I've been wondering why the car won't hook up that well from a dead stop. It actually does better in reverse but starts to hop pretty bad. My first ideas are either too soft or too hard of rear springs. In Forward I should have a descent weight transfer but it just cuntinues to spin until I let off. I'm thinking the rear springs are too weak to take advantage of the weight transfer. Tires hook up well if you don't jump on it but was wanting a bit more at the line. Any ideas?
get it to hook up
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My take is the front spring shock combo is too firm. That combined with the lightweight of the car, open differential and the tire patch contact area. Wider tires don’t seem to help much in this scenario either because there winds up being less weight per square inch on the contact patch if there is no increase in overall weight. Stickier tire do help, along with softer, but not too soft, sidewalls on the rear tires and so does a locking differential. You won’t get hardly any weight transfer from front to the rear unless the rear tires do hook and start to push the car forward. In effect if the rear of the car is traveling faster then the front as in rapid acceleration from a stop, the front either goes up if the suspension will allow it, is pushed forward or it acts like a brake and speeds rear traction loss. If the front won’t lift and rear traction is lost kiss the entire weight transfer idea goodbye. The car also has to be stiff enough to transfer the acceleration force and not absorb it with body or frame twist. If the fine line between front end lift, weight transfer and rigidity is achieved the car darts forward with minimal lift or traction loss. This happens very rarely on any car.
As with just about any car reverse gear is the lowest so the torque multiplication is better but since there is no weight transfer onto the drive wheels(s) and a big drag caused by the weight of the front half of the car you get the hopping affect.
1st generation Mustangs are notorious for the same thing you are experiencing. One of the early drag racing tricks was to disconnect the front shocks to loosen up the front end and allow it to rise more and quicker to get the weight to transfer back before traction was lost under hard acceleration. I’ve tied it and it works.
Something else to consider is simply physics. Any suspension set up can be over powered. Take a look at a top fuel dragster; the engine can easily overpower the suspension and weight transfer. That’s why the driver can’t apply full power at launch and must work up to full power while the car is moving. Same reason why some cars with lower HP and torque out run higher powered competitors, more usable power to the pavement.
As with just about any car reverse gear is the lowest so the torque multiplication is better but since there is no weight transfer onto the drive wheels(s) and a big drag caused by the weight of the front half of the car you get the hopping affect.
1st generation Mustangs are notorious for the same thing you are experiencing. One of the early drag racing tricks was to disconnect the front shocks to loosen up the front end and allow it to rise more and quicker to get the weight to transfer back before traction was lost under hard acceleration. I’ve tied it and it works.
Something else to consider is simply physics. Any suspension set up can be over powered. Take a look at a top fuel dragster; the engine can easily overpower the suspension and weight transfer. That’s why the driver can’t apply full power at launch and must work up to full power while the car is moving. Same reason why some cars with lower HP and torque out run higher powered competitors, more usable power to the pavement.
It may be just too lite in the rear end, Dave. Even with a full tank of gas, 7-8 lbs per gallon, less than 100 lbs, there's not enuff weight on the tires, so they break loose. you could try to run less pressure in the tires, but the handling would go south. Maybe a couple of 100 lb sand bags would help if sitting on top of the gas tank over the wheels. I'l probably have the same problem with Willit? too. JMTCW.
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short of a change of diff to give a higher overall gear ratio all i can think of is try it in 2nd when you are after a quick launch this would tend to make it bog a little and then let the wheels drive it off instead of sending half you tyres up in smoke
My first feeling was not enough weight in the rear or the springs were compressing. A change in ratio would only put the break away point at a different place. I'm thinkin it doesn't have enough lead in it's loafers to really pull it without a solid diff and slicks. I'll try it with alot of weight and see how it hooks up....or doesn't. Um keep in mind I want to lift the front end with a 1.9. Yes I know if I get there it may twist....I'll just hang on...with the LSD it will get very ugly very fast with the front end off the ground. I'm not fond of this diff so a weld up is an option.
Bob want's the LSR, I want one to run draft for the first 1/4 mile for him.
Bob want's the LSR, I want one to run draft for the first 1/4 mile for him.
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730 Posts
Here's a thought for you Dave. If you don't want to put a heavy load on the springs, how about u-bolting some plates on the axle. That way you have grap w/o bottoming out the body.
Dave, Barry's got an idea there. The only thing I would comment on is additional unsprung weight vice sprung weight. I really don't think you'll get the front end off the ground, although it could be a remote possibility, but the torque would have to be tremendous to do it. On the monza, there is a steel plate that is bolted to the differential and runs forward to the tranny, kinda like a traction bar. Might be something to think about. Here's something else to think about also. If Bob is gonna be geared for the LSR, his car won't be a dragster, so there shouldn't be a need for one he could draft behind. IMHO.
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The rear suspension of a GT is not well suited for acceleration. When the rear suspension is lowered it gets worse. Small geometry changes can work wonders for traction, not to mention tire compound, and shock absorber damping....high pressure gas is the worst for drive traction IMO.
Bob
Bob
I was joking about drafting at Bonneville, it's not allowed. I would like to get a speed run there one day in a GT. Just to see for a fact what it can do.
The car is lowered but yesterday I was eyeying it against two bone stock ones and the drop isn't that noticable for some reason. Only the wheel openings look better on it.
Thanks Bob I think the shock issue is it. Rebound rates are more than likely why it hops. At full throttle or close too it I'm surprised how well it all holds together. Just as an odd idea, has anybody ever done a ladder bar type on a GT rear end?
I have another much better rear diff for this car and just for the try and see I was thinking of locking the loud one in it just to see how it would act. Any thoughts on this?
The car is lowered but yesterday I was eyeying it against two bone stock ones and the drop isn't that noticable for some reason. Only the wheel openings look better on it.
Thanks Bob I think the shock issue is it. Rebound rates are more than likely why it hops. At full throttle or close too it I'm surprised how well it all holds together. Just as an odd idea, has anybody ever done a ladder bar type on a GT rear end?
I have another much better rear diff for this car and just for the try and see I was thinking of locking the loud one in it just to see how it would act. Any thoughts on this?
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1,577 Posts
The first question i would have asked is
is the car mechanically sound, but you probably know that already
How much power are you sending to the rear end? what size tires are you using?
the Opel has pretty good setup
The torque tube works just like a torque arm and provides a lot of anti squat.
my Opel Race car has Heim jointed trailing arms where the axle attachment points are move underneath (lowered) from where the stock location attachment point is
i been looking into maybe getting some trick Aluminum trailing arms from a Honda or maybe a Ford mustang but it would mostly be Bling Bling
something like this D2
http://www.modacar.com/products/Honda/Civic/MODACOA/
had the measurements but cant seem to find them right now, also they where kinda wimpy when i saw some last week
Fords have something similar
http://www.online-racer.com/mustang_suspension_components.htm
Davegt27
is the car mechanically sound, but you probably know that already
How much power are you sending to the rear end? what size tires are you using?
the Opel has pretty good setup
The torque tube works just like a torque arm and provides a lot of anti squat.
my Opel Race car has Heim jointed trailing arms where the axle attachment points are move underneath (lowered) from where the stock location attachment point is
i been looking into maybe getting some trick Aluminum trailing arms from a Honda or maybe a Ford mustang but it would mostly be Bling Bling
something like this D2
http://www.modacar.com/products/Honda/Civic/MODACOA/
had the measurements but cant seem to find them right now, also they where kinda wimpy when i saw some last week
Fords have something similar
http://www.online-racer.com/mustang_suspension_components.htm
Davegt27
Mechanicly it's very good. Power wise I'm not sure. I know I'm over the limit for a stock rear diff. The 240 is hanging on so far is my best judge of where the power is. On the other side it's like a semi retired police officer......can eat a doughnut in one bite. 
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What size tires are you using?
i used to run may GT at the Drag strip i got a little wheel spin with 3.67 gears and 205-50-15 Euro TA's
Martin Reimer could almost smoke the tires 1/2 down the strip with his Rotary GT he had 3.44 on Potenza RE71 type R's
his motor was a 12A with a Holly carb
my Race car had 5.28 gears pulled 2.00 short time but not much wheel spin
unless i dumped the clutch power was about 85 WHP
anyway you need some power to have wheel spin problems in a GT
Rally Bob over 100 WHP type power
that's why i was thinking you had something wrong in the rear be it shocks or some kind of binding up
Davegt27
i used to run may GT at the Drag strip i got a little wheel spin with 3.67 gears and 205-50-15 Euro TA's
Martin Reimer could almost smoke the tires 1/2 down the strip with his Rotary GT he had 3.44 on Potenza RE71 type R's
his motor was a 12A with a Holly carb
my Race car had 5.28 gears pulled 2.00 short time but not much wheel spin
unless i dumped the clutch power was about 85 WHP
anyway you need some power to have wheel spin problems in a GT
Rally Bob over 100 WHP type power
that's why i was thinking you had something wrong in the rear be it shocks or some kind of binding up
Davegt27
Darn, Dave, that's the advertised HP on the 3.4 going in Willit? Looks like it will be just like the monza driving in the lower gears. By golly it just might be fun to drive. I had to put the posi in the monza because of excessive wheel spin when the roads got damp here, my saying was "before, I would smoke one tire all the time, now I smoke both tires half the time". I still think the lb. per square inch on the footprint of the tire is a key to traction. There is a point of diminishing return where too wide is not enuff weight and the tires break loose. JMTCW.
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1,577 Posts
you need way more tire i had some 205-40-zr16's on my MR2 i wonder if something like that would do the trick
or a taller tire in the rear,softer springs but i would think that would hurt the handling of the car
What are the specs on your motor sounds like you have close to 190hp at the crank I know Bob is pretty conservative on most of his power estaminets
Davegt27
or a taller tire in the rear,softer springs but i would think that would hurt the handling of the car
What are the specs on your motor sounds like you have close to 190hp at the crank I know Bob is pretty conservative on most of his power estaminets
Davegt27
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I didn't estimate any higher because Dave is (or was) running 40 DCOE's, and they will ultimately limit total airflow and therefore peak hp well below 190 hp. You can get up to about 165 hp with 40 DCOE's and 36 chokes, after that you'll need 45's.davegt27 said:
But like I've always said, even 150 hp in lightweight Opel makes for a very quick car. With 200 hp they're pretty damn fast, and above that you're strictly limited by traction.
Bob
11:1 Venolias via James. .248 and 265 lift at .50 cam, mild porting and 2.0 valves. Twin DCOM on a ported Magnoletsi. Modified 73 ignition with a Crane. Roller rockers and 240 with a S10 clutch all balanced. ok it's a 2.0 too.....I know....it needs more power. kinda wimpy so far
I can up the chokes on the oms but to go back to oes is bad for it to work. Oms have a better flow rate and different chokes....I wasn't playing when I designed it....honest. 2.2 would be 34 chokes at altitude. Bob I can't keep it on the ground now...more power sounds good but I've got to get it to hook up.
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