I was doing some quick measuring on our donor RX7 (51/49) last night. I realized that our completed car will have both the driver and the engine set back with a similar wheelbase. So if I end up say 45/55 how will that affect handling? Is it one of those just don’t put too much on the front but a rear bias is ok?
911s are rear biased. I hear they handle reasonably well. Spring rates abd sway bar selection will matter.
YEA 50/50 is a goal 60/40 IS AS GOOD AS MOST. if you get to far You will be hunting new springs and shocks and Swaybars and alignment, TOO light up front can be as detramental as too heavy.
In reply to DeadSkunk :
Yup...mine is 35/65 and Handles great...
I've always heard that the 911 was proof that German engineers can fix any problem given 50 years.
How do you prepare for for A Tail heavy car, Ask Richard Petty.
When The IROC Series was started they used 911's ,and even with Jay Signor's(mispell) top notch prep he couldn't get around a corner any where. Mark Martin had it down.
Lots of Sevens have a slight rear bias. 40/60 is probably a blast, once sorted. Only thing is you gotta put your foot in it when you get in trouble.
My MGB race car has 55% on rear tires. You will have to adjust tire width, spring rates, sway bars to match the rear bias but once you do that, the car should be amazing
I find a rear weight bias preferable. Braking is greatly improved with a rear bias, as is acceleration. If polar moment is kept low, handling will be great.
What to expect? Good fun.
Depending on how much you have the throttle becomes your rear traction control device. Increase throttle = more grip, decrease throttle = more turn.
Throttle steering is good fun.
There's nothing magical or perfect about 50/50.
Rearward bias uses all wheels for braking more evenly and puts more weight over the drive wheels (assuming RWD).
The car just needs to be set up for it. Tires proportional to the load seem like a good plan.
Yeah a little rearward bias isn't bad at all. The nasty handling trait to watch out for as weight distribution moves rearward is when the car swings toward the outside of a corner if you lift off the throttle mid-corner. The AW11 MR2 and the Lancia Stratos have this, and some rear-engined Porsches, especially older ones, have it so bad that the car initiates a violent spin toward the outside of the corner.
My sbc powered '85 RX-7 weighs 2325 (2525 with driver) and has 43/57 F/R with full tank and 10" of engine setback. Handles surprisingly well with no sway bar up front, TruTrac, torque arm, and Crown Vic bar out back :)
Grant
Grant,
telling us that and not telling how the next one will be built for Challenge money and NOT showing Pics is just WRONG.
Stampie may have something new coming along those lines, OR Why the trip to Huntsville?
In reply to GTXVette :
Don't they build rockets in Huntsville? Maybe I grabbed one cheap from a rocket scientist.
We found that corner weighting is much more important than front to rear bias.
50/50 is perfect for a car with equal size tires on a skid pad at constant speed... only
In every other situation, rear weight bias is beneficial. As others have said, when the brakes are applied, load transfers to the front regardless of car design. Under hard braking with a 50/50 car, the front brakes are doing about 80% of the work. With a 70R/30F car, the front tires are doing maybe 60% of the work. Therefore, braking effectivity is greater for the rear-biased car.
Under hard acceleration, a 50/50 car may have 30% load on the front tires and 70% on the rear. A rear-biased 70R/30F car might have 90% on the rear tires and 10% on the front. (The exception is high-powered drag cars where the front tires lift off the ground; in that case, it doesn't matter what the static weight distribution is.) This example above shows the problem a rear-biased car can have where under acceleration, there may be so little weight on the front tires that understeer becomes a problem at corner exit. Also, and as has been pointed out. In a turn, lifting off the gas transfers weight forward in all cars. At the same time, engine braking is trying to slow the (assumed rear) driven tires. The combination can be a bit like pulling the emergency brake and in an already rear-biased car, makes the rear tires more prone to stepping. As was said, using tires width proportional to weight distribution can do a lot to retain the benefits of rear bias, and also undo some of the tendance to spin.
In short, a rear-biased car is more capable of better track performance, but it also puts the driver closer to the edge of controllability. It comes down to driver skill whether the car is faster in reality or just on paper.
Full disclosure, I'm biased since I designed a mid-engine car using a FWD drivetrain: http://midlana.com/
Stampie said:In reply to GTXVette :
Don't they build rockets in Huntsville? Maybe I grabbed one cheap from a rocket scientist.
If he's Just A Rocket Scientist, Well It can't be much then.
May as Well be A Toll Taker on the Turnpike.
OH YEA , BANG !!!
GTXVette said:Grant,
telling us that and not telling how the next one will be built for Challenge money and NOT showing Pics is just WRONG.
Stampie may have something new coming along those lines, OR Why the trip to Huntsville?
Moving the engine back 10" in a 1st gen RX-7 is a low cost mod if you already have a Sawzall / Mig welder...
Before...
After...
Grant
GameboyRMH said:Yeah a little rearward bias isn't bad at all. The nasty handling trait to watch out for as weight distribution moves rearward is when the car swings toward the outside of a corner if you lift off the throttle mid-corner. The AW11 MR2 and the Lancia Stratos have this, and some rear-engined Porsches, especially older ones, have it so bad that the car initiates a violent spin toward the outside of the corner.
I have owned a couple of cars like that. My 911 (72 vintage) was so bad that even thinking about lifting the throttle was enough for it to try to bite. I never owned a car that so actively tried to kill it's driver.
In reply to weedburner :
when was the last time ya looked at a car and laughed all giddy like, AWESOME!
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