Educate me on weight distribution

[R22tech]

It seems most people say 50/50 weight balance is ideal for nuetral handling, sounds logical. Bias to the rear like a 911 is good for traction, also logical. So.....if you weren't concerned with making the most power to weight, couldn't you load up the front of a 911 with weight in order to achieve a 50/50 balance? If so, would the car then perform more nuetrally in the turns and have less of a tendancy to swap ends?

The reason I ask is because I haven't really experienced the tendancy to swap ends in my stock 3.2. I can corner hard enough for the tail to slide a little, but never has it actually rotated. It seems like having the weight on the rear wheels keeps the tires planted, I don't get how a front-engined 50/50 car has the advantage.

[gumba]

Swapping ends in the middle of a turn is achieved by lifting your foot off the gas. It's called throttle steering until you loose control and swap ends. When you start to loose grip at the front of the car in a turn you compensate by lifting your foot off the gas (modulating the pedal) causing the back end to rotate and getting the car pointed in the direction you want to go. The advantage of having more weight on the front (50/50) is that you will have better traction at the front, resulting in faster cornering.
You literally rotate the car vs turning, it's a lot of fun.

[Superman]

You need to autocross your car, my friend. You are asking a question about how your car might handle. Besides that question, there are a number of other handling characteristics of your car that are also important, that you don't know, and you don't know that you don't know them. It all will become more clear if you autocross. There will be surprizes.

In answer to your question, I am finding that an empty tank of gas is not the thing to bring to autocross, because the car is too light in front to get great steering traction. Recently, I noticed the MASSIVE amount of steering traction I get when I am braking fairly hard. It was startling how violently the front would move under this condition.

Yes, 50/50 cars handle well and that is considered an ideal setup. Our cars are a bit more interesting. Gumba is correct in that the extra tail mass and weight is a very entertaining feature. At autocross, you will seek out and find the delicate balance between what we call "throttle steer" and what we call "lift-throttle spin." My first autocross was in the pouring rain, and I went there determined to spin the car but failed to achieve that goal. Now, I autocross on warm dry pavement with the goal of NOT spinning, and I fail. And laugh out loud.

And Gumba is right about another thing. That 'rotating' thing is really fun, though it takes quite some time to master. You can dive the nose around the cone by lifting and tapping the brakes, at speed this slides the tail around and if you punch it at the right time, you squirt out of the turn a LOT faster than your otherwise could. It's really fun to watch someone who's good at this. You need at least three legs and feet in order to be a champion autocrosser.

[Randy W]

I think maybe I've given my 2 cents on this before, but at the risk of growing another appendage - here goes.

F1 cars have a 47/53 balance or there abouts, so I guess that would be considered ideal. Removing weight from the rear of the 911 and adding suspension tuning is probably the best solution towards achieving that goal. I tried filling my 911 with gas (adding weight to the front) before an autocross once, and found that although the car felt more stable in fast sweepers, I was slower. I asked Greg Fordahl (multi National Champion/suspension guru) about this and he said that besides carrying around an extra 75 + pounds, I was making the car more sluggish to steering inputs and not able to "dance" the car in the slaloms as well. Super did a great job of describing this "dance." It turned out that by adjusting sway bars and shocks, I was able to make the car behave more neutrally and counter some of the pendulum effect, without having to add more weight to the car. Still, when the "dance" stops and my 911 goes, it's ass first! As Super has observed, on a car without this adjustability, adding gas helps the balance of the 911, at the expense of some agility. Autocross is a great place to try all this out!

[island911]

50/50 weight balance is ideal for neutral thinking.
If you "balance" your 911 by putting weight up front, you will increase the polar moment of inertia. (think of how an ice-skater spins; arm-out is slow, arms in is fast) Beyond that, your shocks and spring will be loaded beyond there optimal design point.

Best handling and inherent neutral stability are two different things. Ever see a fighter jet with forward swept wings? . . .that is an example of designing in instability to achieve quicker maneuverability.

I highly recommend, anyone interested in the topic, pick up Vic Elfords book on performance driving.


Pelican has it Here.

[masraum]

Adding weight is generally not a good way to tune any performance characteristics of a performance car. Extra weight is bad for handling and acceleration. By adding more weight you make the car less likely to change it's direction or speed wether that be from steering inputs, braking, or speed. You can get your stock 3.2 to swap ends if you try hard enough

the throttle lift oversteer (which is generally where people get bitten by 911's) is generally a function of getting into a corner faster than you think you can manage so you either lift off of the throttle or you hit the brakes. When this happens you transfer weight from the rear of the car to the front which helps the car steer and plants the front, but also causes the rear to lose traction because of less weight over the rear tires. The problem comes in because all of the mass of the car still is back there hanging out with the engine and tranny and all of that mass has momentum/inertia that is tangetial to the turn that you are making, so the rear tries to go the direction it had been going before you got the front end to quickly and powerfully go a different direction. You can test all of this at an autocross, or if that's a problem you could try testing it in a large flat empty parking lot early one morning.

Get Henry Watts book, Secrets fo Solo Racing if you want to know more. Another good book is Vic Elford's mentioned above or Paul Frere's book Sports Car and Competition Driving or Bob Bondurant's Bob Bondurant on High Performance Driving.

[Wayne 962]

Porsche did that such thing with the early 912s. The weight balance was supposedly thrown off by the lighter engine in the rear, so Porsche put two large weights just under the front bumper to try to balance out the car. I don't think that it worked very well - most people have since removed them.

-Wayne

[dickster]

not sure about the ideal weight distribution but the 911 is famous for its great handling.

as far as what you are describing, its one of the handling traits of the 911 i was worried about - and the rear engine therefore light front end. i have experienced cr*p braking in the wet with other rear engine cars.....

but i have to say the porsche brakes very well in the wet.

i had the chance to push mine on the track and discovered it has near neutral corning. both ends seem to start sliding (i wouldn't call it drifting!) together, and it was all very predictable - no nasty surprises.

overtsteer/understeer characteristics come from loads of variants, from tyre/pressures, to alignment specs, spring rates etc. oversteer being preferable for FUN,fast driving. you can't corner quickly with understeer!

one of the other guys at the track did experience the famous end swapping, but many cars (including front engine) will throw the arse round.

[pete98m3]

well, the more weight you have over a set of tires the more centrifugal force they have to stop in a corner... thats why porsches are tail happy...

that doesn't mean they can't still handle neutrally though... this is achieved by running a "staggered" tire setup, with larger rear then fronts... balences it out...

my current car (for sale) is a 98 bmw m3... they have 50/50 weight distribution... this makes them very, very nice to drive hard through the corners. they came stock with 225/45's up front and 245/40's in the rear, which made them understeer like a big fat lardy pig... most people swap to 235's all the way around, which makes for a car that does perfect 4 wheel drifts...

point being porsche has designed there way out of there weight distribution problems for the most part.

[BYantzer]

So here is an add on.
Somebody backed into my baby in the left front fender and ripped up the bumper valance and fender. I am having the whole thing replaced by the ins. and instead having the RS (or is it the RUF) front bumper put on. It is made of fiberglass so i am losing about fourty pounds in the swap. How do you guys think this will affect the handling. I plan on putting the rear on sooner or later (prob later than sooner)
I just thought that while i had the opportunity i might jump.

While we are on the subject I was wondering also how much of a difference does the turbo tail make?

[dd74]

Because I was wondering what differences in handling would occur when the wheels and tires in the front are increased to the same width as those in the rear, i.e. 15x7 wheels and 205 60 15s. So does having equal size wheels and tires all around promote more neutral handling? Someone once told me wider wheels and tires in the front promotes understeer. I think it'd be closer to neutrality or possibly oversteer.

What are your opinions?

[billwagnon]

Porsche did that such thing with the early 912s. The weight balance was supposedly thrown off by the lighter engine in the rear, so Porsche put two large weights just under the front bumper to try to balance out the car.

The first 911s were very back heavy, so before the wheelbase was lengthened etc cast iron weights were put in the bumpers.

I believe the 912 has always enjoyed a reputation for better handling than the 911 because of its lighter go-parts.

[Bill Verburg]

The weights were added in the bumper ends to increase the polar moment. The engineers wanted to slow down the yaw response so that less than experts had a chance to catch it before disaster struck. Other later efforts included longer wheelbase and wider rear rubber.

[autobonrun]

F1 race cars do place weights at different locations around their car to achieve a desired balance for a given track. One of the tricks I learned while at Spa was that competitors like to watch how a car hangs when lifted after an accident. Some tend to tilt forward and others backwards. Supposedly, this tilt can tell knowledgable mechanics something about how the weight is distributed. I've started to watch for this now at every F1 race.

As far as adding weight to a 911, I wouldn't do it. Instead, I'm trying to lose weight where possible.

[CamB]

If you even up the tyre size it will make the car more likely to lose traction at the rear. Your car should already have this anyway (a '74 with no fender flares, yes?).

Speaking of adding weight, the WRC just passed through NZ (Gronholm has now won the drivers championship).

The cars (all manufacturers) are listed in the specifications as "1230kg" - presumably a minimum weight. The Peugeots are physically tiny, and I was talking to a friend of mine who said the body work seems to be as insubstantial as possble.

I would not at all be surprised to find they actually weigh substantially less and the weight to bring them up to the minimum is added wherever it makes the cars handle the best.

The Peugeots have completely cleaned up the manufacturers championship, so it stands to reason the handling must be pretty good....

[Deeks]

I have a Peugeot 106 Rally. Handles like a dream. Suspension set up is perfect and grip is sensational.
Even though it's front engined, front wheel drive I've NEVER suffered understeer, only oversteered a couple of times in the wet.... and boy when it snaps out, your hands have got to be as quick as lightening !!!

[CamB]

Haha - I had one of those. Awesome little car and I wish I hadn't sold it.

Mine went slightly more neutral when I changed the wheels to 7x15" and the tyres to 195/50 and lowered and stiffened the front slightly. Not quite as much fun but more grip overall. More importantly, always 7500rpm in every gear

[dd74]

That's interesting you say this about losing traction in the rear: yes, it is a '74 narrow body. The car (as far as I can feel) seems to want to go into four-wheel drifts a little more since I put 15x7s all around. I can definitely put more power down in the turns though. What's interesting is the car tends to tuck in a bit more when letting off the throttle in the center of a turn. I actually like this action -- as the car seems to be speaking and demanding more. As far as losing traction - I don't know. I've been driving it a lot harder since the increase from 15x6s to 15x7s up front. BTW, tire size is still 205-15s. All-in-all, with my limited experience with this change, though I haven't tracked the car, I suggest the increase. By looks alone, the wider wheels are worth the cost. Just check out Adam's '74 911. His is sweet!

I wonder what Adam's experienced with his?

Adam?

P.S. Never had a Peugeot 106 (N/A in The States), but I did have a built Ford Fiesta many years ago. Sachs/Bilstein suspension, dual Webers with a .410 lift Kent camshaft and 9.5:1 compression. 13x6s with Phoenix Stalflex tires. Now that was a canyon burner! Fun! Fun! Fun!

[speeder]

This is all fairly well traveled territory, but suffice it to say that the Porsche 911 is an engineering marvel. No one else has EVER tried to build a sports car with the engine behind the rear wheels, and Porsche is STILL doing it.

The reason that it is an engineering marvel, (also refered to as, "a triumph of engineering over design"), is that the engine location is probably the least favorable from a handling perspective, but they have made it work. (Understatement). What the location IS good for is that it allows for the aesthetic design of the car, (low front end, small overall size w/ rear seat), and THAT part of the design is widely considered the best ever by the lion's share of auto designers.

A couple of points: the cast iron weights were added to early SWB 911s, not 912s which did not need them due to much lighter motors. The original Porsche rear engine design had a tiny air-cooled 4 cylinder, (356), so the handling was not a large issue. Someone mentioned braking; that is one of the areas where rear engine, or rear weight bias, is an absolute advantage. With great brakes, light overall weight and great suspension design and tires, the 993turbo and 996turbo stop like, well, like you just hit a brick wall. The rear engine acts like an anchor holding the rear end down, this unweighting is where most vehicles lose braking traction.

Rear weight biased cars will naturally understeer due to their light front ends, and oversteer, (a nice way of saying that the rear will become the front), under the wrong conditions, such as lift-throttle. Porsche has engineered these characteristics out of the 911 as much as possible w/o changing the basic design of the car, but it is not possible to eliminate them. You can only beat physics to a point, and they have been dancing on that point for decades. Larger rear tires and extensive suspension engineering are the most of it. 911s also have understeer designed into them for safety/handling reasons, (otherwise a grippy rear weight bias car would go directly to hard oversteer), still this feature is maddening to many enthusiasts and racers. It requires very skillful manipulation to get the handling potential out of our cars.

All of the above is concerned with driving the 911 beyond the traction limits of the tires, which it does not sound like you,(R22), do yet. BTW, the handling of a 911 below that threshold is beyond perfect.

[pete98m3]

hahaha, whoever learned the tilt idea at spa has been fed a tall tale... even the most basic of homegrown racing shops have 4 wheel scales... the only thing thats useful for is examining other peoples cars. Even me and my dad have a set of scales, we even corner weighed my m3 one day when we were bored. you can get a set for less then a grand, or if your car is under 1200 lbs (more likely to be a kart or a FF) you can use bathroom scales. (300lbs cap. ea)

summit racing sells them i think, not a bad idea if you have coilovers but its cheaper to have it done if you don't plan on doing it a few times... personally i'd rather buy the tools and DIY then pay, this way i can tinker and such later...