel-to-ground distance) then you can do a couple of things...
1) Move the upper bar axle housing hole closer to the axle centerline, this will give you more pull through the upper bar.
2) Put a little up angle in the lower bar, being careful not to change the wheelbase of the car and also taking extra care to keep your rod ends clean. I realize this is against much of what I've written about in the past, but it will plant the tire harder, yet create new problems for you, mostly in the form of roll-steer (the wheelbase will change when you grab a gear.)
3) Eventually, you may want to raise the lower bar one hole at both ends (after trying step 2), as this will give you more push through the lower bars.
Now I have a couple of questions for you...
1) What's your frontend weight and how heavy are your springs?
2) What's your mph?
Please let me know and I'll be looking for an announcement in my e-mail from the system administrator.
Stay Tuned,
Dave

[tduffy]

Hi Dave,
My 4-link setup picture is attached.

My front springs are 425 lbs/in. These are standard big block springs that come with the TCI Mustang II type front end. Spring choices are few. Spring spacers don't exist, and travel is short. It will take some serious customization to do better.

My mph in the 1/8 mile is 91 1/2. Estimated 112 mph in the 14 mile.

The car weighs 3259 lbs loaded; the front weight is 1756 lbs.

Thanks,
Tim

[BillyShope]

1. My best ET now is 11.85. I see much faster cars at the track and on TV that get from A to B with very little chassis comotion (twist and rise). Shouldn't I be able to achieve better performance by tuning out this chassis comotion?

Definitely!! Any twisting of the front end is a sign that your rear tires are not being loaded equally. If the rear tires are equally loaded, there is NO front end twist and the left front and right front rise equally.

2. I can't understand the mathimatical derivation for the neutral line. Not that the rise or squat is minimal for ICs locations along that line - but why? How do the force vector's magnidute and direction add up along that line that makes it neutral?

The easiest way to visualize the effect of the 100% antisquat line (neutral line) is based on the fact that a link, freely pivotable at both ends, can only support a net force which passes through the two pivot points. The entire rear axle assembly can be considered a very strangely shaped link with one pivot at the tire patch and the other at the instant center(IC). The vector sum of the weight transfer and tractive force must then act on a line through the rear tire patch and the IC. If the IC is on the 100% antisquat line (which also passes through the rear tire patch), it has the same effect...on the chassis during launch...if it is located ANYWHERE along that line. So, let's move it up that line until it is directly above the front tire patch. When situated here, it is obvious that the inertial force, acting at the CG, and the tractive force are canceling each other and that the weight transfer and the unloading of the front are doing the same. Since there are no forces or moments left to cause the rear of the car to either squat or rise, we can be certain that an IC located anywhere on the 100% antisquat line will do the same.

3. With 4-Link wizard I can see many bar/hole location combinations that result in identical, or very close, IC locations. These must result in significantly different bar forces. How can I evaluate/compare these similar locations?

You need concern yourself only with the percent antisquat. As I recall, your first post showed an antisquat of 19.6%. If you draw a line through the rear tire patch and that IC, any hole combination that yields an IC anywhere on that line will give you exactly the same launch performance. (An exception to that last statement: The squat or rise of the car can change the IC location as the link angles change relative to the chassis. The preceding assumes this effect is negligible.)

Note that the distance forward of the IC is not important. A misunderstanding has arisen here due to the manner in which most racers adjust their 4links. If the lower link is nearly horizontal and the IC is moved forward by raising the front of the upper link, the result will be a decrease in percent antisquat. This decrease in percent antisquat not only causes the rear to squat, but also causes more rise at the front. The wrong conclusion is that you get more front end rise as you move the IC forward. Actually, if the adjustment change described had been accomplished by adjusting both links in a manner which kept the IC on the same line drawn through the rear tire patch, there would have been no change in front end rise.

An excellent proof is provided by the unique situation when the links are perfectly parallel. Parallel lines meet at infinity, so, with the links parallel, 100% antisquat is achieved when both links are angled up at an angle with a tangent equal to the CG height divided by the wheelbase. But, note that the IC is at an INFINITE distance forward. If that misconception were true, the slightest tap of the throttle would cause a violent blowover!

[tduffy]

.
Wow - That was an excellent post!
.
I've re-read you're multi page suspension lessons many times trying to learn.
.
Mine was an old post. Since then I came to the conclusion that my frame holes did not get me the IC locations I wanted so I had the frame hole brackets redone. The story is here -->
http://racetec.cc/1967Nova/Four%20Link%20Mods.html
.
I now have more rearward IC locations around the neutral line. Here is how it looks now:

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The car now hooks every time, every track. 60 ft time is 1.530, doen from 1.6's.
.
More details are here --> http://racetec.cc/1967%20Nova.htm
.
I'm not sure I agree with you (I think it was you) about energy used to rotate or lift the car or hit the tires is not lost. Would you mond taking a moment to go over that?
.
Thanks for the excellent explaination. Very helpful.

Cheers,
Tim

[BillyShope]

[quote Tim Duffy]60 ft time is 1.530, doen from 1.6's.[/quote]

Could be better. Work on equalizing rear tire loads. The right side of the 4link should have more antisquat than the left. When done properly, this will result in equal rear tire loading for any value of driveshaft torque. See my site for details.