small 1/4mile oval tips, transitioning from road racing

[rob burgoon]

Ok, made a bunch of changes. In the past I had raised the chassis mount of the J bar without adjusting length. I put the J bar back down to about 7" (I think). Lowered the right side ride height to 6". Changed the bite from -70 to +10. Left the 850 RF spring in place. Changed the RF UBJ to a +1/2" model. These changes should lower the rear roll center and raise the front roll center, and hopefully tighten the car up.

RR was a little worn so I put some sipes in what was left and did a test day.

At the test day the first 5 min session was good (but that session is always good since the dirt is softer). The second session wasn't amazing, but seemed to be better than two weeks ago. The car was worse in the next session, got out and saw the RR had corded. Put the spare on the RR and the car seemed "not terrible" in the next session, got into the rev limiter a few times (5500rpm), then it went to crap again in the next session or two. Noticed I had corded the LR and was out of tires so went home.

I would have liked to have swapped the left side tires to put the bad one up front to see how that felt. Also if the tires were a little more stable in life, I would have tried changing the cross weight to see if I could eliminate mild to moderate looseness from mid to exit.

Conditions started out as lightly watered dirt that was good for sliding in, then shortly after the dirt seemed to pack, get hard, and a black ribbon appeared that only got wider. As the track blacked over, holding a slide got harder and harder and towards the end I was experimenting with driving straight in hard and thinking about shifting brake bias forwards. The rear tires were conking out as well, so perhaps that played into it. Tire temps were 145-150*F across both front tires, 190*F on rears. I was going down most straights sideways the length of the straight.

[rob burgoon]

Under car video here:

http://www.youtube.com/watch?v=EjwDOwqpDdQThe

LR appears to be bottoming out on a tube, I think I need to add a droop chain or strap so the tube doesn't get beaten up further. Anyone know where those are for sale or what they are usually called?

[Bcollins82]

I think you should start off by listening really, really close to what Just Add Dirt says. You are getting info from one of the fastest guys in the country, you should pay very close attention to the advice you get from him. You are hitting the chip because you are spinning the tires.... Start with a gear change that gets you in the ballpark. A 4.70 on a 1/4 mile is not in the ballpark. Take the 850 out of the RF, its keeping you from getting over on the RF and letting the LR travel to add drive and sidebite. Im not sure where you are measuring your ride height from, but normally the right side is lower than the left. 2" higher on the right side is a ton of tilt the wrong direction. As a rule of thumb, center of balljoint level with center of the RF inner pivot bolt of the lower control arm, left side inner pivot bolt 1/4" higher than the right, and 1" of rake from front to rear measured at the bottom of the front crossmember to the bottom of the framerail on the RR in front of the lower link mounts. Thats not exact, but will get you close. Also, from your under car video, you need to change the LR underrail bar because its not letting the LR travel enough. Adding a limit chain to stop it shorter will make your problems worse. Limit chains are good, and a great tuning tool. Set up a limit chain thats adjustable once you get the underrail dropped so your suspension travel isn't being limited by it. My advice to you is to start by taking the car to someone that understands how a dirt car works, and is fast in your area. Let them go through the car and get it right, and explain to you why they are making the changes. Once the car is closer you will be able to concentrate on driving more, and making small tuning changes to the chassis and tires. Good luck!

[oldtrackchamp4x]

Agree with 82. Also if you know who built your chassis, call them and they will get you dialed in on setup if no one will help you.Get a baseline setup and drive the car like the fast guys at your track drive since you are just starting out on dirt. Don't slide the car. Finally, ONLY MAKE ONE CHANGE AT A TIME. JMO.

[rob burgoon]

My engine makes 350hp and reds at 5500. If you divide your highest not spinning RPM by diff ratio, you get rear axle RPM. People I race against are at about 1200 axle rpm at the end of the straight. My rear axle rpm at redline is 5,500/4.7 1170rpm. It's already geared a little short, so when you guys say to shorten it way further, it sounds like crazytalk.

The LR underrail is a welded in tube, I'd have to get aggressive to lower it. The rear upper bar currently sees 42* at full droop. That's in the ballpark, right?

The car is a dirtworks from the 90s. I have what I believe to be the factory setup guide, and my flurry of changes all brought the car back towards baseline (big RF spring included).

Ride height measured from where the frame moves out a foot behind the front "wheel wheel", and where the frame rail stops before the rear "wheel well" (as described in the old dirt works setup guide).

I'm not saying I don't believe you guys when you say that 850 is too much, right now I'd be giddy if I could feel a push just once while the throttle is on, so I'm screwing around to see if I can get that. Since 850 is in the setup guide, I figured why not.

[rob burgoon]

"Take the 850 out of the RF, its keeping you from getting over on the RF and letting the LR travel to add drive and sidebite."

bcollins, let's dig into this a little if you don't mind.

Let say I soften RF from 850 to 650. On entry the car will rock further towards the RF, drooping the LR a little. However, mid corner, the softer RF means more weight from body roll on the RR which can overload the RR causing oversteer.

How does a softer RF create drive and sidebite at mid and exit? Seems like it does the opposite.

[Bcollins82]

"Take the 850 out of the RF, its keeping you from getting over on the RF and letting the LR travel to add drive and sidebite."

bcollins, let's dig into this a little if you don't mind.

Let say I soften RF from 850 to 650. On entry the car will rock further towards the RF, drooping the LR a little. However, mid corner, the softer RF means more weight from body roll on the RR which can overload the RR causing oversteer.

How does a softer RF create drive and sidebite at mid and exit? Seems like it does the opposite.

I understand that the underrail bar is welded in. You are dealing with a chassis that was never designed to travel that far. Same as the ride heights, things are being done differently now. Cut that underrail and bend up a new one and weld it in... Don't worry, all older underrail cars have had this done. Bolt in sections work well also, just in case you don't go far enough the first time. Things have progressed, and even though it may have been designed for it then, it doesn't mean its the best way to do it now. 42 degrees at full drop is close, I usually aim for 44* with both sides hanging, car up and level. Its the only way to do it consistently IMO, but its at 42* when dropped because it is probably starting at a steeper angle than most to begin with. Softening the RF by itself will not add forward drive or sidebite. It is part of a package most cars are running now that uses a softer RF to get the LR up easier, farther, and keeping it up. Dynamic bar angle in the LR also seems to load the LR harder than stiffening the RF spring. And by letting the LR drop farther will raise the rear of the car, and that raises not only the LR bar angle, but also the weight, spoiler height, and adds J bar angle dynamically increasing drive and sidebite.

As far as the gear, you said it was hitting the chip more when you describe the track as dry and black and with the rear hung out all the way down the straight. That leads me to believe that you are hitting it by spinning the tires. 5500 rpm seems incredibly low for a limit. Is that really the high end safe RPM or is that just where power falls off?

[RRR_Products]

Hey Rob what springs do you have? We will def ditch the 850 though. Depending on the night you want to run dry vs tacky have a few springs available for the front end and let's see the valving on the shocks. Depending on what shocks are at or springs we can go from there. Throw a 175 in the RR and 200 LR. Don't recall if you have a brake floater or not on that car. Get rid of the LR under slung and put a strap on it. 4.5-5.5" drop you can start with. With the amount of power you're running you'll be fine on a dry night and tacky may struggle. Nothing wrong with less hp as most of the time too much can mask handling issues anyways. If I were to suggest where to start and what I know works well is a 750 RF and 550 LF in dry or if very dry also lighter RF but again the shock valving is something we would need to see. Just get a 550 LF though to start and get those shock numbers.

[rob burgoon]

Sounds good, looks like I'll be cutting the bar out and adding a droop chain soon then.

5,500 is where GM says to stop.

"RPM limits are critical to engine life. Catastrophic engine failure can occur if the engines are run
above the factory recommended limits. Extensive dyno & track testing has determined the limits of
the engine. Under no circumstances is it recommended to exceed these limits. GM recommends
that all sanctioning bodies, track operators or promoters have rev-limits written in their rule book.
Maximum limit for 602 engine is 5500 rpm."

If I was spinning the tires hard, I'd expect to zing the engine at corner exit when there's still a side load, not at the end of the straight when I am the most hooked up out of the entire lap. As the track got blacker, I was playing with a more diamond driving pattern and driving in deeper rather than trying to rotate early. (since sliding in became impractical). That might have extended the straight a little.

[oldtrackchamp4x]

Does the car have a metric or chevelle front clip? If it is chevelle then the 850 is way to stiff.JMO

[rob burgoon]

Does the car have a metric or chevelle front clip? If it is chevelle then the 850 is way to stiff.JMO

Chevelle I think, but I'm just guessing. What would the symptoms of a way too stiff RF be?

[rob burgoon]

602 crate produces nice useable power past 5500.. around here they run 6200 chip in them so they are chiping at approx. 6000.. just because the sticker from gm says 5500 doesn't mean that's the way to go.

Nice usable power is good. How long does the engine last going to 6k?

[rob burgoon]

Just checked droop on LR, 3 inches of droop travel.

[rob burgoon]

I see them for sale all the time around here 100 plus nights...

the speed limit is 55 on a lot of highways, you don't actually drive around at exactly 55 do you? same thing with gm sticker on these made in mexico motors.

Doesn't the strain on engine parts go up with the square or cube of RPM? My street car isn't at redline at 55mph!

But broken analogy aside, there's probably a safety margin to exploit. I'll consider upping the limiter instead of a gear change once we get the handling issues sorted and the speeds go up.

[charcoal01]

Many times I've dropped my total rpm by adding a taller gear, why? Because you're able to control wheel spin better and keep tires hooked up. I went to barona with my open motor ina mod and ran a 6.2 gear. You should be in the high fives at least.