Well, I gotta jump in on this too. I agree that the "porpoise" launch isn't optimal, however, I used to run a street/strip '90 Mustang Vert with a road-handling suspension down the 1/8th mile. At launch, that thing would jump up so much I actually thought the rear tires were going to come off the ground! Remember, the car was set up for handling - NOT drag racing. Anyway, I was getting 1.5 60's with it jumping up like a kangaroo and then down the track! A lot of guys spent hundreds of dollars and many man hours on their drag-only setup just to get something close to a 1.6 flat. I had a lot of guys compliment me on my selection of drag springs and shocks and adjustmen. "Yeah, thanks" :confused: I have no intention of setting my strip-only Maverick up that way, it's just a matter of fact. Don't ask me how, it just happened. :confused: See this VIDEO. If you can watch it in slo-mo, you can see the rear tires almost leap off the ground (which is probably why this particular run was a little gnarly).
Do you have a reference you can point to on this theory? I'm not calling B.S., I'd find it interesting reading. Huh? If you're saying the pinion angle is supposed to be pointing down during the first 60' or so, then I'd find that interesting reading as well. I thought the pinion angle was purposely set to a negative 5-7 degreess (sometimes more) to counteract the upward roational force of the rear end. The entire rear end wants to rotate upwards under load. It does this because it is reacting to the (opposing) force of the tires. Also, the pinion angle, I thought, was determine by the angle of the engine and tranny so that, when the car is launched, the engine, drive shaft and pinon gears are all parallel to reduce the stress on the u-joints (wicked vibration if angles are way off) and to prevent them from stealing precious horse-power. Imagine launching in reverse, the rearend housing would want to point downards in that case. I'm so :confused: Thanks, Rick
Rick Book, after watching the video I've changed my mind. I don't have a traction problem compared to your Mustang, at lease I go straight when I spin my tires. Your tires didn't look like they stopped spinning until you changed gears. I can see by all the responses that we now have a small debate on how a leaf spring rear should be set up; well this is not my area of expertise and I’m only going to sit back and read from here on out. Once again a BIG thank you to everyone. Terry Gates AKA Bossmav
Having fun now A good book to read is "Door Slammers" helps alot to explain how things work. Well anytime any of you folks are in the southern Utah or LV area give me a heads up. Meeting someone is not a problem. And to think in H/S I drove a beefed up 66 Stang 289, C-4 and around this time 30 years ago I traded it in for a 71 Torino Cobra 429 SCJ, 4 spd, and my sweetheart could shift for me like a pro.
This is from a site Full of very knowledgable people when it comes to Drag racing... The drivers name is Melissa! http://mustangsandmore.50megs.com/Temp/MelissaLaunching.MPG I think the way Steven Moores comet launches in the "welcome to the jungle "video ,is the way a car should leave the line! Come on CometGt 1974 post the video again?
My choice of word specifically "leap" was poor . In a perfect world ,yes you would want the rear of the car to rotate perfectly on the rear axis . But leaf spring cars are far from perfect as far as racing suspensions go . You are trying to find common ground between what your springs are supposed to do ( hold up the car ) and what they dont do well ( drive the car forward and drive the wheels down to gain traction ) Light cars are hard to make hook with leaf springs . Light cars without a racing suspension ( ladders , 4 link ) are hard to make run to their full potential in the 11s let alone in the 10 s. Think about it this way . If you stomp on the gas and you assume that you have no weight transfer and your car sags down in back ,what is REALLY happening ? The wheels are going up because the crown gear is trying to climb up the pinion forcing the whole axle assembly to twist up . The leafs are trying to both hold the car up AND stop the rotation . Rick Book also misunderstood and confused my statements about the WHOLE axle assembly and the pinion angle . He is perfectly correct about why the pinion angle should be set so , under load , the ujoints should parallel or whatever your preferred term is to both reduce vibration and wasted horse power . As a side note I read a paper about the dynamics of applied power to offset shafts . Although it was more geared toward machinery in the steel industry the physics should still apply . I am using my memory here so the numbers may be a little fuzzy . Offset drives shafts using ujoint2 use up horsepower or energy after only 3 degrees of misalignment ( something around 5% of the total energy ) But when the alignment gets to be off around 15 degrees you can lose between 30 to 40% of total torsional energy ! Keep that in mind the next time you are welding your mounts on the diff . My fingers really hurt now
starting line Boss what is your starting line RPMs and your shift rpms? you can make a big difference with just 100 rpms on either point.
I'm leaving the line at 2000 and I'm shifting at 6000 my stall flashes at 4600. I now have a new set up (taller tires and modified traction bars) and last night I ran a 1.42, 1.46 and a 1.47 sixty foot times, now that’s not what I was hoping for but not bad either. The one thing that make me wonder if I'm going in the right direction is I'm now not carrying my front tires at all, before I could lift my left front about 8 inches and the right about 2 inches and carry them about 12 to 15 feet. I was only at a 1/8 track and my car is set up more for 1/4, so I'm keeping my fingers cross that with a 3 inch taller tire I will now cross the traps closer to 6000 (67 to 6800 before). The motor was broke in on the dyno this year because of a new cam I installed (easier to change the inner springs on the dyno) and on almost every pull the motor topped out at 6000, so there is where I'm shifting. Last night I had no one to video tape for me so I'm still a little confused as to how much this has helped, but with that said it sure does beat the $#@& out of the 1.60 plus I was in. Thanks and any input is gratefully appreciated. Bossmav
torque curve Where is your engine torque working at, (cam specs)? Might try raising your starting line rpms to around 2600-2800 and see what happens. Also shaft at 6200 to lower your finish line rpms. You want to pull until the very last 50ft of the race. You actually don't your front end off the ground. To just lift up and barely touch. Otherwise you shock load the rear suspension and break the tires lose.
Someone else pointed out that you are wasting time and energy going up instead of ahead which is right . Just concentrate on your 60 foot times and dont judge your launch by how far you carry the wheels . Wheelies ARE cool though .
Yeah... I've been told that picking up the front tires only means you are wasting energy, some old time racers told me this as well. Lightning said it… it is cool to watch and the pix looks good on my refrigerator. Ricky, I’ve thought about rising my launch rpm to around 3000 and see what happens. According to the dyno my peak torque is at 4500 and my stall is 4600 - 4800 to flash but I’ve only seen it flash to 4600. So I’m real close to peak torque to stall speed. I’ll see what happens this coming weekend, I’m going for 3 days with one of those being test and tune so I’ll going to try what has been recommended. Thanks again everyone, It always nice to helped by the best. Terry Gates AKA Bossmav
Kind of interesting that you point out that your 60' times are lacking. Today my time trial run I had a 1.78 60' on and off the throttle trying to find some sort of traction. Came back to the pit and loosened up the front suspension to make it rise more on the launch, and added an old battery to the trunk on the LH side to help with rear weight. Went up for the next run, left exactly the same @ 2500, converter flashed to 4800...ran a 1.49 out of the groove. Now the front wheels are picking up out of the beams and I'm redlighting. Came back, tightened up the front end limiters, but left the ballast battery where it was, next run was a 1.41. I was happy, and there was no tire spin, at least not for the first 80-100 feet. There is normally a bit of spin when I shift from low-high gear. I felt that today would be a contest of who could get their car to hook--and I was right. I made the mav hook well, averaging a 1.45-1.46 60' and took home some money. Point being, stick with it--and all the great folks on here have good suggestions that might be worth a look.
Those are some great 60' times mavman! Jeez! Ever get a 1.3X? Sounds like your on your way to getting it if not! Good info btw, Rick
Yes, I ran a couple 1.39's earlier this season. I'm working on some other chassis tuning as we speak...and hopefully soon I'll be able to stand on the transbrake and see what happens. I did it once last season on T&T day, my firse experience with a transbrake---and I must say, it was pretty neat. 60' was the same, though...but tire spin was HUGE problem from 40-80' down track (when the front tires settled down.) Also, I'm working on putting the mav on a diet...it's heavy, and getting rid of some excess body reinforcements, etc. It's still all steel, with all of the factory panels, everything...I have never weighed the car, but I would guess it'd be around 3000-3100 lbs. Sure would like to get rid of about 500 lbs....
the way i have heard this launching thing is the body should raise because the more the body raises it is pushing that much harder on the tires,, i know im not a expert in this either and im not even running leaf springs instead shortened ladder bars and coil overs but i bet my car raises 2.5+ inches in the rear but the front comes up too, it had a couple of 1.42 60' times footbraking it,,, and im running 28x10 Goodyears with no modifications...
