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Slicks or Shocks? Help tuning launch

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Jeff65SS

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Discussion starter · #1 · (Edited)
I need some help deciding which is the next move to obtain a consistent launch. My goal is have a more consistent leave so that I may be able to do a little trophy/bracket weekend racing.

Current setup as follows:
65 Chevelle
498 CI BBC
350 Turbo, tight 10" BTE converter - approx 3000 stall (maybe less)
12 bolt, 3.42 gear, Moser axles, Ford housing ends
Lakewood No-Hop bars
Factory style 1" rear sway bar
Boxed factory lower control arms, urethane bushings
Edelbrock adjustable upper control arms
Pinion angle, -2 degrees (relative to crankshaft angle)
Rear springs - 165#/in eibach (cut to achieve current stance)
Air Bag - pass side spring only 8-10 psi
Front springs - 365 #/in TRW (also cut to get ride height), can adjust front ride height via adjustable spring spacer.
Urethane front bushings
Shocks - KYB frt & rear, Tom Baird (MC71454) shock nut trick used on fronts.
Front sway bar - 79 camaro 1-1/4"
Front suspension travel - 5-1/2"
Rear tires - M/T ET Street Radial 275/50/15, 10" thread width
Ballast - 100# sand bag in trunk

My best 60' and et are shown in my sig. I can get about one good leave out of three. On a bad leave it will slip the tires until I feather the throttle. Usually lose about 2 tenths on a bad leave. Current launch technique is to power brake to 2500 rpm plus. If I try to leave with any less rpms it will spin for sure. More power brake rpm seemed to kill the converter hit. I get alot of rear seperation on power brake and launch, see photos for difference between at rest and launch.
http://www.chevelles.com/showroom/data/500/medium/DSC01368.JPG
http://www.chevelles.com/showroom/data/500/medium/DSC01375.JPG

I'm considering trying some bias ply tires (true slicks or street slicks) or keeping my ET Street Radials and trying some better rear shocks (adjustable). I've read on here that the bias ply tires are more forgiving than the radials. Currently the street radials are 11.2" wide on an 8" rim and I can't go any wider on section width. If the section was narrower I could go a little taller but maybe only 27", I don't think 28" tall tires would fit. I've also read about how important the rear shocks can be to the setup.

So my question is which way would you go and why?
 
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#3 ·
Take off the KYBs and get some drag shocks. Not just the rear, but all the way around. I went the cheap route with CE shocks, they're the bottom of the barrle, but way better than the KYBs I pulled off.

Why shocks over tires? I've seen the difference cheap shocks can make, it's huge, even on a car already running slicks. Do me one better and buy good shocks. You already have drag radials, which are better than street tires, and will wear out with use. When they do wear out, then buy better tires. build the suspension to hook better now, get better tires later and hook even more!

Devin
 
Discussion starter · #4 · (Edited)
Mike, I forgot to mention that I use Tom Bairds shock nut trick on the fronts (rubber bushing cut in half and two thin jam nuts). I can jack up the front of the car and the shock bushing will never touch the frame. I've tried removing the front sway bar and it made no difference at all. I had to cut the front snubbers to get the 5-1/2" of travel, no limiters.
 
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Discussion starter · #6 ·
Dave, thats one of the tires that I was considering. Also looking at the Hooiser 26 x 10 x 15 slick,

#18131
10" tread width
26.2 diamater
11" section width on a 9" rim (a little narrower on a 8" rim)
 
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Discussion starter · #8 ·
Bob, can you sucessfully street drive with #200 front springs?

I know that there are trade offs when trying to drive on the street and still have a car that works well at the strip. My car sits pretty low all around and I love the look/stance.
 
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#9 ·
Yes, the front end rises if I want it to. I can also stiffen the shocks, but yes it is street friendly. I drove it 5 miles to the "street wars" race we had a while back, it handles suprisingly well with drag radials and drag fronts on the street ;)
 
#10 ·
Jeff,

There are a couple things that come to mind reading your info.

As Bob said the front springs having such a high rate and then being cut on top of that are definelty a limiting factor in weight transfer as they do not get the front end lifting before the rear end is reacting for a few reasons.

1. the rear springs are likely too short and are likely too stiff

2. The powerbraking makes the rear separate since you have the No-Hop Bars and when combined with the small amount of stored energy on the front springs, there is no chance that you will get the weight transfer you need no matter what shock you have. Also those Lakewoods make the rear separate to much by moving the Instant Center back to far for your power level. You need to have the front rise Before the rear separates, you are starting with the rear up already with the front having virtually no chance to transfer weight. So it is a combination of things going on working against each other.

3. One of the major reasons you spin without powerbraking is that Torque of your motor combined with the high mounting point of the rear upper control arms with the Lakewoods you have is hitting the rear tires too hard with the initial hit and then add to that very little weight transfer, you spin. When you are powerbraking, the hit is less because you have already consumed a lot of energy in lifting the chassis before you hit the pedal. What this leads to is a possibility of getting traction but really it leads to inconsistency as you are experiencing.

4. Right rear air bag typically works best around 20 to 25 psi.

5. With a lot of Torque the Drag Radials are pretty finicky and have a much narrower band of fforgiveness if they don't have the optimum amount of heat and pressure into them. Not saying they won't work, but it is harder to find the zone than if you had a Quick Time Pro of ET Street tire.

But, the real need is to swap out your springs and you may need to ditch the Lakewoods, as I have to ditch my Edelbrock No-Hops due to the high mounting point being too much of a hit in my Chevelle. Just got my Dick Miller kit for the Chevelle on Friday.

I see that the 12 bolt you have has Moser axles and Ford ends, is it a Moser Housing as well?

Please digest and I will get back later, gotta run to a meeting.

Tom
 
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Discussion starter · #12 ·
Tom, thanks for the explainations.

I could swap out the no-hops for a set of jegster lift bars that I have. I'm not sure how much they lower the mounting hole, I'll measure that tonight.

The rear end is a GM 12 bolt, not a Moser. I think that the Lakewood No-Hops raise the upper mounting point +3.5", alot for sure.

I thought that stiff rear springs were perferred over soft, isn't that what the Dick Miller set up uses?

For the air bag, I add just enough air to get the car to come out straight. Without the bag it would try and crawl to the right side of the lane on launch. If I were to put that much air in the right rear side would really be jacked up in the air. Maybe its because the springs are cut some (shorter space to fill inside of the spring). I'll try measuring how much different pressure values raise the right rear quarter off the ground and report back.

Front springs would be the toughest to change but looks like it might be the path to follow. What rate spring are you running on your 70?
 
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#13 ·
I have a few theories, most aligned pretty well with Bob and Tom.

One thing different that comes to mind is your gear ratio. I have 3.55's, my absolute best 60' is 1.594 on a 10.691 run that nosed over on the top end because of fuel level. My next best is 1.623 on a 10.591 run. Both were with 275/60-15 MT Street Radials and can be seen on the links in my signature. 1.65 - 1.7's and 10.7's are pretty common for me during warmer temps, which is the norm around here. My point is, 1.67's are pretty close to optimal for 3.42 gearing. Last fall I tried (currently still on the car) 275/50-15 Hoosier DR's with next to no negligible change in either 60' or 1/4 ET's.

What tire pressure are you running? I've tried everything from 14-20 psi. Found 16.5psi (digital guage) to be the most consistent.

How much does the car weigh? Mine, with me, is 3770 lbs. Removing a 1 1/4" front sway bar will knock 10-15 lbs off of the nose and free the action of the suspension, however, it shouldn't have any effect on the rise height. Stiff front springs hamper rise, cutting them has a progressive effect on the amount of travel. Using hypothetic figures, each +/- inch of spring (installed) height, is tripled at the ball joint because of the length and leverage of the control arm. A taller squishier (small block) spring will be longer and have more coils compressed, effectively giving a smooth long stroke of energy and weight transfer that seems to work well (coordinate) with the "action" of the rear suspension.

Which brings me to this. By launching with the suspension loaded (powerbraking), you're losing all of the energy stored, as well as, the "action". The action is the mechanical advantage created. It is tuned by optimizing the geometry of the collective components so that their reactive motion allows the mechanical energy to be transfered or directed in whatever particular manner that the engineer has deemed detrimental to the effect desired. Weight transfer, instant center and lift (seperation) are all the effects of a dynamic reaction to applied energy.

Translated: launch with the throttle just off idle.
 
#14 ·
Tom B. & Rowdy have very good points & they are correct!! Also with the 3.42 gear & big block low end torque you will have too much wheel speed & fry the tires past recovery. a 4.1 -4.30 gear will limit the early wheel speed & the car can catch up to the wheel speed earlier, when I went from 3.73 to 4.56 I started wining championships

Good luck
 
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#15 ·
Trying to hurry out the door, I screwed up in my previous post. Instant center is not a reactive motion, rather, the hypothetical intersection created by the positioning of the control arms. The location is critical because it effectively demontrates where on the chassis the rotational energy is applied. Altering the control arm angularity, therefore, the IC, results in a mechanical advantange by basically leveraging the push and pull force transfered to the chassis.
 
Discussion starter · #16 ·
Rowdy, I found that 17-18 psi worked the best for me and the tires had to be hot (good burnout) to work right. Small burnout usually provided best chance for spin. on a good pass i could leave with zero tire spin, pretty much dead hook.

I'm sure the car weighs close to yours, never weighed it though. I should scale it the next time I'm at the track.

The power brake deal is a bandaid to get it to work. I want to get it to where I can leave just off idle like you suggest.

grpweld
I want to try and make what I got work as well as it can without changing the gear. I love the way the car drives on the street and don't want to compromise that just yet, just want to make it more consistent. If I get more serious then I'll consider a new gear and probably a converter.
 
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#17 ·
Jeff,

I hear ya on the 3.42's as that is what I run in my Chevelle, any more gear and I really run out of camshaft. ;)

Just for comparison, I leave off idle (1000 to 1050 ) with the Chevelle and 1200 in the Monte.

On your question about the front springs in my chevelle, I really do not know, as they were in the car when I bought it 3 years ago. It did have a small block in it, and the way it handles it seems like they are stock SB springs.
 
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#18 ·
The pass in the first link was at LVMS, during Car Craft's 2007 AntiTour visit. All that smoke was from me. I was experiencing similar hook ratios (1:4) and exploring different burnout procedures. That particular run cost me dearly, having gone around the water box, then burning it in hard and long. With a great launch and personal bests across the board, this became my new ritual. Hook consistency still eluded me and necessary tire replacement tripled.

Another racers snafu on the line forced me to have to improvise one time and thankfully, I stumbled onto the staging ritual that I now swear by. With the car in drive and my tires just passed the water box, I'll start them spinning slowly, increasing rpm through the gears. Once in third I'll wind it up hard and let it roll out. Sometimes, if I question the feel (traction), I'll give it another quick blip before coming to a stop. I shift into neutral for another blip to clear it out (might be psychosymatic or superstitious) before dropping it back into first and staging.

Whatever the reason, all I know is that my hook ratio went from 1:4 to 12:13 regardless of track condition and with no other changes
 
Discussion starter · #19 ·
Rowdy, what suspension parts are under your car? Seems to work pretty well.
 
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Discussion starter · #20 ·
Tom, what do you think about switching to the lift bars? I would think that they would not change the IC as much as the No-Hops do.
 
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