Just dropped 40 lbs from the rear of the m3

[ChewToy]

Hey guys. I just dropped 40 lbs from my m3 and would like to hear some thoughts on what I can expect. Do you think it'll make a NOTICEABLE difference while I'm autoxing? If so in what way? Acceleration? Grip? Overall handling? More Understeer? More Oversteer? Please EXPLAIN why you think the way you do. I've been racing with this custom bass box for the past 2 years, and now that I'm a little more competitive I figured I might as well prep the car the best I can... and reducing weight I felt could only help.

I've been wishing for a little more oversteer and I'm thinking that dropping the 40 lbs that was sitting right over the rear shocks might help in this dept.. maybe tooo much. What do you think? I'll find out at the next race Sunday.

Anyway I was just looking for some thoughts from the fellow enthusiasts on the pros and cons of what I just did.

Thanks for reading.
///Matt

Click Here to see pictures of the bass box... which incidentally maybe for sale soon

[GotBHP?]

dropping weight from the rear of the car will cause more understeer.

[ChewToy]

Please post reasons to your assumptions for those that don't know the obvious... such as myself I'm not doubting or challenging you, I just want to understand the reasoning behind everyone's views as well. Tia.

[EMAXX]

It will cause more understeer because there isn't as much weight to hold the rear wheels down.

[ChewToy]

And what would remedy this? Dropping or raising tire pressures come to mind. I' was running 41 lbs in the front hot and 35 lbs in the rear. I'm not sure what I'll be doing on Sunday. Any suggestions? Should I raise or lower the psi? I typically mark my tires and judge by the rollover.

[NegativeCambre]

Actually, my experience with removing weight in the rear is opposite... and your explanation, EMAXX, only confirms that...

If you are losing grip in the rear... why would this lead to understeer? It wouldn't. Understeer is when you max out the front tires before the rear...

...this is the reason that staggered wheel systems promote understeer, albeit at a higher g-load then a consistent wheel set-up.

It will, however, promote understeer in certain situations. The weight bias will be shifted slightly to the front of the car, meaning that the front tires will be asked to do a lot more...

Examples:

If you enter a turn too hot, with this new, lighter setting, you will encounter understeer quicker.

If you liftoff during said turn, your car will rotate a lot quicker and may provoke a spin.

So, it is a trade-off... neither of those situations are desirable, but the advantages are also tangible... less weight does a lot of very good things... shorter braking, faster accleration, quicker transitions.

You'll be able to alter this a bit with tire pressure, but not by much.

Steve

[EMAXX]

Oh, I knew there would be something different, but I wasnt' quite sure. I've never driven before, just read about and rode in cars.

[GotBHP?]

I thought it was more of a function of the component of the force pushing your car towards the outside of the particular turn (centrifugal - hell i might be totally off on this one). With less mass in the rear of the car, there will be less force causing the back end to break loose, as the mass is centered more towards the front of the car. bah i dont know i was just thinking about it a little

[novulari]

You don't need to put weight just to centralize the weight distribution, that what the wing is for, to keep the rear tires on road surface at all time, any speed. Why would someone put weight into his car because he thinks his car is oversteer/understeer? that does not sound right, and I have never ever seen one race car actually have extra weight just to push his car to the ground....

remember inertia? the more mass you have, the more distance and time you need to stop a moving object or to accelerate an object

weight is something you want to eliminate if you want performance, what's the purpose of CF hood, Aluminium engine, LTW wheels, LTW wings, and etc, it's to eliminate extra weight.

[GotBHP?]

there is no question that reducing weight will do wonders, you just want to make sure that if you eliminate a whole lot from the rear, try doing the same to the front to keep things balanced the way they should be on these cars

[novulari]

40 lbs is nothing, it's not even equal to adult normal weight. so removing 40 lbs, you will not feel much differences.

[John (PA)]

You can gain or loose 40 lbs by having an empty or full gas tank.

Work on the driving skills.. read books on autoX, study others who are good, ride with the best.. etc.

That is, if you really want to improve your autox times.

There's a guy in our local club. He can wipe anybody out.. driving their car. It's always a humbling experience. :

Not to disregard your question, but there are so many factors involved. More than most engineers can even tackle. Improving your driving is your best bet.

[Gene V]

Originally posted by EMAXX
Oh, I knew there would be something different, but I wasnt' quite sure. I've never driven before, just read about and rode in cars.

[ibizam3]

Also, DRIVER WEIGHT is important. For a 900-lbs example of a human being, that 40-lbs won't matter much.

[NegativeCambre]

Originally posted by novulari
You don't need to put weight just to centralize the weight distribution, that what the wing is for, to keep the rear tires on road surface at all time, any speed. Why would someone put weight into his car because he thinks his car is oversteer/understeer? that does not sound right, and I have never ever seen one race car actually have extra weight just to push his car to the ground....

remember inertia? the more mass you have, the more distance and time you need to stop a moving object or to accelerate an object

weight is something you want to eliminate if you want performance, what's the purpose of CF hood, Aluminium engine, LTW wheels, LTW wings, and etc, it's to eliminate extra weight.

Sure, lightening a car is all fair and good, but destroying a near 50/50 weight distribution is not. In this case, 40 lbs aint gonna do much harm... and its benefits will far outweight the detriments... but in extreme cases...

And wings? "at all speeds?" No way, man! It takes a whole lotta air to gather up on a wing to make it any use to the driver... and given that autocross speeds don't generally exceed 60mph (although there is this GT2 at SCCA... more on that later), wings are essentially superfluous. The most useful wings are big, not-very-cool-looking plastic sheets that stick up at 45degree angles on the back of dedicated autocross cars. These wings would create a hell of a lot of drag at even freeway speeds, but at the moderate speeds autocrosses are held at they make a different.

In summary... less weight is good, but be careful that you try to take away uniform weight... and like was stated above, driving ability is paramount... weight distribution, downforce, and power/weight mean crap if you got junk in the helmet!

Steve

[stjobs]

Wow, some of the posters on this thread have ZERO physics knowledge. (not directed towards the few who do).

Less weight in the rear generally leads to an oversteering condition, for the same reason that drivers brake to rotate the car - with the weight transferred forward, the tire's mechanical grip potential is reduced because there is less mass pushing the tire into the pavement. As you may or may not recall. frictional force equals mass (or is it weight? either way, same effect) times the coefficient of friction. If you reduce the mass, the amount of force the tires can redirect to counteract the outward motion of the car is reduced, and therefore the grip potential is exceeded sooner. Assuming all conditions at the front are equal, a 40 lb. lighter car with all that weight taken off the rear will oversteer slightly more.

stjobs

[novulari]

not all car has 50/50 weight distribution, but they handled as good as 50/50 or even better, depending on variable of suspension setup, tire width, height of the vehicle and of course wing ( you are not going slow in the track, aren't you? )

some of you may have physics knowledge, but in the track, every one knows that you don't brake during cornering, it's a potential hazard no matter how good is the handling of your car.

Physics always help you to engineer a car, but driver's skill and knowledge are very important attribute in winning the race.


ps: some of the greatest racer out there actually have no idea/knowledge about physics, and some nerds who knows a lot about physics have no idea how to drive a car or even race it.

[Def]

It will make different effects in either steady state or transitional manuevers.

I'd say oversteer in transitional states due to the natural axis of spin being moved farther forward in the car. This will make the ass feel "more lively" when throwing a car into a corner.

More understeer in steady state cornering because of the larger relative load placed on the front tires.

With all that though, it takes an in-tune driver to notice a 40lb weight loss on the tail of a 3300lb car in alot of situations.

some of you may have physics knowledge, but in the track, every one knows that you don't brake during cornering, it's a potential hazard no matter how good is the handling of your car.

Lots of people that are much better drivers than you or I brake in the turning in phase of cornering. It is called trail braking.

[b0rf]

Originally posted by stjobs
Wow, some of the posters on this thread have ZERO physics knowledge. (not directed towards the few who do).

Less weight in the rear generally leads to an oversteering condition, for the same reason that drivers brake to rotate the car - with the weight transferred forward, the tire's mechanical grip potential is reduced because there is less mass pushing the tire into the pavement. As you may or may not recall. frictional force equals mass (or is it weight? either way, same effect) times the coefficient of friction. If you reduce the mass, the amount of force the tires can redirect to counteract the outward motion of the car is reduced, and therefore the grip potential is exceeded sooner. Assuming all conditions at the front are equal, a 40 lb. lighter car with all that weight taken off the rear will oversteer slightly more.

stjobs

Then why do cars with more weight in the rear, such as the Porsche 911, have lift-off oversteer problems?

[NegativeCambre]

The 911 is a special case because of its terrible, terrible weight distribution. Think of it like a pendelum, it has so much weight in the rear, that it is almost always going to max out its rear tires in a high-G situation then its fronts, and when it does, all that mass is going to want to keep moving...

Its inertia, and by the same token it is why I stated that front-heavy cars will tend to understeer a bit more in in a high-G corner, because they will max out their front tires since the front of their car wants to keep going straight... but lift off, and you've got big oversteer problems because now that light front wants to come around.

911 is something like that, just more extreme... because once that tail starts rotating, very little will stop it!

Steve

[Dj Waffelz]

steve ryan,,, my friend, you are incorrect.. download the topgear video of the green porsche 911 turbo... he tries to get the rear end out... and can't do it ...

[NegativeCambre]

Originally posted by Dj Waffelz
steve ryan,,, my friend, you are incorrect.. download the topgear video of the green porsche 911 turbo... he tries to get the rear end out... and can't do it ...

Haha, you didn't specify which Posrche, Dj Waffelz... I'll stand by my claim:

I'm very familiar with that movie, in which Tif Needles drives the new, 996 TT Porsche, righto? Well, there are a few special things about the current generation of Porsches:

The new turbo uses AWD, which greatly culls the lift-off oversteer problem. It doesn't do as much, however, as adding weight (yes, off-the-shelf-brand weight) to the front of the car to even up the weight distribution, which they did, in great numbers.

What resulted was a car that is not very Porsche-like... its not a car that will break your heart, like the 993 and below, but its also a car that doesn't have the wicked charm and dancing ability as its older siblings.

The 996 Porsche, and I can only assume all future rear-engined Porsches, do not suffer from the dangerous handling characteristcs of their old brethren.

And please, DJ Waffelz, just do a search on the 911 on the internet... you'll see I'm telling the truth about the old air-cooled Porsches.

And if you recall, Tiff Needles gets into a wicked, 4-wheel drift around the previous generation turbos at the very end of that episode.

I'd probably consider that getting the tail out.

Steve

[mrclam]

are you talking about the blue one? with tiff needell?

one video shouldn't be the basis for your judgement about a car's characteristics.

[b0rf]

Originally posted by StevenRyan
The 911 is a special case because of its terrible, terrible weight distribution. Think of it like a pendelum, it has so much weight in the rear, that it is almost always going to max out its rear tires in a high-G situation then its fronts, and when it does, all that mass is going to want to keep moving...

Its inertia, and by the same token it is why I stated that front-heavy cars will tend to understeer a bit more in in a high-G corner, because they will max out their front tires since the front of their car wants to keep going straight... but lift off, and you've got big oversteer problems because now that light front wants to come around.

911 is something like that, just more extreme... because once that tail starts rotating, very little will stop it!

Steve

No, I understand WHY the 911 has lift-off oversteer problems, but i'm wondering why the same principle of weight distribution doesn't apply to the M3?