a few tire psi is difference between spin and hop - Ford Muscle Forums : Ford Muscle Cars Tech Forum
 
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post #1 of 8 (permalink) Old 04-10-2018, 08:02 PM Thread Starter
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a few tire psi is difference between spin and hop

Any comments are appreciated.
getting ready for track this year.
over last season i was wrestling with wheel spin and wheel hop with no really good launch.
i have 62 falcon with 400HP 306CID, Gforce 5 speed (2.94 1st), 9 inch rear w/4.11 gear. Have leaf spring with caltracs and calvert drag shocks front and rear.
I run Hoosier bias ply 26/8.5 inch slicks. I usually go to the track when they prep it for slicks. I usually launch at 4000 rpm. My usual approach when i go to the track is to start with tire high around 20psi. what usually happens is the tire hits and unloads and then spins. Here's a video


I then start to lower tire pressure about 2 lbs and run again. spin is reduced but then starts to hop with a few more reductions in air psi. here's a video


The hop was much worse when i had stock leaf springs. I actually started to unwrap the front spring eye! I had some stiffer custom springs made and it got better but videos shown are with the new springs. I've tried leaving at higher RPM to try and avoid unloading but it just hops harder.
I will probably try an even stiffer leaf spring.I also want to try a drag radial.

thoughts?
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post #2 of 8 (permalink) Old 04-11-2018, 01:06 AM
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Re: a few tire psi is difference between spin and hop

What do you have your rear shocks set at and how do you have the Caltracs setup? I suggest trying stiff (7-9 on the shocks) and using the lower hole of the Caltracs with minimum contact (actually 1 flat from contact) of the cam to the spring when the car is at race weight, sitting level, and with the driver's weight in the seat.

You may need to add some ballast to the rear of the car.

It a stick car is spinning off the line, drag radials will make the situation worse as once they start spinning they will not stop it on a run. A bias slick will do a better job of hooking up again. Your Hoosiers should hook good if the side walls are not wore out and they are not several years old.

Dennis

65' Stang Street/Strip. Dart 434W NA, Victor heads, G101A 4 Spd, 4:56 rear gear, on 93 octane pushing 3550lbs. [email protected]
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post #3 of 8 (permalink) Old 04-11-2018, 06:27 AM Thread Starter
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Re: a few tire psi is difference between spin and hop

thanks Dennis. I was hoping you'd reply. Our cars are very similar so its good to hear your advice. Thanks for the advise on the tires, I was ready to spend $500 on the radials. My thinking was maybe they'd be less prone to hop.
I have the rear shocks set at 9 or the stiffest rebound. I showed Calvert some videos and they thought the shocks were bad. They had me do a crude test by hand to check for greater rebound resistance with setting 9 vs setting 1. They were stiffer at 9 so we deemed them good. Calvert also told me to lower the preload on the bars. I was at 1 turn but in the videos I'm at 1/4 turn. I'm in the car when we set them.
I'm on the lower hole of the front shackle.
I'm on my second set of bias slicks. both acted similarly.
When you say ballast do you mean weight? I was thinking that too and I changed the front springs to moog 8088 which brought the front end up another few inches. hopefully that will help weight transfer. In the posted videos I have moog 8088 but 1 coil is cut off. I also welded in a roll bar over last winter so added weight center and rearward too.
When I put the new springs on I put the car on the ground and moved it through the travel with the help of some friends. I did this without the shocks installed. There was much more rebound travel as compared to the shocks installed. I think the shocks were limiting rebound travel. I was wondering if there was a shorter upper shock mount that may leave more rebound shock travel available at launch. I did add about 1 inch of spacers under the upper shock mount to try to have more rebound travel available. I better double check to make sure the spring remains retained at fully rebound!
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post #4 of 8 (permalink) Old 04-11-2018, 09:32 AM
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Re: a few tire psi is difference between spin and hop

The videos"to me" show what I call "bunny hop". I used to get that when the "instant center" was too far back. A car with Calvert bars should "leave" like Dennis's car.

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post #5 of 8 (permalink) Old 04-11-2018, 01:55 PM Thread Starter
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Re: a few tire psi is difference between spin and hop

hehe "bunny hop". some years ago it was so bad that I had all 4 wheels off. In slow motion it looked like something from Disney's "Cars" animation. Good for a laugh but hard on parts!
Thanks for the reply. I have checked my pinion angle and it is about 4 degrees downward. I bought a Gopro I'm going to aim at the rear suspension to try to get a better idea of what's going on.
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post #6 of 8 (permalink) Old 04-11-2018, 02:55 PM
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Re: a few tire psi is difference between spin and hop

I just had some time to look at the videos. The first one might have been better with a lower RPM launch or maybe 50lbs added to the trunk. The burnout is fair as you can see a little smoke.

The 2nd one looked rough and you can see the leaf spring jumping. The front looks like it has good travel with the 8088's but the rear acted up before the weight was transferred. Its not really crushing the tire like you'd think it would with less air and there is not much separation in the rear either. The rear acts stiff, but I don't know if a stiffer spring is warranted. It is possible that the shocks bottomed out. The burnout sucked as there was no smoke-that didn't cause the hop.

The rear shocks could be bottoming out as you deducted. I setup my shocks with 1 to 1 1/2" of travel before the shock is fully compressed when the car is at rest. That allows for more shock extension movement. Simply crawl under the car, unbolt the bottom of the shock, set shocks to "1" and then compress the shock as much as possible to see how far it rises off the perch. You can also check how far the shock extends by removing it out of the perch and extending it as far as possible.

The rear suspension could also be binding. It is imperative that it move freely. You should loosen the front and rear leaf spring mounting bolts, remove them, and then put a little grease on the length of the bolts. Reinstall the bolts and replace the nuts with nylocks. Don't fully tighten the nuts on the bolts (allowing the bolts to be able to spin in the springs' mounts. The cams of the Caltracs should also move freely and hot be grabbing the spring.

The least amount of hit on the Caltracs has been proven to be with a slight air gap of the cam bolt from hitting the spring.

Another issue you may have is clutch related. If the clutch hits hard when you dump it the abrupt hit will cause wheel hop with slicks at the track. The slicks should be fine as I started with 26x10 Hossier QPT's and they worked well when my car had similar power as yours. (Typically 1.6x 60'ers using stock springs, Calvert shocks, Caltracs and a 4500-4800 rpm launch.) I didn't know anything about messing with the front suspension at that time so it was also stock.

What type of 60'ers do you have at what MPH to you cross the stripe?

Dennis

65' Stang Street/Strip. Dart 434W NA, Victor heads, G101A 4 Spd, 4:56 rear gear, on 93 octane pushing 3550lbs. [email protected]
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post #7 of 8 (permalink) Old 04-11-2018, 04:01 PM
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Re: a few tire psi is difference between spin and hop

Thought of a little more. When you do changes in slick pressure, do it in 1/2lb increments. That can be enough to make a noticeable difference. I went through my old logs from 2007 and saw that I was using 15lbs of air in the little Hoosier QTP's. I ran them as low as 12lbs but it was literally crushing the slicks into the track.

Also, you'll never want the car to dead hook so always expect (or create) a little spin off the line. 1 1/2-3 revolutions is about right. It will help prevent drivetrain breakage.

Do you have subframe connectors?

A couple of things can help the front end. The front suspension should move very freely when the coil springs are removed. Also roller front spring perches will react faster than the factory bushings. Removing the front sway bar (or at least the link on 1 side) will help too. I ran the Calvert front shocks for several years but they are not much better than having no springs at all.

Additionally on the rear I replaced my double stud Calvert shocks with ones that had an eyelet on the bottom. That helps free up the rear. Its not hard to weld on a new mount on the spring perch for it. My best run with the Calverts gave me 1.37 60'ers and 9.8x timeslips.

For ballast I stopped by Lowes on the way to the track and bought a 50lb bag of sand. Its not really track legal to do it that way so I put it in double contractor bags and didn't advertise it. I put it on the RH rear side of the trunk. The difference was felt immediately and I ran it that way for many years until I started loosening things up and figured out how to hook without it. 50lbs gave about .05 reduction in 1/4 mile ET, but the gain in consistency made it worth it.

Dennis

65' Stang Street/Strip. Dart 434W NA, Victor heads, G101A 4 Spd, 4:56 rear gear, on 93 octane pushing 3550lbs. [email protected]

Last edited by dennis111; 04-11-2018 at 04:03 PM.
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post #8 of 8 (permalink) Old 04-12-2018, 04:10 PM Thread Starter
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Re: a few tire psi is difference between spin and hop

thanks for the input.
1.7 sec 60 ft times and 115 MPH are what i ran in the videos.
I do have sub frame connectors and i did check for any binding in the rear suspension. It was all good. I have the spring eye bolts double-nutted without sqweezing the shackles. The front sway bar is removed.

Its interesting you mention the rear shock bottom-out. I asked Calvert when i installed them and noticed only about 1.5 inch of the compression available before bottom-out. They said if it doesn't get street miles than its ok. So, I kept it the way it is. You have me rethinking that. I was thinking it wasn't the problem because when I watch the videos in slow motion and it looks there there is no compression only extension. It appears the rear end and tires go downward and the body stays stationary or moves up. I going to put my Gopro on it. It has a nice remote app i can start and stop video from inside the car. Its new for this year hopefully i can really see what's happening.
Ill bring some weight to the track next time i go. ill try the 50lb on RH side trunk.

Good point about the tire pressure increments. Ill try that. I think i may also be wrestling with, what some call, the spring rate of the tire. I think there is a rebound happening with more or less force based on the tire psi. Does that sound reasonable? I've heard of road racers speak of tire spring rate but have not read much in drag racing.

Can i see some video of your car? Are you going to race it this year?
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