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.

SGMP

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.

 

 

The Really Weird Thing About Modern American Autocross- Revisited

Notice of update: I realized after publication that I’d used an entry speed of 55mph rather than the 50mph used in the previous installment of this series. So, I’ve got back up at 12:30 A.M. and changed it back to 50mph. Didn’t make much difference to the results data and no difference to the conclusions. (I also tried to make Figure 4 less of an eye test.) My apologies to the over 400 of you who have already read this in the first few hours of publication.

I received some good comments on the earlier post “The Really Weird Thing About Modern American Autocross- Part 2”. Charles Krampert, for one, pointed out that I wasn’t taking into account the angle one must drive after leaving the first cone to be set up for the same radius around the next cone. I finally got some time to put that extra turning into the graphics and the spreadsheet. Turns out it makes a significant difference.

Remember that I’m trying to figure out what radius is fastest around a cone, depending on the acceleration capability of the car. (If you don’t remember, you might want to go back and read the earlier posts.) I assume an infinite procession of cones 150 feet apart which require a nominal 90 degree change of direction around each cone. I also assume 1.2G lateral capability and 1.0G braking, which is typical for many street-class autocross cars. The basic idea is shown in Figure 1, below.

 

proper path

Figure 1

More specifically, Figure 2 shows what I’m analyzing and, in fact, represents the actual fastest radius for a car with 0.3G acceleration capability, namely a radius of 15 feet.

tic3 .3g curve

Figure 2

From A to B the car has to take an angle to the outside of B to produce the 15 foot radius. The car then has to go more than 90 degrees in order to exit B to allow it to go around the next cone, 150 feet away (not shown) at the same 15 foot radius. I  calculate the time from the start at A, going 50 mph, to the finish at C, 75 feet beyond B.

It turns out that considering the extra turning to get to the proper angle for the next cone reduced all the answers. Here are the results, with the old radii results, then the new radii results. The remaining data is all for the new radii.

TIC3 results updated.png

Figure 3

We can make a few observations:

-As before, as the accelerative capability of the car goes up, the best radius goes down. This agrees with what most people think.

-As before, we see an immediate issue with the curve velocities: they are too low for most cars to accelerate strongly from in 2nd gear. More on this below.

-The new best radius values are lower and more compressed over the G range than calculated previously. The extra turning required to be oriented correctly for the next cone greatly penalizes big arcs. A lot of time is lost going around at minimum radius at minimum speed. This goes a long way to answer the doubt I had expressed about the large radius values that the previous analysis showed as best and which I have not seen being used in practice. (I’m a big believer in the idea the most experienced people are doing it  mostly right most of the time.) By the same token, if you don’t have to turn the car as much for the next cone you are better off with a somewhat larger radius. The data seems to be very sensitive on this point.

The very low curve velocities associated with very small turning radii mean that there’s a big problem with using this data to make firm conclusions. I began this study thinking that I was using acceleration ranges that were typical of peak torque in 2nd gear. What this has shown is that we can’t think of it that way. The best theoretical radius is always too small. Instead, we have to think of the acceleration that is actually available at the particular curve velocity required by such small radii.

For instance, my BS Corvette can accelerate at 0.45G at peak torque in 2nd gear. But at 25 mph it may struggle to reach 0.3 G. The results chart says the best radius for 0.3G is 15 feet, but my car will only be going 16.4 mph around a 15 foot radius and will really struggle to accelerate in 2nd gear from that low speed.

On the other hand, what this data may be telling me is that I’d be better off taking a smaller radius and downshifting to 1st. I know, this is sacrilege! I’m going to lose roughly 0.2 seconds when I upshift and probably lost some time downshifting as well. Will it ever be worth it to downshift?

Looking at the sensitivity of the results may shed some light. Here is the spreadsheet set for 0.3G in Figure 4, below.

TIC3 graph updated

Figure 4

Looking at the Total Time row you can see that the minimum time is 5.959 seconds underneath the 15 foot radius column. That’s how I get that 15 feet is the best radius for a 0.3G car… by comparing it to the results for radii both bigger and smaller. It’s a brute-force optimization technique. (It’s also 5.959 seconds underneath the 17.5 foot column, so the real minimum is somewhere in between. I just chose one.)

If I change the A 2nd acceleration parameter in the upper-right corner all the columns recalculate and I find the minimum time for that G-level. I graphically determine the arc distance and straight length within the columns for each case.

Now, how far off of that best 15 foot radius do I have to get to equal a 0.2 second downshift loss? I’d have to go beyond a 30 foot radius before losing 0.2 seconds. But, the bigger radius I take the less I will need to downshift. If we assume that at a 30 feet radius I definitely don’t need to downshift, then if I choose to take a 15 foot radius and do downshift I am, at best, breaking even. I think this confirms the majority view that downshifting with a relatively slow-shifting car like mine is almost never a good idea. The corner’s gotta be really tight to consider it.

If we consider a motor with really poor low-RPM torque, say an S2000 that will drop out of V-tec, then no way it should take the smaller radius. Unless…

Here’s the real rub and why I entitled this series of blog posts the way I did. For corners where the car is forced by the course design to take a very tight radius S2000 drivers have learned that it is better to downshift to first when I would not in my Corvette. At least that has been my observation after competing against them the last few years. They “know” that they lose too much trying to get off a slow corner in 2nd. (When I see an S2000 struggling to accelerate below the V-tec RPM switch it just warms my heart!) I think we all know that there is some point that we should shift to first gear if the corner is very tight.

Let’s level set and get our bearings. When do we know that it is definitely advantageous to downshift? Pin cones. All of us who have done 180-degree pin cones know that the best way to take them is absolutely as tight as possible and downshift to 1st by all means. (Thank you, Randall Wilcox.) We never see these at National events, but they had one at every event in Nashville at the Superspeedway lot that I ever attended and we have them quite often in Huntsville when we run at the old airport. I’ve done a lot of those suckers.

Alternatively, when do know we would never downshift? How about a 45 degree turn? No way we would downshift to first. We all know that we cannot even create a radius small enough to force the car so slow to even think about downshifting.

So, somewhere between 180 degrees of turn and 45 degrees of turn is a middle ground where it may or may not pay to downshift from 2nd gear at autocross speeds. We happen to be concerned here with 90 degrees and somewhat more, so we are probably right in the no-man’s land.

Of course, there’s another issue with downshifting. Can you get the power down? If you can’t get enough power down to create the fantastic acceleration promised by 1st gear, no point in downshifting. Also, as soon as the rear end steps out you’ve lost 0.2 seconds, or more.

All this implies that, in certain situations, the right radius for a car on R-comps is not the same as the right radius for a car on street tires. It also means that as tires get better at putting down power, as we saw with the Bridgestone RE71R last year, it may affect the proper radius. Yes, autocross is complicated.

What about dual-clutch transmissions? What if you can downshift and get 0.7G in 1st gear even in a relatively low-powered car and then lose next to nothing when you upshift to second? I think this data says that for large degree turns you should consider braking down to a very small radius and downshift, within the limits of you car’s ability to put power down. You might even alter your whole style of taking large-degree corners in order to use the quick-shift capability.

Wilmington Data Crunch Part 3: Killer Oversteer

After the big error discussed in Part 2 of this series, I got mad, got fast and made up some time. Then, it happened again!

Recall that this is what the course looks like from the GPS data, with sectors designated by dots. Run 1 is in blue, Run 3, overall slightly faster, is in red.

Fig 1 Overall Course Day 1

Fig. 1 Overall Course Day 1

I want to focus now on the revised seventh segment from the start (green dot), which is the one preceding the cross-hairs, i.e. the last one before the finish segment.

A closeup of segment 7 looks like this. It contains three sweeping corners.

Fig. 2 Segment 7 Close-up

Fig. 2 Segment 7 Close-up

Right off we can see significant differences between the two tracks. There even appears to be a kink in the red path, Run 3. Uh-oh! That’s probably not good.

Continue reading

Wilmington Data Crunch – Part 2: Showcase Turn

The showcase turn was an increasing radius on Day 1, the way most people drove it .  I lost of lot of time in it on my third run. The data can tell us how much.

If you recall from Part 1 we are looking at 20Hz GPS data. I don’t have engine parameters, steering angle, brake pressure… just position, velocity and acceleration and what can be derived from those.  For Part 2 I’ve slightly redefined the segments as compared to Part 1. They look like this, with the start being the lower left green dot:

Full 1 Mile GPS Track In 7 Segments

Full 1 Mile GPS Track In 7 Segments

The cross-mark is in the middle of the showcase turn, which is Segment 5. Here’s the segment time data, with Run 3 set as the baseline:

Segment Times- Part 2

Segment Times- Part 2

The Run 1 S5 delta is -00.40, meaning 4 tenths of a second faster (less time) than in Run 3. What happened to lose so much time the third time through this corner?

Continue reading

Get Your Transients In Order

Most people rank three qualities in order of importance for an autocross car: 1) peak lateral grip, 2) transient response, and 3) power-to-weight ratio. Let’s talk now about transient response.

What we mean by transient response is how fast a car can change direction, that is, how quickly can turning be initiated or reversed. All forms of automobile racing value transient response, but probably none as highly as autocross. Does any other form of motor-racing regularly negotiate slaloms? The closest are probably the chicanes incorporated into road-race circuits for the purpose of slowing the cars. Here’s some data from my car in a long slalom at TAC/TVR #6 this past Sunday.

slalomsnip

The top graph is speed. Notice that it is constant at about 45 mph until I accelerate at the end. (You may also notice that I start accelerating even before the peak lat-g is reached. I “steal” a little cornering to do this, but I also think it means I was under-driving this slalom.) The lower graph is lateral G’s. They alternate plus and minus at about .58 G’s with a little over 1s peak to peak. The slope of the line between the peaks is a measure of how fast the car is transitioning from left to right… how fast it takes to get from max lateral G right to max lateral G left. The steeper the slope, the faster the transition.

What we’d really like would be perfectly vertical lines separating broad plateaus of max lateral G, as shown below.

instant transient.001

How do we get closer to that? Also, if we could get to max lateral G faster, we’d get to both a higher maximum value and higher average velocity. The car did not have time to get to even 0.6 Gs lateral in the slalom before having to reverse, but the car regularly reached 1.1Gs on that lowish-grip surface on that day on longer corners. [Update: I now know that the data device I was using does not capture the lateral-G peaks in a slalom. EF 2020]

The first thing to remember is that if you want the car to transition quickly you must ask it. How do you do that? You turn the steering wheel fast! Fast hands shorten the transition from a larger radius to a smaller radius, or from turning left to turning right, by quickly establishing useful slip angles at both front tires, thus creating maximum lateral Gs as soon as possible. In theory, a slalom can be taken the fastest with alternating steady-state maximum lateral G turns with instantaneous reversals. The faster you turn the wheel, the more you mimic an instantaneous reversal. However, the more over-steery the car, and the faster the steering ratio, the more you may have to limit your hand speed. It is also perfectly valid to do like many top drivers: turn the wheel too fast and too far initially, intentionally causing excess rotation, then turn back to catch the oversteer, applying throttle to shift the weight to the rear and increase rear grip.

Besides fast hands, high roll stiffness is necessary to achieve a quick change of direction, which is one of the major advantages of stiff springs in autocross cars where the class rules allow it. How long it takes for the sprung mass to roll to its new attitude directly affects how fast a car can change direction. In classes where you cannot change the springs to increase roll stiffness some cars can use an extra-stiff front anti-sway bar allied with shocks valved to produce high levels of compression damping with a knee at low (1 to 2 in/s) shaft speeds and digress (blow off) at higher velocity. (The digression is necessary to keep the car from becoming unstable over sharp bumps at high speed.) The figure below is a dyno plot of my present shocks, showing average Force vs. Velocity. Notice the knee at 1 in/s in the compression data, which are the top half of the chart. (The bottom half is rebound. I have linear, not digressive behavior in rebound.)

shockdyno1

The compression damping resists and slows the compression of the outside shock, thus limiting roll and speeding up weight transfer. This produces higher transient roll stiffness than otherwise. It has no effect on the maximum roll angle reached after the car takes a set because shocks only creating damping force when the shaft is moving.

Rebound damping also helps turn-in by slowing the extension of the inside shock, thus again resisting roll and speeding weight transfer across the axle. However, the autocross car is limited in the usable level of rebound damping because, in excess, it hurts grip by binding up wheel motion. The effect is that the tire does not stay fully in contact with the pavement. Even on a substantially flat and smooth surface too much rebound can “pull” weight off the contact patch and reduce grip. For one thing, you are rockin’ & rollin’ out there, creating your own pitch and roll dynamics. Every pound of force a shock produces in rebound is obtained from the contact patch of that tire.

Let me repeat that, and put it in italics, because I’m not completely sure you really got it the first time and I haven’t seen it put quite this way before: All rebound forces developed by a shock absorber are achieved by reducing the load at the contact patch of that tire, which in turn reduces the grip available from that tire.

To an extent this is what we want when turning, because that pound of force also acts on the mass of the car to limit it’s upward motion at the shock for good transient response. When rebound damping is used to excessively control body motions we may regret it. For instance, when we try to turn after a ripple in the pavement we may find a momentary decrease in grip has exchanged our normally sharp turn-in for a half second of understeer.

Toe-out in the front can also speed turn-in by quickly creating slip-angle at the inside tire. This pulls the front into the corner even before any weight has shifted to the outside tire.