Launch problems

Bin fighting this all year. Only difference from last year is the flywheel. Last year and years before I had been running an aluminum wheel. Found it was warped over the past winter. Installed a steel wheel which is about 20# heavier. Now the car blows the tires off at launch. I've dropped my 2 step all the way down to 3K from 5K. I've dropped tire pressure down from 15 to 12 psi. I've loosened the rear shocks from 9 to 7. I can't get the car to plant or even control the amount of off line spin. I'm pretty confused at this time..

car weight with me in 3926#
slicks 29 12.50x15
gears 4.56
Rancho 9 ways out back
90/10 up front
Mopar SS springs

Any suggestion?

I'm no expert but I can see 20lbs of additional rotating mass at the flywheel driving the tires much harder at the initial hit exposing shortcomings in the suspension.

There is a lot of knowledge on this site so I'm sure you'll get lots of good info but the common thread is always get some video showing what's happening. I'm in the same boat and making progress thanks to this forum.

Take care.

How old are the tires and how many runs on them?

Yes. 20 pound increase is significant in a flywheel. Lots of extra inertia. You've already changed your shock settings and air pressure. It might be time for new tires, maybe a little wider, if you have the room. What clutch and trans are you running? Might need a little lighter clutch plate to allow some slip.

Kind of figured that adding 20#'s to the flywheel would mess up my set up, but also thought that I could adjust for it with 2 step, shocks and tire pressure. Tires.... They are old, BUT, still have good depth and I treat them with VHT before every race. I can put my finger nail into the tread area and it leaves and indentation. Like I said, last year I had no issues 'til toward the end of the season when the flywheel started going bad.
I have ALWAYS run an 11" CFDF clutch. I drive my car on the street. I don't want to get into adjustments and such. Too old to be crawling under the car before a street cruise or race event. I also realize that what I do with my car is different in a lot of ways than most. There are certain things that I can do and won't do. It's one of those "nitch" things that only a few of us fall into. A good friend of mine just suggested lowering the tire pressure down to 10 psi.

On the tires..........how old is OLD. If they are more than a season old, they are likely junk, no matter what you treat them with or the depth of the holes. The sidewalls are SHOT on a 3900lb stick car....BANK on it

Oh! they are way older than one year. Probably closer to 5 years, but I don't race every weekend. Maybe 4-6 times a year is all.

The slicks are removed after every race and stored. Last year they were working good for me except for the flywheel issue I had.

I remember one season when I was having troubles going any faster than 9.6s. Kept doing things to make more power. MPH would go up, but never a better E.T.
I ended up getting new tires. When I took the old ones off, they were so structurally wasted, that the tire wouldn't stand up and keep it's shape when off of the rim. These were 31w tires(stiff sidewall) The new tires with no other changes went 9.20 on the first hit.

Remember, I said that I started with the shocks on the highest setting (9). Went lighter thinking that it would hit the tires harder. Usually I run the shocks on the highest setting (9).
From what I've read so far, it seems that a new set of slicks maybe the ticket. I'm going to hate pitching these slicks, even though they look good to me.
What about,...I also run tubes in the slicks. Maybe dropping the tire pressure way down (for me), to about 9 or 10 psi? Do you all think maybe this might work?

Sell the old slicks on craigslist for 100 bux

9 or 10 pounds is kind of light for the weight of your car. Be aware of the way the car handles down the track when trying. Maybe try adding 100 pounds ballast to the trunk first. New tires does seem like the ticket, though.

I'd try a set of stiff walls, with tubes.

'thought soft sidewalls were the way to go with a stick car????

You might try letting the clutch out under 3000 RPM to se if the car will bog or go My old 1970 NHRA A/S Hemi Cuda( a 383 car originally converted to the hemi drivetraiin and motor) would bog with 4.10 and the 1968/69 OEM 173 tooth heavy flywheel on a good track on 9x28 inch slicks. I had Bob Lambeck put a set of 4.89 ratio gears in the rear and it never bogged again umtil I put a 18 lbs 130 tooth aluminum flywheel in it I would have the old aluminum wheel surfaced to see if it would be useable agian or not

Sounds like it's time to step up to a shock with stiffer rebound valving. You were full tight and now you're hitting the tire even harder. A good slow motion close-up at launch will lead you to what is needed.
Doug

Cab,
My aluminum wheel cannot be resurfaced. It's a Mcleod wheel, and they want way too much to either replace the insert or for me to buy it outright and replace it myself. (I do have that capability). Anyway, from what I've learned, aluminum wheels don't last long in a street/strip application. mine lasted only 2 or 3 years. (can't remember exactly}.
If I have to drop the 2 step below 3K, what's the use in having a slick?
Last season, my 2 step was set between 48 and 5200. Tire pressure at 15 psi. No bog. Launched hard, but due to the aluminum flywheel issues, the clutch would slip in high gear. Hell, I'm using the same clutch assembly from last year with the steel wheel and it grabs hard.
Nothing has changed from last year 'til now except the steel flywheel. This is what's so confusing....

I think replacing your 5-year-old slicks would be my first step..

Negative, there ghost rider........LOL!!!

3900lb stick car, I would want stiff sidewall tire, tubes, beadlocks and anything else I could do to stiffen em up.......LOL!!!

As to your 5 year old slicks. The sidewalls were probably toast about 4 years ago depending on how many licks they had

If they are Hoosiers, the sidewalls were too soft when first aired up.

They are Hoosiers. So if I buy new slicks, should I go with MT's then, if the Hoosiers are too soft? If so, I don't see an exact match to the size I have now. I see a 29.5 X 10.5-15 with a stiff sidewall. I also see a 29.5 X 11.5 - 15 which would be within a 1/2" of what I have now. Which would you choose?

Your clutch hits the tires/chassis harder than they can currently handle.

When your engine loses rpm on launch, it's dumping stored inertia energy along side the torque the engine is making, creating an intense torque spike that will be gone as soon as rpm stabilizes (heavier flywheel just made it worse as it stores more energy). That rpm loss probably only lasts for .3 of a second or so, so to me optimizing the tires/shocks/chassis to counter that .3 second spike results in the tires/shocks/chassis being less than optimum for the rest of the run. If you reduce the hit of your clutch just enough to eliminate the rpm loss on launch, your tires/shocks/chassis can be optimized for the entire duration of 1st gear instead of just the first .3 seconds. No need for super stiff shocks if your clutch does not hit super hard.

The resulting launch will be softer, but it won't be slower. You can likely do this with the clutch & steel flywheel you currently have, just requires thinking outside the box.

I hear what you are sayin', but the engine does not loose rpm at launch. When I leave out on the clutch the tires spin instantly. Sometimes the car will walk out with the spin, other times the car just sits there with the tires spinning... I'm seeing the light that I need a new set of slicks. I just have to decide what make and size to go with..

I would call M/T and get a recommendation. But would do what Monty suggest. That's what I did over the winter.
Monty helped me get rid of my tire shake issue.

You probably don't notice the loss. Do you log rpm? With no changes other than going from aluminum flywheel to heavier steel version, that indicates to me that a more intense torque spike on launch is what's knocking your tires loose.

When you get the clutch to hit just right, you will be able to drop the slicks and step right up to radials.

ALWAYS go with a stiff sidewall slick. For your car, I would go with a M/T 29.5x9 STICK tire. They are similar to a 10.5W tire, and have a stiffer sidewall.

Not sure that going with a tire that will be 3" narrower than I run now would be the right choice? I did find a set of Hoosier stiff wall 29 10.50 X15"s that are supposed to be 1" narrower than I have now. I'm leaning toward them at this time. The MT 29" stiff walls are supposed to actually measure 10.2" wide but are 29.9" tall. The Hoosiers are supposed to measure the same as I have now at 29.3"

3062S M/T 29.5x10.5 STIFF
3062W M/T 29.5x10.5W
18175 Hoosier 29x10.5W

I've run all 3 with no problems, all are stiff sidewall tires. Buy whatever one you wish, they'll be better than what you have. Like everyone else has said, RUN A TUBE!!!! 3900# stick car is hard on tires.

[quote=Triple Threat]3062S M/T 29.5x10.5 STIFF
3062W M/T 29.5x10.5W
18175 Hoosier 29x10.5W

I've run all 3 with no problems, all are stiff sidewall tires. Buy whatever one you wish, they'll be better than what you have. Like everyone else has said, RUN A TUBE!!!! 3900# stick car is hard on tires. [/quot

I already run a tube. I'm actually leaning toward the Hoosier 18175. It should be a direct fit.

Perfect, you'll be happy with the new tires.

Hey Larry, what is your wheel width?
For what it is worth, ran both the hoosiers and MTs of approx the same size. Seemed to get more life out of the MTs, but the Hoosiers didn't leak w/o a tube. That was nice, but makes no dif w/tubes

Buy the MT. It's a better tire

What about the difference in width?

My wheels are 10". I already run tubes, so that's not an issue. The MT's are taller but narrower.

The 10" rim is perfect for the 9" tire. It will allow you to run more air. Almost every stocker runs a 9" tire with a 10" wide rim.

Just checked with Summit. Out of Stock til August on the MT's. So I guess I'm going with the Hoosiers.

Jeff,
I'm not getting that warm and comfy feeling about running a tire that is that narrow.

Summit is out of stock, but Jegs has them. I just ordered the 29 10.5 x 15 stiff walls.
Thanks guys for all the suggestions. Really appreciate it.

another FWIW
The first time I ever dead hooked consistently on the convertible was with a 9" tire. Then could adjust the clutch properly. Also, best time and MPH. The stockers seem to get them to work pretty well

also, you might check w/mccormack racing. had good dealings with them, tires come in measured/matched sets, and same age. They are worth calling and pretty fast

http://www.mccormackracing.com/cgi-bin/l...p&order=123

also, if you are an amazon prime member, same price, in stock, in fri
but.....crap shoot w/size/age but can return

Harlow tire

2x been using them for 25 years

"Launched hard, but due to the aluminum flywheel issues, the clutch would slip in high gear. Hell, I'm using the same clutch assembly from last year with the steel wheel and it grabs hard."

You just answered your question. I guarantee you if the clutch was slipping down track, it was also slipping on the starting line enough to let it leave without destroying the tires. I'd buy an adjustable cover (Ram/Mcleod) and throw the centerforce away. Mike

At the risk of offending any of the hardcore guys like Monte or Mike-there is a product called Clutch Tamer-you can google it. Because your's is pimarilly a street car a soft-lok or one of Rob Youngbloods deals would be impractical. It appears that this is much like an adjustable shock absorber that would allow the clutch to slip when you launched thus avoiding knocking the tires off. Maybe its cheezy but it should be more consistant that trying to feather with your left foot

Same thoughts here. New tires, better shocks. Once you make it work, I would think the 60 ft will be better than ever with the heavier flywheel.

If the clutch slips in a way that there is no gain or loss of rpm during launch, flywheel weight won't affect the launch at all.

Where flywheel weight will make a difference is when you are balancing stored inertia against the clutch's torque capacity. Here's a simplified explanation- generally the clutch's torque capacity is higher than the torque that the engine makes. Let's assume the engine makes 500ft/lbs and the clutch's capacity is 700ft/lbs. When you launch the car, the clutch is going to draw 700ft/lbs…the 500ft/lbs that the engine is making plus another 200ft/lbs of stored inertia energy that will cause the rotating assy to lose rpm. This is where the balancing act comes in- after engine rpm is drawn down to the point that engine rpm sync's up with vehicle speed, rpm ceases to drop and that transfer of an additional 200ft/lbs of inertia energy stops. If you raise rpm or add a heavier flywheel, more energy is stored in the rotating assy which extends the amount of time that the clutch will be forced to slip. When you extend the time that the clutch slips, you are delaying the clutch's lockup to a point farther down the track where the car is traveling faster, which means less "bog" at the point when speed and rpm sync up.

In the end, installing a heavier flywheel or raising staging rpm are just ways of forcing the clutch to slip longer, minimizing the bog. The downside is that after you have lost the rpm and used that inertia energy to force the clutch to slip longer, that spent energy then has to be paid back in full before the engine can recover the rpm that it lost. That inertia energy transfer which made the car launch harder initially now slows the car, as it reverses and some of the engine's power must be used to recharge spent inertia energy back into the rotating assy.

This is why temporarily holding back some clutch clamp pressure at the throwout bearing to prevent rpm loss, is a better way to launch. The initial hit of the clutch is softer as you are not borrowing inertia energy from the rotating assy, but then again you won't have to pay that borrowed energy back either. The softer launch is easier on parts, easier to make the tires stick, and actually quicker as you can now easily extend the clutch lockup point far enough down the track that you won't lose any launch rpm at all. Remember- the car actually gains speed at a faster rate during that period of time before the clutch locks up. After the clutch locks up and the rotating assy is gaining rpm, some engine power is siphoned off and stored in the rotating assy, which means the overall acceleration rate will be slower after the clutch locks up because the rotating assy is being accelerated as well. If you want proof and collect rpm data, all you have to do is lay a straight edge along your rpm graph from the point that the clutch locks up in 1st gear until the shift point. The angle of that straight edge represents the average rate that the car gained speed after the clutch is locked up in 1st gear. Then lay a straight edge from the "0" beginning point to that same point where the clutch locked up...the angle (rate of acceleration) before the clutch locked up will be steeper. To a point the longer you delay clutch lockup, the longer you can ride that steeper rate of acceleration.

My ClutchTamer is just a simple way to temporarily hold back some clutch clamp pressure at the throwout bearing, but you can do it with your foot if you put in enough practice.

Moving on.... I've got my new slicks mounted and balanced. Went with the 29 10.5 x 15 stiff wall MT's.

Be ready for the wheelie

Larry, will you be at the Mopar Nats?

Probably on Saturday only, but not racing...

Pulling them up 1-3" is normal, even with some tire spin. Here's hoping that we're back to those type of launches...

Hopefully I will see you. I'll be there Saturday and Sunday with my Cuda.