Suspension tune and handling journey

[GavinT]

G’day BDA,

Looks like you’re making some inroads (pardon the pun) on this.
Front camber can be tricky, I reckon, but it’s also needed less with stiffer springs.

I’d set front camber to 0º.
Reduce front toe to a poofteenth (engineering term).
Raise the front ride height perhaps 1/2”.
Go up another two clicks on the front dampers and leave the rears alone for now (though I do like EuropaTC’s method of taking big handfulls of clicks to establish a pattern).

All those should help the car to wash off some of the corner entry oversteer and make it easier to drive as well. And all within specs too, if I recall.

I’ve lost track but what tyre pressures are you currently at?

[jbcollier]

Hmm, you rebuilt the rack... did you shim the inner tie-rods correctly?  If they are not under proper tension, they will "steer" the car as they load and unload.

[BDA]

Thanks Gavin. I think I'll do my posts after a few tests. I guess after a while it seems silly to say that I made one small change and here are the results. I forgot to say in my replay to EuropaTC that the other reason I'm sneaking up on it is I am sort of feeling my way in my darkness here!

JB. Yes I did rebuild my rack - more than once! The first time (they all weren't complete rebuilds), I didn't pay attention to the tension on the tie rods you note and you didn't have to go very fast before it would try to jump into the next lane on the slightest steering input! It was diabolical! The second time, I estimated the tie rod tension and when it still didn't feel right (actually, some of that feeling was probably due to the shock/spring situation), I rebuilt the entire rack. I got a 40,000 mile old Spitfire rack basically adding the Lotus bits to it because it was in such good shape. This time, I used a trigger gauge (basically a spring scale) to test the tension on the tie rods and I made an 8" lever to press on the pinion splines to test the tension on it. Old racks end up being compromises when you set up the pinion resistance because the center ends up being a little too loose and the ends end up being a little too tight. I think what I have now is about as good as it's going to get.

[GavinT]

Oh, I dunno, BDA.
Sneaking up in the darkness never worked on the girls I went out with. 

Just on the off chance, when swapping over to the Spitfire rack, presumably you also swapped the correct TCS tie rod sleeve nuts to the rack as well?

[BDA]

Yep! The tie rod nuts, the tie rods, the tube to limit the amount of steering input, and the extension for the rack housing.

We went out with similar girls! 

Edit: I forgot to mention the tie rods!

[Lotusjps]

Hey DBA,

I've been working on tuning my suspension for a while, so I'll pass on what I've learned so for, keeping in mind my car is used almost exclusively for autocross and track days. 

Bit about my car:
Tires - 185/60/13 Hoosiers
Shocks/Springs - AVO adjustable
Adjustable front sway bar (had an adjustable rear as well but removed it)
Front toe - 1/16"
Rear toe - 1/8"
Front camber  -2.4 deg
Rear camber  -2.2 deg
Front pressure - 22lbs
Rear pressure - 24lbs

After each run I use a Longacre probe pyrometer to take tire temps which has been invaluable for tuning setups.

Like you have had people recommend higher tire pressures, and I've tried on multiple occasions (clear up to 35lbs).  Though in my experience there's a lot less grip at higher pressure.  Also when I've tried lower pressures the tire tend to roll too much and heat up the outsides.  I also bleed off pressure right before running so that I always start 22/24 (unless I make adjustments based on the pyrometer readings, though usually not more than 1.5 lbs up or down).

The toe is going to make the biggest difference as far a how stable the car feels.  Running 1/16" in front does make the car a bit darty, though that works out great for autocross (for street use I'd probably bring it up to 1/8").  I've also found that running less than 1/8" rear makes the car a bit unstable at high speeds or very hard braking.

As far as shocks, start on the softer side and work up as needed.  Currently I probably have my car stiffer than ideal, though I've been trying to reduce the amount the front inside tire lifts when accelerating out of a tight corner.

Anyway, best of luck (and highly recommend a good probe pyrometer)

[BDA]

Great information to chew on, JPS! You wouldn't know your spring rates would you? Any notes on shock tuning?

I've been concentrating on tuning the shocks lately. So far that has had the biggest change. Currently the front is at 9 clicks and the rear at 3. On the highway, its better than it's ever been but in small steering inputs make the car feel like the front is very stable and the rear seems to pivot from the front. I'm thinking that some more stiffness in the rear shocks might be in order.

I'm not sure I'm going to go as far as taking tire temps as I don't expect to track or autocross my car. I just want it to feel comfortable when I push it in a corner or drive on the highway.

There are lots of things I want to try but I haven't been able to devote the time to it I would like. I'm also on the lookout for a twisty road to test on. The one I use now is very handy and near my house but if I loose it on one side, I got into woods and maybe into a ditch. If I loose it the other way, I'm in oncoming traffic!

[literarymadness]

BDA  Good luck with dialing in your TCS.  Can't wait to hear the details. Here is an interesting cross-reference chart on understeer and oversteer.

[BDA]

Thanks, 'Madness! That's a nice chart to keep handy!

[literarymadness]

Lotusjps What spring rates do you recommend for autocross?

[BDA]

I did some more experimenting over the last several days. As I mentioned in an earlier post, I set my front shocks up four more clicks to 9. That helped a lot with oversteer - though there is still a lot more than I want. Highway driving was better again but still sensitive to small steering inputs, though not as much as it had been. Small steering inputs made the car feel somewhat like a pendulum. The front was the pivot and the rear wanted to step out a little. 80 mph required your attention but was a lot more relaxing than it had been - especially before I had started monkeying with the shocks.

I added 2 clicks to the rear shocks and that was an improvement everywhere. I don't know that I was faster through the twisty road I was testing on, but it feel a lot more stable. On the highway, it was a big improvement again. It was more stable and again less sensitive to small steering inputs. When I turned the wheel a little, it was much more comfortable and the pendulum feeling seem to be pretty much gone. I still had more oversteer in sweepers, such as highway on ramp, than I liked.

The latest test was to lower tire pressures. I have forced myself to only change one thing at a time and I figured that tire pressures would probably produce less change so I waited till the car was pretty good before changing them. I was running 30 in the front and 35 in the rear. I lowered them to 22 in the front and 24 in the rear. Everything was better still. There was a bit less oversteer on the on ramp but still more than I wanted.

It's a lot more fun to drive now. There's still room for a fair amount of improvement - especially in reducing oversteer. I'm not excited about stiffening the front shocks any further because it's starting to impact the ride quality.

[Clifton]

When you say oversteer. Are you saying the rear is loosing traction and sliding to where you have to steer it back straight? I ask because they understeer from the factory as most cars, especially mid engine ones for safety reasons. 

[BDA]

Yeah, the normal thing is to default to under steer but then I don't have a normal Europa, at least in terms of springs, ride height, wheel & tire size, and shocks.

Well, I didn't let it get as far as having to correct but you can feel the rear end try to swing around. I didn't want to push it that hard and potentially lose it entering a freeway! It is definitely not under steering. I wish I had a place to test that wasn't on city streets!

I'm open to any suggestions. This is what I'm thinking about at the moment:
I could reduce rear tire pressures a little, increase rear negative camber, lower the rear, or reduce rear spring rates. However, my tire pressures are 22/24 so reducing the rear would bring it equal to the front or lower, increasing rear negative camber would make it greater than -1°. Both of those seem out norm from what I've been reading. My ride heights are F: 4 1/2", R: 5 3/4". That's lower than most TCs I've heard of and the rear is higher than the "1/2" higher than the front" Richard suggested to me. Playing with ride heights may be something to look at. As for changing the rear springs, I do have a pair of 100# rear springs I could swap for my 130# springs...

[EuropaTC]

My ride heights are F: 4 1/2", R: 5 3/4". That's lower than most TCs I've heard of and the rear is higher than the "1/2" higher than the front" Richard suggested to me. Playing with ride heights may be something to look at. As for changing the rear springs, I do have a pair of 100# rear springs I could swap for my 130# springs...

I don't think it's the springs at fault, our cars are fairly similar and I'm running 160lb rears which gives wheel frequencies of 1.8/2.0 (F/R). On the same spreadsheet assumptions the S2 has OEM values of 1.1/1.4, the TC 1.2/1.4 so they also had a lower front ratio.

I started off with 1.8/1.8 and that didn't feel right so I increased the rear springs, aiming for similar balance to the OEM cars.  I can't say the improvement was only down to the springs because I also moved the ride heights as the front was very low and I felt there was too much rake on the car.   I also changed damper settings so again, it might not be just a spring thing.

If you change to 100lb springs then it will certainly alter the rear grip but on the same basis as the previous calcs I get 1.8/1.6 F/R ratios which sounds contrary to what Lotus did.

Next - where are you measuring ride height from ?  If it's chassis based then that sounds like a lot of rake but to be fair I've no experience of such an angle.

Measuring under the front chassis cross member I have 5.875", at the rear of the central spine just before the "V" I have 6.125" and the rear hoop is 8.625". The latter might not be relevant to yourself because we have different gearboxes and presumably different rear hoops ?  I have a garage pit so can measure from the chassis easily but it's also possible with old fashioned external calipers, lying down & stretching your arm underneath to get a rough idea of where you are.

Unfortunately I didn't record the initial settings (no idea why not  ) but the earliest numbers I have for the three locations above are  5"/6.5"/10". I went through several permutations up to 6"/6.5"/9" before ending up at the ones I have now.

I've never successfully figured out where the F/R roll centers are on the car but I'm wondering if because there's 1.25" higher at the rear (4.5"/5.75") then the rear roll is too high compared with the front.  Don't know and I've no idea how critical that aspect is on our cars so that last sentence is just me rambling.....

Brian

[literarymadness]

BDA: Based on the chart I posted, raising the front-end height would reduce oversteer. I think getting closer to that recommended 1/2 inch differential between your front and rear height might reduce the oversteer.  It is certainly worth a try.