understeer

[GeoffBob]

I think there is confusion over definitions of terms..

I think where the confusion comes in (speaking for myself at least) is understanding that the distribution of weight is only minimally related to the actual position/orientation of the car. Because the body is tipping over with soft springs it is easy/tempting to think that the orientation of the body accounts for the relocation of the weight. In reality (as Caneswell indicated) that only represents a very small amount of relocated weight. More specifically, the weight distribution is due to the forces acting upon the car.

If I may be permitted to explain further - When the concept of weight shift (while braking, cornering and accelerating) was first explained to me I imagined my wife and shopping sliding back and forth in the car. But with them both nailed down I failed to understand how any weight shift (other than due to the slight reorientation of the car) could be possible.

The truth is that there isn't any shifting of "weight" (well very little anyway). What there is, is the "forces" that result (while braking, cornering and accelerating) that, if we were to put scales under the wheels (if such a thing were possible while driving) would appear to make the weight at each wheel go up or down.

In truth these changes in "weight" shouldn't actually be recorded in units of lbs, but more accurately (to be pedantic about it) in units of lb-force, in order to indicate that this "weight" has its origin in the forces acting upon the vehicle. And since this force (pushing down on each wheel) acts in the same way as if we were to physically add or remove real mass from above each wheel, we tend to think of these forces as real "mass", as opposed to just a force.

I hope that this is explanation is of some use to someone. Sorry to be so long winded. Pleasure not to argue Caneswell, God knows I can argue when I feel the need to stand my ground

[UweM3]

The rest of you are still struggling! Uwe i thought you knew more than that!

The only thing I know is that I bolt other bits to my car and I can drive faster.

I can see where I get the terms wrong after reading Geoffbob's last post. The actual weight doesn't shift of course, it's the forces that change.

So do we agree that fitting stiffer suspension let's you drive faster because it manages the FORCES better?

[e301988325i]

I think it's also interesting that when you lower the front of an E30 - the lower arm gets closer to horizontal, which is it's max neg camber position. As the spring compresses the lower arm effectively gets shorter and that starts to push the tyre in to positive camber.

I wonder sometimes if it might be better to have much stiffer front springs, but only slightly lower - so that when the spring is maximum compressed, the lower arm is horizontal and giving the max amount of neg camber. I guess it's a trade-off between small optimisation of dynamic camber and slightly(?) lower CoG.

Any thoughts on that point?

Very true, also once the lower arm passes above the horizontal, the sideways forces acting on it will act to compress the suspension further. Raising the suspension, will effect negatively the COG and aerodynamics, high at the front isn't good of course with more air under the car. What would be a better solution would be to raise the inner pivot points of the lower arms. In a technical, theoretical sense. Largely impracticle I suspect.

[caneswell]

The rest of you are still struggling! Uwe i thought you knew more than that!

The only thing I know is that I bolt other bits to my car and I can drive faster.

I can see where I get the terms wrong after reading Geoffbob's last post. The actual weight doesn't shift of course, it's the forces that change.

So do we agree that fitting stiffer suspension let's you drive faster because it manages the FORCES better?

Hmm....not entirely. The forces/weight (whatever you want to call it) going through the tyre patch in steady state cornering are independent to the springs. You could replace them with an RSJ and it wouldn't make a difference. Dynamically and over bumps it all gets much more complicated, but we won't go there, apart from saying that stiffer springs can be a disadvantage!

Put a car on a constant radius turn (or skidpan to the Americans) and the grip generated has nothing to do with the spring rate (so long as you can control the camber)

Does this help?: http://en.wikipedia.org/wiki/Load_transfer

[caneswell]

What would be a better solution would be to raise the inner pivot points of the lower arms. In a technical, theoretical sense. Largely impracticle I suspect.

Funny you should say that! DTM E30 M3:

[e301988325i]

bloody hell, I got something right for once.

[caneswell]

If this doesn't answer it i'm giving up!!

http://www.neohio-scca.org/comp_clinic/ ... ed%202.pdf

"When thinking of load transfer it may help to consider body roll. As the body rolls the
outside springs are compressed and place more load on the outside tires. In reality, body
roll is a result of cornering force, not a cause of load transfer. If you double the spring
rate you will NOT significantly change the load transfer, but you will reduce body roll."

[rix313]

I imagined my wife and shopping sliding back and forth in the car. But with them both nailed down I failed to understand how any weight shift (other than due to the slight reorientation of the car) could be possible.

I like that lol!


Interesting read this

[GeoffBob]

What would be a better solution would be to raise the inner pivot points of the lower arms. In a technical, theoretical sense. Largely impracticle I suspect.

This is what I did when I designed the frame for my track car by mounting my front and rear E30 sub-frames higher than normal. Alternatively you could see this as if I lowered the body, engine and middle section of the frame down relative to the E30 sub-frames. Either way you look at it, the effect is no different to as shown on the DTM E30 M3. The down-side of this was that I had to reduce the height of the struts to stop them from coming out through the top of the bonnet, but it does mean that my car is effectively lowered without technically having actually lowered the suspension.

I still wish BMW could have built the E30 with double-wishbone suspension all round.

[driftmonkey]

The understeer was a pain, i wasnt really enjoying going round track, had a play on a handling circuit, basically just a big area with cones and you can do whatever you want..... so i was driving like a joyrider, i couldnt drift, slide very well, or do doughnuts, i need to fit my lsd, and think i'm going to go more towards drift events other than trackdays as had so much fun trying to go sideways.

This advice is all great, will have a play over the xmas break, thanks people.

[GeoffBob]

I think it's also interesting that when you lower the front of an E30 - the lower arm gets closer to horizontal, which is it's max neg camber position. As the spring compresses the lower arm effectively gets shorter and that starts to push the tyre in to positive camber.

Help, I'm confused I can't see how this is possible? The more I think about it the more it seems to me that the lower the front of an E30 (by fitting shorter springs, say) the greater the degree of negative camber. The control arm may be getting shorter relative to the horizontal (obviously its actual length remains unchanged) but does this have as much effect upon the change in camber compared to that due to the compression of the strut? And not forgetting the position of the hub is fixed relative to the strut.

Hmmmm, I think I need to draw a picture. Three years ago when I layed out my front suspension I could have answered this off the top of my head, now I have forgotten even the basics. Some help anyone?

[GeoffBob]

The understeer was a pain, i wasnt really enjoying going round track, had a play on a handling circuit, basically just a big area with cones and you can do whatever you want..... so i was driving like a joyrider, i couldnt drift, slide very well, or do doughnuts, i need to fit my lsd, and think i'm going to go more towards drift events other than trackdays as had so much fun trying to go sideways.

Since your interest lies with drifting then understeer is definitely not your friend! You should possibly consider a few easy techniques to make it easier for you to tempt your car into oversteer. Can I suggest to you that you look at stiffening up your rear ARB? I believe there are a number of stiffer aftermarket rear ARB's available as well as a number of threads on the zone with regard to how to modify your own by relocating the mounting positions. Have a look at Handpapers thread here for example. By stiffening up the rear ARB you will both increase the tendency of the rear to oversteer while decreasing the tendency of the front to understeer. Fitting your LSD is also a very good idea for drifting. I would also suggest that, while you are learning to drift, you fit a pair of semi-slick track tyres at the front, but run your old road tyres on the rear to extinction. This is basically the advice to beginners that you will find in the book How to Drift: The Art of Oversteer available from http://www.cartechbooks.com

[caneswell]

Help, I'm confused I can't see how this is possible? The more I think about it the more it seems to me that the lower the front of an E30 (by fitting shorter springs, say) the greater the degree of negative camber. The control arm may be getting shorter relative to the horizontal (obviously its actual length remains unchanged) but does this have as much effect upon the change in camber compared to that due to the compression of the strut? And not forgetting the position of the hub is fixed relative to the strut.

As you say the hub is fixed relative to the strut so the only possible camber gain you can get is the small amount of change in horizontal length of the control arm over its arc. This is one of the main disadvantages with macpherson strut, which is why you will see touring cars running rediculous static camber to make up for this. As the arm goes from droop to horizontal it pushes the bottom of the strut out a small amount(giving some negative camber gain), probably not as much as the roll of the car is leaning it over, but i'd need a diagram for that.

[rix313]

Geoff you're like this epic car book librarian, always makes me smile that no matter the subject you always know of a book on it

[GeoffBob]

Geoff you're like this epic car book librarian, always makes me smile that no matter the subject you always know of a book on it

Thanks Rich, I love my books. Contrary to what some may think, I don't work for or have shares in www.cartechbooks.com, but I do like there books (although some of them read like they never saw the desk of an editor, the grammar and spelling can be atrocious).

[hoshy]

Geoff you're like this epic car book librarian, always makes me smile that no matter the subject you always know of a book on it

Thanks Rich, I love my books. Contrary to what some may think, I don't work for or have shares in www.cartechbooks.com, but I do like there books (although some of them read like they never saw the desk of an editor, the grammar and spelling can be atrocious).

Almost as bad as getting their and there wrong.

[GeoffBob]

Geoff you're like this epic car book librarian, always makes me smile that no matter the subject you always know of a book on it

Thanks Rich, I love my books. Contrary to what some may think, I don't work for or have shares in www.cartechbooks.com, but I do like there books (although some of them read like they never saw the desk of an editor, the grammar and spelling can be atrocious).

Almost as bad as getting their and there wrong.

Hey, I don't write them, I just read them .

[driftmonkey]

Bringing up an old thread, on my latest e30, a 335i h&r cup suspension; adjustable arbs front on back on hardest setting, front sturtbrace, 195 45 toyo t1rs all round on 15". This one understeers too! But with a stab of throttle oversteers. It's not very controllable tho, can't hold the slide on the throttle, not that I'm much of a drifter anyway. Camber plates needed for front I think, and wider tyres?

[MillRat]

Better tyres might be a good start. It does sound like a traction problem, considering the car feels uncontrollable.

I have only ever used 205/50R16 tyres, and I somethimes feel like I run out of grip with them, so I can image what the 195/45R15 would be like.

Altenatively, I wonder if your front suspension is too stiff, or too soft, allowing the car to skate through, or wallow through a corner, then stabing the throttle highlights the very stiff (with respect to the ARB), rear end introducing loads of uncontrollable oversteer.

In my opinion, camber/caster plates help to fine tune a car, if the car is always understeery, then I do not think they will sort your problem.