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GTV27 · Post by **GTV27** » Wed May 30, 2007 1:45 am

Grant, if all else fails and it still understeers too much, increase the rear spring rate.

A set of coils will have it sliding like a Tokyo Drift junky with a minimum of fuss!

grant · Post by **grant** » Wed May 30, 2007 2:03 am

Hmm, stiffen the front end, stiffen the rear end. Two contradicting suggestions. Unless Mats knows somethign we're missing, historically, increasing the rear wheel rates has given me a car that wants to understeer less.

GTV27, or anyone for that matter, where can I buy stiffer rear springs?

Echo Leader · Post by **Echo Leader** » Wed May 30, 2007 3:34 am

Whoa, I don't pay attention for an evening and suddenly there's an explosion of activity!

Grant/Potenziato: Thanks for posting/providing the effective spring rates for the T-bars!

That's handy information and saves me much time in calculating...

In general I think I'm starting to get a bit confused with this whole concept again. I thought I was getting a good handle on it, but it's sliding away from me. We've got suggestions to leave well enough alone, tighten the front, tighten the rear, modify spindles, change UCA angles...

All of it fun, but half of it seemingly contradictory.

I think the main thing we were attempting to figure out is how to best mitigate dynamic camber loss. If that is the biggest problem with the front end, as postulated by a number of wise folks, then the suggestions to 'tighten' the front should work to counter understeer by eliminating some roll and thus maintaining a better camber angle. Correct? However, is it possible that Grant is now too stiff in the front (no jokes please

) compared to the rear and is thus out of balance?

The drop spindle attempts to do reducing the chassis' natural tendency towards roll (specifically when lowered) by adjusting the geometry to shorten the front roll couple. Again, an attempt to mitigate dynamic camber loss when cornering. But I'm starting to wonder how varied the designs implemented are. Grant, are your LCAs back to parallel with the ground? Does the drop spindle design you have raise the UCA to spindle mount point?

Speaking of the UCA mount point, Mats, shouldn't increasing the height of said joint (by inverting the ball joint) make the camber curve a bit more "negative". Thus with wheel bound, additional negative camber is added (or at least some of the camber loss is negated)?

Grant, maybe reduce the amount of front AR-bar you're running as a test? I know it is not a 1:1 (apples to apples?) comparison, but it would be easier then buying new higher rate springs in the rear.

Mats · Post by **Mats** » Wed May 30, 2007 3:55 am

Before we go into a lengthy discussion about this specific car do the simple check. Have a friend take a picture of the car from the front when you are driving hard in a turn, that will clear up all the confusion.

My bet is that the front end is still way soft and that the camber therefore is out the window. And thats why the front needs to be stiffer before the regular tuning tools work (i.e. stiffer front -> more grip at the rear).
If the car looks like a steam boat on the Atlantic in the pic odds are I'm right.

Angle of any control arm is between the center of the joints, everything in between is only there to fixate the distance between the joints and the angle of that part is simply not interesting. in the case of the UCA the joint is always on top pointing upwards and the only thin changed is the mounting surface on the control arm itself, below instead of above, possibly to gain som clearance to the spring seat on the shock?
Correct me if I'm wrong.

In my opinion the first priority is to stiffen the front end considerably, then if the problem is still there look at the RC. Lots of cars are plenty fast with stock (but heavily lowered) suspension and they're not even too hard for the street.

Barry · Post by **Barry** » Wed May 30, 2007 8:11 am

The tool simply takes the place of the jack under the lca..

Micke · Post by **Micke** » Wed May 30, 2007 10:10 am

Just put in some stiffer rear springs. They will help in two ways:
reduce roll (thanks for the tip mats)
reduce load transfer front -> more front grip

BTW, everything else being equal:
When you raise the front RC the load transfer there will INCREASE which reduces grip. To maintain same grip the spring ratio should be reduced.

grant · Post by **grant** » Wed May 30, 2007 10:55 am

My car doesn't roll that much. I'll try to get a pic from the front, but I was at the track a month ago and I saw some pictures from a professional photographer. Under full cornering load, it rolled about as much as a base model Boxster or something? It wasn't bad. But even then, my camber was noticeably positive. I'm hoping it should atleast be at 0 or slightly positive now. I'm only running ~-1 degree of camber right now.

My LCA is slightly, slightly above horizontal. My front end is pretty low for a street car. The LCA was a lot worse before the drop spindle conversion.

About raising the front RC, why would that cause less grip up front?
I'm sure I could have a lot less understeer if I used a 24mm front AR bar, but that sort of defeats the purpose of having the 28mm AR bar?

As a reminder, I have 27mm T-bars, 143 lb/in rear springs (pretty low I think?), and 28mm front and 26mm rear adjustable sway bar (it' on the second to stiffest setting).

I"ll try to snap some pictures this weekend.

AHa, Barry, I totally forgot about that. But what video was that? I don't remember what I was writing about.

Micke · Post by **Micke** » Wed May 30, 2007 11:18 pm

OK. So your doesn't roll too much.

Then, as I said earlier, stiffer rear springs (or AR bar).
For records my front is about as stiff as yours (600 lbs+24 mm AR) and my rear springs are 520 lbs. No wonder you have horrible US.

If you raise the RC, you should know what effects it has.
When you do is you get more weight transfer unsprung through the RC. Still the amount of weight transfer through the springs remain constant. So, the sum will increase = less grip

The same for guys lowering the rear RC. The spring stiffness needs to be increased to compensate for this.

Echo Leader · Post by **Echo Leader** » Thu May 31, 2007 3:41 am

Micke wrote:If you raise the RC, you should know what effects it has.
When you do is you get more weight transfer unsprung through the RC. Still the amount of weight transfer through the springs remain constant. So, the sum will increase = less grip

The same for guys lowering the rear RC. The spring stiffness needs to be increased to compensate for this.

Are you talking about the effect known sometimes as 'jacking'? I thought this only occured when the RC was above ground? In grant's case, I would guess that the RC is still below ground or close to it as his front is low and I doubt the drop spindle produces a massive movement in the RC.

I really need to get that SusProg3D program and model this stuff...otherwise we're all just doing our best to guess at a solution. Which might work given time and iterations.

Micke · Post by **Micke** » Thu May 31, 2007 5:52 am

Nope.

Here some theoretical stuff for anyone interested.

For the whole car you have 2 sources of weight transfer.
1) WT(lat acc) = mass * lat_acc (G) * CoG / Track
2) WT(roll) = mass * sin(roll) * CoG / Track

The first one is the one always used. Very easy to see what can be done. Lower weight, center of gravity or increase track
The second one is usualy omitted but plays a big role in cars which roll a lot. This is how much the center of gravity moves sideways due to the roll of the car. Same cure as first point plus reducing body roll.
There's still one element which is flexing of tires. I don't have enough data to simulate this accurate enough. Wide and low aspect ratio tires on wide rims help this of course.

Then for the front and rear end we have 4 elements of weight transfer.
1) Through the springs
2) Through roll centers
3) Unsprung mass
4) Car moving sideways like above.

# 3 and 4 are quite small (in case of track cars anyway) and can be skipped for now.

#2 you can easily imagine using the rear suspension. You have the tires on the road at a distance from each other (track). then you have the roll center at a height from the ground. Now imagine this a bit higher. Push that sideways (gee_force) and you have a nice lever trying to lift the inner wheel with a force WT=a*RC/Track.
The same applies front as well. And if the RC is under ground it is negative.

Remember that the total amount is constant as of the formula explained first. Only the ratio front/rear changes.

I made an analysis sheet in MathCad ages ago and if I get extra energy one day (fat chance) I might convert it to excel to be more useful.

TS_turbo · Post by **TS_turbo** » Thu May 31, 2007 8:14 am

Micke you are talking about jacking force caused by G force that acting from tire contact patch through RC ... but front suspension is independent and if RC is above ground that jacking force is acting vertical through RC = reduced vertical overal load ... similar suspension setups at same cornering G's first one with RC at the ground second one RC 300mm above ground will have very similar weight transfer BUT different vertical load(overal too) values on outside tire caused by jacking force ... as far as i know of course

grant · Post by **grant** » Thu May 31, 2007 9:33 am

Micke,

What is your suspension setup again?

I don't think my rear end is quite as soft as you make it sound, I do have a 26mm rear sway bar back there on nearly the stiffest setting....

Also, I need to do some more reading, but how can a higher roll center create more roll? Wouldn't that give the CG less leverage to induce roll?

We'll see how the increase in camber works out this weekend.

Where can I get stiffer rear springs? I think Ron's offerings are all too stiff and a bit expensive. Any other sources?

Micke · Post by **Micke** » Thu May 31, 2007 10:55 am

TS:
I'm not 100% sure what you mean but "different vertical load values on outside tires" is weight transfer or what do you mean?

Grant:
My suspension is so modified that it can't be compared. As I said 600 lbs front and 520 rear. Rear springs are moved outwards for less reduction. 24 mm AR bar front and none rear.
The rear AR bar has such a shitty reduction that I'm not happy using one at all at least in the stock position.

I didn't read my post but I hope I didn't say a higher RC creates more roll. It doesn't everything else being equal the roll is reduced. Weight transfer is incresed though.

Stiffer rear springs are available at the tuning shop near you. 2.25" is a std size and nice to work with. I think Ron uses the same.

Mats · Post by **Mats** » Thu May 31, 2007 11:01 am

Tell me something, how can weight transfer be different if the CG is unchanged? Do you mean the ratio front/rear?

Micke · Post by **Micke** » Thu May 31, 2007 11:16 am

Let me quote myself some rows earlier

Remember that the total amount is constant as of the formula explained first. Only the ratio front/rear changes.