Autocross shock clickies and their uses?

We autocrossed the One Lap Civic this past weekend. Impressions were that it's a billion times better than last year, but not perfect. Turn in is great, mid corner on sweepers the car breaks away evenly with good rotation by LFB. On corner entry though the back end is dead, just trailing along for the ride and no amount of LFB on entry could change that. Any thoughts on what we could change to keep the mid corner behavior while inducing a bit of playfulness on entry? Shocks have Bilstein single adjustable guts and we have adjustability in both directions. 

Single adjustable means you only have general rebound adjustment, so there's nothing you can do with those shocks to target corner entry behavior alone. If you don't have any rear negative camber, adding some would reduce corner entry/exit grip and increase mid-corner grip (assuming typical production car suspension with insufficient camber gain).

We're actually somewhere between 2.5 and 3 degrees all around now. Grip was an issue on 225's and we'll be on 275's for One Lap. I'm hesitant to change anything knowing how much things will be changing with the new tires and the fact that we're adding about two inches to track width front and rear when that happens. This is more of a "I wonder if I had tools at my disposal that I didn't use?" question. I probably could have gone up a couple psi in back. 

In reply to mazdeuce - Seth :

Two options, which may work individually or combined, but you'll need to test to reveal what works best. 

1.) Crank in as much rebound in the rear as you can. That will transfer weight to the outside tire quicker, which may help.

2.) Give the car a bit of rear toe-out. 

A lot of people run staggered tires on a setup like that.  275's up front and 225's in the back to let it rotate.

Pete Gossett said:

In reply to mazdeuce - Seth :

Two options, which may work individually or combined, but you'll need to test to reveal what works best. 

1.) Crank in as much rebound in the rear as you can. That will transfer weight to the outside tire quicker, which may help.

2.) Give the car a bit of rear toe-out. 

I thought about rear toe-out, and while it will improve rotation on corner entry, on most suspensions which toe-out under compression, it will have an even greater effect mid-corner. Increasing single-adjustable rear damping isn't a bad idea but it will affect the car throughout the entire corner...in fact from my experience it seems to have the most effect in mid-corner.

KyAllroad (Jeremy) said:

A lot of people run staggered tires on a setup like that.  275's up front and 225's in the back to let it rotate.

To me, reducing tire width is like adding weight: if it solves a problem, there's always a better way to get the same effect. Increase tire pressure or negative camber, or redistribute weight instead. The first two could cause uneven tire wear, but the higher cost/inconvenience of staggered tires should be accounted for too.

What about rear brake bias?

Assuming in the context of OLOA you only have rebound to play with since some of the typical autox tweaks to alignment/geometry would make the car more twitchy and possibly not as enjoyable to road course...

Tire pressure.

If you could stand a bit more rotation mid/steady state corner or could alter your LFBing to get less rotation I would do the below:

A couple test runs with the rear clickers set full soft and then full stiff in the rear. See if either extreme fixes what you feel, you may find this helpful to know what each end of the adjustment feels like before OLOA anyways so you know how much you can "fix" the car on the fly and how much you'll have to drive around. Then work your way back towards the middle from whichever end of the clickers felt better until you strike a balance of turn in and steady state that you are happy with.

If you wind up with the rear shocks maxed out and still want more turn in then I'd do the same for the front, try full soft and full stiff just to see what changes that makes to the car. On singles I always have a hard time picking the sweet spot on front shocks where it still responds well in slaloms without giving up too much on turn in and braking.

As you alluded too, with much bigger/stickier tires inbound I'd do the above tests just so you have an idea of how much change you can inflict on the car but would then wait and expect to tweak settings (possibly drastically) once the new tires come in.

We thought last year we had the car pretty well dialed in heading into OLOA but the auto-x was our first time actually driving on the Conti's compared to the re71's we typically ran. We wound up chasing our tails the first 2 days and wound up quite far away from our usual baseline starting settings.

Unless your shocks are very difficult to adjust, try adjusting them. Get a notebook and make changes. You'll learn exactly how the car will be affected far faster than asking a forum.

Shock changes should make more difference in transitions than in steady state cornering unless you're on a very rough track. Then it depends on exactly how they change, and this comes down to how the valving is set up. In other words, you can't generalize.

Experiment! Nothing beats experimenting.

Thanks guys. I'm still "teaching" the car owner the difference between autocross and rallycross and I was trying to keep the car as consistent as I could to help him work on technique. I will experiment next time to try and get the car to behave. I think we can get at least one more autocross in before One Lap and I'll have a notebook with me. 

mazdeuce - Seth said:

 Turn in is great, mid corner on sweepers the car breaks away evenly with good rotation by LFB. On corner entry though the back end is dead, just trailing along for the ride and no amount of LFB on entry could change that. Any thoughts on what we could change to keep the mid corner behavior while inducing a bit of playfulness on entry? Shocks have Bilstein single adjustable guts and we have adjustability in both directions. 

 let's break a corner into 3 zones:  entry aka turn-in (transition); mid-corner (steady state); and exit (transition).

and let's break the entry transition into its components (considering a constant radius turn taken at a constant speed):

1. Driver turns steering wheel, at some rate, to some angle
2. Compliance is taken up in steering system, and front wheels achieve some angle relative to direction of travel
3. Front tires deflect as they achieve some slip angle relative to direction of travel
4. Front tires generate lateral force in response to this slip angle
5. Weight is transferred laterally from inside front tire to outside front tire
6. Vehicle yaws (rotates) about (vertical) yaw axis of vehicle body
7. This yaw generates a slip angle of the rear tires relative to the road
8. Rear tires deflect as they achieve some slip angle relative to direction of travel
9. Rear tires generate lateral force in response to this slip angle
10. Weight is transferred laterally from inside rear tire to outside rear tire
11. As rear lateral force increases from zero to steady-state, the yaw center moves from the Yaw axis of vehicle body to the center of the turn.

Steps 1-6 are called Phase 1, and steps 7-11 are called Phase 2.

i interpret "inducing a bit of playfulness on entry" to mean "i want a larger rear slip angle, earlier", so let's consider trackside adjustments to the steps of Phase 2:

7 / 8 / 9. Increasing rear tire pressure will increase rear tire cornering stiffness, which will reduce time needed to generate lateral force.

10. Softer jounce / stiffer rebound damping will decrease the time it takes for the lateral weight transfer to occur.

As noted by others, rear toe out will decrease grip of inside rear tire, which means the outside rear will have to go to a larger slip angle to generate enough lateral force.   And increasing static negative camber will decrease grip of inside rear tire while increasing grip of outside rear tire, again having net effect of requiring larger rear slip angles to generate enough lateral force.

For a test plan, decide which variables you're going to play with, and determine the bookends of each variable.   Dampers are easy, full soft to full firm with a couple points between.  Tire pressure and camber:  Check with the tire manufacturer for recommendations for your suspension type and corner weights.  Otherwise, for pressure i'd go with door sticker, +3, +6, -3, and -6.  these coarse adjustments should show you in which pressure bands you want to make fine adjustments.  if results of +6  > +3 > 0, then try +9.  ie keep going one direction until performance stops getting better.  IIRC with the Neon RT way back in the day, Per had the rear tires at something like 56 psi, and something like +3* camber to get the rear end transient where he wanted it.  So don't be afraid to test Positive Camber.

In the end, whatever changes you make have to work everywhere, not just constant-radius sweepers.  optimizing sweepers could FUBAR your slaloms.  test test test, and keep good notes.

I swear classic Mini guys would install casters in place of the rear wheels if they were allowed.

BTW, when I was having trouble with my CRX being a bit inert on corner entry, the solution turned out to be lifting the rear. This should increase the amount of weight transfer across that axle, but more importantly it got me away from some travel-related problems. Edge conditions can do weird things.

In reply to GameboyRMH :

The front wheels of a FWD car are expected to do 100% of the accelerating, 80% of the braking, and something like 70% of the turning.  The rear tires are just along for the ride and to keep the bumper from dragging.

A big front tire stagger is widely accepted among top level drivers (national champion level) as being a good option.  And the smaller rear tires weigh less as a bonus, free power!

The main problem with tire width stagger on the one lap a the tire rule. You're only allowed five tires for the entire trip, so if you choose the wrong size tire to bring as a spare, you're fuscrewed. 

Actually the OLOA rule states you can bring 1 spare for each size on the car so people that run staggered actually have a bit of advantage should the worst happen as they'll have 2 spares vs the folks running square setups only having 1.

In reply to klodkrawler05 :

i'd run a different size at each corner...

My thoughts went right to rear toe because it is so easy to change on those cars.

AngryCorvair said:

In reply to klodkrawler05 :

i'd run a different size at each corner...

you'd probably want to run some kind of minivan/truck class entry to pull that off

also, to note... you have to get Brocker's approval to swap tires... as "they're worn down" and "I flat spotted them in 3" aren't considered good enough reasons to change

In reply to AngryCorvair :

Thanks for that breakdown. It kept me awake last night thinking about elementally tearing down what the car is doing at any given point. Good stuff. I think you're right that I want the rear to come into play quicker. Testing is forthcoming. 

To everyone else, thanks for the suggestions. This all has relavance to me in the Accord too as I'm going to be attempting to make that behave as well in the future and the rear suspension is largely the same. I have a lot to think about with these cars in the rear and some more adjustable links are in my future. 

In reply to mazdeuce - Seth :

you'd really like the SAE Applied Vehicle Dynamics Seminar I help instruct

I always used air pressure to get the rear to rotate earlier/faster. I was elated when I "only" had to run a 5-7psi difference front to rear on the Forte. Previous FWD vehicles were 15-20+psi. 

Bob the REAL oil guy. said:

I always used air pressure to get the rear to rotate earlier/faster. I was elated when I "only" had to run a 5-7psi difference front to rear on the Forte. Previous FWD vehicles were 15-20+psi. 

You were running higher pressures on the rear than the front?

In reply to GameboyRMH :

yes. I know some go lower, I preferred to go up. Especially on the heavier cars with smaller tires. 

The Forte would get stupid loose at 10psi difference. Like "Holy crap it's coming around and there isn't a damn thing I can ... wait.... nope we're good."

AngryCorvair said:

In reply to mazdeuce - Seth :

you'd really like the SAE Applied Vehicle Dynamics Seminar I help instruct

I think I need to take that course.

Hang on, you're James Walker?