04 May How to Make a Streetcar Launch
This article assumes that most who will read this wants to, or has, a real street car not running slicks and has 200 or less hp.
The whole idea is to get as much weight transfer to the rear wheels and keep it there. There are many things that can be done to accomplish this. First and the easiest way would be to add 90/10 front shocks. (This produced a tenth of a second better 60ft time when I was using 165 radials. Best 60 ft with these tires was a 1.90 and a best of 12.47) These shocks let the front of the car raise up easy and tries to hold the front end up by not letting the front come down nearly as fast. (Chrysler 64-74 model works great on ball joint and an adapter/extension will be needed on a link pin) The first thought that a lot of people have is drill out the existing shocks so it will come up easier. Well this does accomplish the up travel and yanks the front end up but most of us don’t have enough power to carry it. Thus, the front end comes right back down and unloads the rear tires. In most cases, a little tire spin will out 60ft a wheelie any day. Don’t take this the wrong way, because there are exceptions, and even I have experienced this exception. If you try to do a wheelie and bog the motor with a bigger than normal cam, the rpm will drop right out of its power band and your time goes right out the window. The whole idea is to have a little tire spin and keep the weight on the rear tires and it will move out. Another area to look at is the front beam itself and how your adjusters are set up. E.g.: I use the old (very old) style select-a-drop. This keeps the bottom leaf springs in their stock location and pulls or twists the top leaf springs down. This crates a binding situation. When putting a jack under the front end, the tires will not fall all the way down. With out down travel, you have defeated the whole point, unless you have the hp and rear tires to pull. Verify that the front will drop out all the way.
The rear end is pretty much a simple set up. There really is no need for big aftermarket torsion bars or any other fancy items. Although when using in the area of 190-200 and above with slicks, and a heavy car, this is around the cut off of when to switch to some bigger torsion bars. If you don’t have enough power when using aftermarket torsion bars, the car will have a tendency to unload and bounce back up and this is not what you want. You are looking to keep the rear end squatted. Front up and rear down. At what height should you set your rear end up? Well in my opinion, it’s not all that critical. I personally have my car set at a slight positive camber. But more importantly is the down stop that is needed to keep most of the tread on the ground. I have been using some old bus snubbers. A better bet would be to make some adjustable snubbers using the type 1 rubber. There has been talk about the bus snubbers being too soft and the rear bouncing off and unloading. So this will be on a car to car basis of what works best. Another good reason to have some sort of positive stop is to keep the fulcrum angel of a swing axle from becoming to great and hurting parts. Obviously an IRS suspension is another story all together. When using more rear travel, you have reduced the shock load in the gearbox as well. It takes longer to come up to full load on the gearbox as the suspension is going into compression. The longer you can delay the full weight of your car on the gearbox the longer it should live. One of Gary Larsen’s methods of setting rear ride height is can you stick a finger between the snubber and the stop and that’s right about where I have mine. A good set of stock shocks should be adequate but 10/90s are definitely not out of the question. This is something I am looking into.
I have noticed that during a good launch seen on video and pictures that the rear end will squat and then slightly unload. So this is telling me that it is not perfect. If at all possible, get as much video from the side as possible. This is a great tool, especially if you don’t have any knowledgeable crewmembers helping you out. In other words get that friend or wife to take that camera out there. A couple of other items that will help keep a consistent launch will be the use of a heavier flywheel and even a heavy front pulley. My personal technique of driving out of a heavy spin is to just blip the throttle and put my foot back in it. Using a heavy flywheel will keep your motor from just falling on its face completely. Using a heavy flywheel also accomplishes the option of using a lower launch rpm which in turn, give the tires time to plant themselves instead of just shocking them into a spin, and the motor will not die off because the weight of the flywheel will keep the motor spinning.
I have also found that when using the small tires, that making your accelerator pedal have more stroke then stock will help control feathering the throttle out of the hole. I have a cheap roller pedal, and drilled a new hole about a ½” lower then the original, and shaved down the stops on it. This will give more resolution in the throttle movement. So as you back pedal and your foot instinctively moves 1” you will only close the butterflies maybe ¼” of the way instead of ½ way closed.
Tires selection is another consideration. When using radials, I have found that going anything below 16-17 psi on pressure makes them go the other way. We can all usually blow off a set of radials on the street, but before you decide that you don’t have enough tire, try taking it to the track, they will react completely different with all the rubber laid down. When moving into some bias ply or maybe even a slick or recap. my opinion is only get what you need, bigger is not always better. Bigger can be a little more consistent, but we also want to drive our car back into the garage under its own power. My feeling is that if you get too big a tire, that will hook up, the pressure it pumped up in them as well and will make the shock load happen that much quicker in the gear box. If you use a 6” slick you might have the pressure set at 12 lbs., and if you have seen any pictures of slicks under a good load you have noticed that there are wrinkles in them. This is much more time to reduce the shock in your box.
A soft clutch will also help reduce the shock load to the box. And the less the shock load on the box, the less on the tires to brake them loose. Personally I use an old 2300# Kennedy clutch and a 5 year old stock rigid disk. Yes my flywheel and clutch have small blue marks here and there, but it works well. I must be doing something right, because I have heard that using a 4.37 in my car is a no no. But I can get 100 passes out of one before I change it out. Not brake but change before it brakes.
The above has come from trial and error on my part and other people that I know. What I have written wasn’t intended to be the only way to go or the only way that it has to be. I know some will read this and say, that’s BS because I have done this or that. It is only intended to give some ideas, besides they are only my opinion. Use this and what others have said and make your own decision on how you might want to approach your car. If we are all willing to copy one another, then how can we make it better? I myself am looking into switching to 10/90’s in the rear even though my car works well already. Keep it off the guardrail and I will see you at the track.
