First T&T: This thing feels good!

Just back from two days of testing & tuning down in Birmingham at the Hoover Met(ropolitan Stadium) parking lot. What a great site- big with very good, very grippy asphalt. As usual, the ALSCCA folks were very welcoming and did great job. The Iron City Match Tour scheduled for June 17-19 should be a fantastic event. Be sure to put it on your calendar.

But, before I get to how the car handled, let me catch you up. I borrowed a set of corner weight scales from a friend (Thanks Tom!) and got another friend (thanks Glenn!) to help me do the corner weighting. The empty car weighed 3043 lbs on 3/16 of a tank and the cross weights were way off somewhere near Saturn… around 1.5% out. We got it close to 50% but I’d only disconnected the rear roll bar, not the front. I felt like I was fighting the front bar, so I knew we’d have to do it again.

The next morning Clem Tire aligned the car. They found that last years’ home alignment was good, but the big thing I had them do was reduce the toe-in in the rear. Up until now I’d wanted a very stable car so I ran 0.28 degrees of rear toe-in per tire which is 1/4″ total. This year I had them reduce it to 0.22 degrees per tire, which is right at 3/16″ total. Not a lot of difference, I grant you, but along with some other changes I was hoping to get a little more slithering from the back end this year. When a well-driven Corvette slithers like a big lizard through a slalom it looks from the rear like the car is bending in the middle around each cone.

That night Glenn and I measured the corner weights with both roll bars disconnected. We had to crank up the left-rear corner even more to get close to 50%. Here are the results, with driver and helmet in the car and 3/16 of a tank of gas:


Corvette Corner Weights After Adjustment

What does 50% cross weight mean, you may ask? For one thing, it doesn’t mean that there is equal weight on each tire. The values in the chart show that clearly and the % left and % rear numbers tell you what the static weight distribution is. Cross weight is something different.

What 50% cross actually means is that the differential left to right is equal front and back. (It’s calculated by adding the right-front weight to the left-rear weight and dividing by the total weight.) So, if the left-front has 51% of the front axle weight, then the left-rear will also have 51% of the rear axle weight when the cross weight is 50%. Per the numbers above the car actually has 51.5% on the left-front and 51.2% on the left-rear. So, the cross isn’t perfect (49.74%) but it’s less than 0.5% from 50, which is usually the target.

Why is cross weight percentage important? It helps make the handling of the car symmetrical. That is, it will have the same characteristics turning left as turning right. Equal cross can’t do this all by itself, however. Remember I said my car has more weight on the left side than the right with the driver in place? Most production cars are that way. And this condition will forever affect the handling. (Not to mention the 54% that’s on the front axle!)

How do you adjust the corner weights to get 50% cross? Well, in Street class, there’s only one way: adjust the ride height at a corner. (We can’t rearrange components.) Not all cars have such an adjustment from the factory. Luckily, the Corvette has an adjustment bolt at each corner. What I’ve done is set both fronts low and equal. Then, I adjust the rears. The right-rear was already as low as it would go and the left-rear about in the middle. So, I had to crank up the left-rear, physically lifting that corner of the car. Doing this also sends weight to the diagonally opposite corner, the right-front, and removes weight from the other two corners. Think of your car like a 4-legged table. If you shim up one leg, it teeters on that one and the diagonal. With a sprung suspension it’s not all or nothing like it is with the rigid legs of a table, so even a little air pressure difference in the tires has an effect. Set your pressures before you measure the corner weights!

Lifting the right front would have the same effect on the cross, and maybe I should have. I’ll have to think about that and what difference it might make. I had a specific reason for making all the adjustment at the rear. Raising the rear of a corvette increases the roll stiffness at the back, promoting more of that slithering talked about earlier.

So, now the car is aligned with healthy front toe-out set at the event, cross-weight balanced, shocks set stiff, more air pressure than last year, a narrower (better supported) front tire than last year… wow, it drove good!

I couldn’t believe the turn-in rate! I was early on every corner for the first three runs. The back end was wagging left and right way too much, but once I slowed the input to the steering wheel, it all came together for top PAX time for the day.

The car is definitely better in transition, slithering through the slalom the way a Corvette should. It is less stable but more fun to drive and more than one person commented to me how good the car looked on-course. I can’t tell if any peak lateral G has been lost… the site and courses didn’t allow me to figure that out, but I’m not worried. I’ve achieved what I set out to do, which was to improve transient response. If I can’t do well at Dixie Tour a month from now it won’t be the fault of the car.

With all the runs today (day 2 of the Test & Tune had very few cars) I was able to play with tire pressures and figure out the sweet spots. Minus 4 psi from baseline in the front didn’t seem to reduce peak grip, but it definitely slowed the transitions. Plus 4 psi in the front reduced front grip and induced understeer. Plus or minus 2 psi around the baseline in the front and I can’t tell the difference.

In the rear, minus 2.5 psi from the baseline was a disaster… totally uncontrollable at the 1-2 shift! Plus 2 psi and I could feel some loss of grip. So, I think I have a +/- 1 psi band figured out for the rear. Of course, this is all for one surface, on one particular day, without changing shock settings, but it gives confidence in the starting point to use in the next events leading up to Dixie Tour.



Autocross Season Prep- Assembly Complete

Big progress this weekend. First thing I did was remove the tow hitch… 15.5 lbs of steel that won’t be needed as I plan to drive on the race tires or tow to all future events. Anyone need a tire trailer?

Here’s a sanded rear disk, with Carbotech pad installed:


Red Carbotech Pads Installed- Sanded Brake Disk

Flushed out the Motul 600 brake fluid installed for the last track day and pushed in some fresh ATE Super Blue Racing fluid. Love that color!


ATE Super Blue Racing Brake Fluid

As most 5th generation Corvette owners know, you’ve got to regularly suck the clutch fluid from the reservoir with a turkey baster and replace it. It gets dirty from clutch wear material and eventually causes a sticking clutch pedal. In the pic below, you can see thru the new golden Motul 600 fluid to the bottom. (The reservoir is full to the mark.) It won’t be long before it’s so dirty you can’t see below the surface.


Clutch Fluid Reservoir

Here she sits after the test drive and initial brake pad bedding. Now she’s ready to start tuning the handling.


Ready To Go- Mismatched Wheels and All

Upcoming this weekend is two days of Test & Tune in Birmingham at Hoover Met Stadium, site of the Iron City Showdown Match Tour later in the Summer. The weekend after is our TAC/TVR-SCCA Test & Tune at Milton Frank Stadium in Huntsville, my local autocross site, and the following Saturday is our TAC Driving School with a practice autocross on Sunday, also at MFS. Then I plan to go to Knoxville for ETR-SCCA’s first points event on Sunday, March 13th. Four weekends in a row leading up to Dixie Tour March 18 -20… should be enough to shake off the rust and hopefully get the car handling the way I imagine it can.

Eight drivers in three different car types have registered in B-Street for Dixie Tour as of this moment. I know six of the other seven personally and they can all be fast. The seventh I haven’t met, but with his track record he might very well be the one to beat!

Autocross Season Prep Gets Started

Prep for the 2016 autocross season started this weekend. A few pictures to show the plans and progress.

First up, new rubber: RE71Rs in 265/275-18. They’re so clean and sticky if it was summer they’d catch flies.


New Bridgestones

All disks have been sanded, ready for bedding-in new pads, Carbotech Bobcats that come painted red. I’ll replace the brake fluid as well.


Sanded Brake Disk with New Pads in the Caliper

The Pfadt/Ohlins shocks are reinstalled. No changes since RE Suspension worked on them last year. I’ve routed the line to the remote reservoirs a little differently than last year to better keep it away from any sharp edges. The upper right A-arm bushing has migrated a tad, but it should be good for another year.


Pfadt/Ohlins Shock Rebuilt by RE Suspension

The shock remotes are again strapped to the front roll bar, but reversed in orientation from last year. If I lay down in front of the car I can just reach them to turn the adjuster knobs.


Most modern roll bar bushings don’t really need any lubrication, but to reduce friction to a minimum I cleaned and lubed them with Energy Suspension super-sticky grease intended for polyurethane bushings. I noticed that the inner surface of the bushings have circumferential ridges that seem like they will hold some grease in place quite well. I don’t think this stuff will wash out.


Cleaned & Lubed Roll Bar Bushing

After the rears are finished, I’ll get the car aligned and corner balanced. Then, I’ll disassemble the driver’s seat again and figure out what’s broken in it this time.


2016 Plan: Amended

I think I got something wrong in the last post.

I said “However, the sway bar, unlike the springs, acts across the car to create an increase in the total weight transferred from the inside to the outside, which tends to decrease total lateral G capability.”

Did you agree or disagree?

Some people have definitely thought this in the past. They (and I) may have been misinterpreting such statements as this, from Carroll Smith, in Tune To Win, page 38: “…the stiffness of the anti-roll bar  will both decrease roll angle and increase lateral load transfer.”

We’ve got to be careful with that statement.

The sprung mass rolls. If there is no sway bar, the inside spring extends, reducing weight on the inside tire. The outside spring compresses, increasing weight on that tire. If you could put a scale under each wheel you will measure what looks like a “weight transfer”, but it’s not, really. It’s a differential in forces at the tires and it will disappear once the car stops cornering. The amount of force differential is controlled by the mass, the lateral acceleration, the track distance and the moment arm which is the distance from the CG to the roll center. The amount of roll doesn’t matter. The stiffness of the spring doesn’t matter. The existence of a sway bar doesn’t matter.

If there is a sway bar, some of the roll energy goes into it instead of the springs. The roll is reduced, but that energy creates another force differential at the tires. The sum of the spring differential forces and the roll bar differential forces exactly equals the previous   amount with springs alone.

So, there is no negative effect on maximum cornering force due to using a stiff roll bar. There is no increase in the total amount of weight transfer, or what I call force differential.

But, during the transient, that is, during turn-in while the lateral-Gs are rising, roll stiffness from springs and bars is a good thing because it makes things happen faster. Energy absorbed by the shocks is a good thing as the effect is to (temporarily) increase roll stiffness and make things happen even faster. The force differential at the tires is going to do what it’s going to do, which is to decrease the maximum lateral-G capability due to the shape of the tire load sensitivity curve. Not much I can do about that in Street class other than set up the car as low as possible. Or, go on a diet and get a lighter helmet!

2016 Plan: Improve Transient Response

I have a simple goal for the near future: do better at Dixie Tour, the first National Tour event of the year.

I’ve always done poorly at that event, held for many years at South Georgia Motorsports Park near lovely Adel, GA. I think at least part of the problem has been the site: it’s a long, thin parking lot. As a result, the courses have been similar, transient-heavy things. Wiggle your way to one end, turn around and wiggle it back. Almost a constant speed. Not much of anywhere to use much power. Not especially good for a Corvette against S2000s and MSR Miatas. In fact, in 2012 Jadrice Toussant won B-Stock in an S2000 with a time that would have been 3rd in Super Stock. He beat every Lotus, every GT3 and every Z06 except for Strano and Braun. Now, Jadrice is a heck of a driver and National Champ and he flat tore it up that day, but I think the course had something to do with it.

Here’s what it looks like. You can even see some tire marks.


South Georgia Motorsports Park

So, the plan is to improve transient response. Mostly to see if I can do it and maybe do better at Dixie.

Two years ago I concentrated on maximizing lateral grip. Then last year I got some better shocks that I’d hoped would improve transient response via higher low-shaft-speed compression damping, but I was still focused on lateral grip. It worked to some extent, but not enough to place in the top 3 in class at Dixie. ( I was a miserably slow 4th.)

Before I start making changes, I figure I should review what I think I know about what happens when you turn a car and what factors control the transient response. So, I tried to write it down. (You’ll note that I like to start an analysis at the very beginning.) I focus on the front of the car.

  1. Turn the steering wheel. (We all do this pretty well, I guess.)
  2. The tires turn to an angle with respect to the car direction.
  3. Due to friction provide by gravity and proportional to the weight on the tires, the tire contact patches deform and twist, producing slip angles and lateral forces at each contact patch.
  4. Tire patch lateral forces transfer to and act on the sprung and unsprung forward masses and do two things: change the direction of the front of the car by creating a lateral acceleration, and create lateral weight transfer.
  5. Lateral force acting at the unsprung mass CG acts to instantaneously create weight transfer (like on a kart) and tends to reduce the ultimate lateral acceleration possible as weight is transferred from inside tire to outside tire, reducing the total contribution of all the tires added together.
  6. Lateral force acting on the sprung mass through the roll center rolls the sprung mass on the springs and bars to create (what Dennis Grant calls) elastic weight transfer, neglecting any small lateral translation of the CG. (I’m going to simplify things and neglect jacking force, or Geometric weight transfer, again per Dennis Grant, as it is small in the Corvette because the roll center is close to the ground.) The amount of roll does not affect the [total] amount of weight transfer. Update: I think I wasn’t clear, as Mr. Glagola has pointed out. What I mean is, the total amount of weight transfer is not dependent upon roll stiffness. Yes, the weight transfer build-up is proportional to roll angle, but that is because both the roll angle and the weight transfer are proportional to lateral acceleration. When the car gets to it’s final roll angle, whether it be 1 degree or 15 degrees, weight transfer is complete.
  7. Compression damping turns part of the sprung mass roll, during the roll transient, into increased downward force on the outside tire contact patch. This temporarily increases the lateral force capability of that tire. (It also turns some of this roll energy into heat, which leaves the system, so it never gets into the springs or bars.) This is why high levels of low-shaft-speed compression damping assists transient response. It acts as if the spring temporarily got stiffer. At the extreme it would allow the outside spring to compress only very slowly. In a slalom, with enough compression damping, very little roll might have time to actually occur before the car is asked to change direction again.
  8. Rebound damping on the inside wheel also resists sprung mass roll during the roll transient and once again some energy is turned into heat. The rebound force tends to hold back the roll of the sprung mass. To do this it picks up weight from the inside tire and tire patch, which tends to decrease the lateral turning force produced by that tire. In the extreme case the tire might leave the ground as the body rolls and the wheel follows. (I don’t think any shocks have that much rebound damping, but it could be done. If you were an idiot. Or an engineer trying to prove a point. Or some mixture of both, as is the usual case.)
  9. Roll of the sprung mass extends the inside spring, thus reducing the load on the inside tire. Roll compresses the outside springs, increasing the load on the outside tires. Without shocks, achieving maximum cornering force is delayed until the sprung mass roll is complete, if for no other reason because the outside tire doesn’t get to it’s final, proper camber until then. By resisting roll both rebound and compression damping forces speed up weight transfer across the front axle, getting the car into a cornering attitude faster and with slower roll, and thus less total roll during the transient. In this way, we don’t have to wait for the sprung mass to roll to it’s maximum before achieving high lateral cornering forces, though reaching the maximum cornering force is probably not going to happen. I suspect this will increase the maximum achievable cornering force when time is short, such as in a slalom. However, almost all of the increased energy in the compressed spring will be delivered back into the sprung mass when the turn is reversed, helping to roll the car the other way. The shocks absorb some of this energy in both rebound and compression, turning it into heat, and thus assist in keeping the car controllable during repeated maneuvers.
  10. All roll twists the front sway bar. Like the springs, most of the energy absorbed by the sway bar is given back when the car is turned the other way. Therefore, during transient maneuvers the energy put into the bar during whatever roll occurs wants to come right back out and roll the car the other way. Once again, shock rebound and compression damping absorbs some of the energy, keeping the car from rolling uncontrollably during repeated maneuvers. (Unless you are a certain production SUV and fail the Scandinavian Moose Test.) By limiting maximum roll, and thus camber loss, the sway bar may increase maximum lateral G forces in a sweeper by keeping all tires working better than otherwise. The sway bar also slows the rate of roll, assisting transient response. [However, the sway bar, unlike the springs, acts across the car to create an increase in the total weight transferred from the inside to the outside, which tends to decrease total lateral G capability.] Update: I now believe the statement in brackets to be false. In softly sprung production cars it is almost always better to limit camber loss by limiting roll with stiff sway bar(s). However, it may be that during the roll transient of a production car (with soft springs) it may be better to trade sway bar stiffness for an increase in shock damping, especially compression damping, in order to limit weight pulled off the inside tire.

So, based on this assuredly imperfect understanding of what happens when a car turns, I’ve come up with an action plan:

  1. Choose a tire known for it’s lateral stiffness. The RE71R is known to be one of the stiffest & most responsive. That’s what I’ve been running.
  2. Properly support the tire with a wide-enough wheel. I’m  down from an oversized 275mm to a less-oversized 265mm this year on the required 8.5” wide front wheel.
  3. Increase support to the tire with air pressure. The past two years I found a relatively low pressure was needed to maximize lateral grip in sweepers. This year I will test using higher pressure in front to maximize tire support and hopefully produce faster transient response at the contact patch.
  4. Keep using significant toe-out on the front tires to more rapidly establish a bigger slip-angle on what will be the inside tire in the turn. This worked well last year. This allows the inside tire to more quickly create a lateral force, pulling the front end of the car into the turn. As the weight shifts to the outside tire, it has now developed a good slip angle and can really drive the front end into the turn. Because of weight shift, the inside tire is of lesser importance by then. I reset the toe before driving home after out-of-town events. The poorer front-end response with no toe-out is palpable.
  5. Test using the softer setting on the front anti-sway bar. The final roll angle may be less important because the final roll angle will not be reached in a slalom situation. I used the stiffer setting last year on a stiff bar to maintain proper camber of the tire during sweepers. For best transient response, it may be better to reduce the stiffness to reduce weight transfer off the inside tire. It may be possible to keep the inside tire working longer in the initial part of the turn. I will try to figure this out at an upcoming Test & Tune event. Update: As noted in an update above, the basis of this is false. So, I won’t be reducing roll bar stiffness, at least not for this reason. I might reduce roll bar stiffness for balance or stability reasons.
  6. Test with the shocks adjusted for higher front rebound and compression damping than used before.

Of course, all this may completely unbalance the car and I’ll be even slower than last year. I expect I’ll learn some things either way.