Ariel Atom Autocross Development

[762]

I just recently picked up a N/A K24 Atom 3 from Franz at Forman Motorworks and I will be developing the car for use at Arizona SCCA autocross events.

The car is not classed competitively, but I have wanted to own an Atom for years so I figured it was better to just deal with the classing issue. Although I am no national champion I generally hold my own within a few tenths of the national trophy winners when I have competed in the same class (STR in a S2000).

The purpose of this thread will be to share my experience in developing the car. With the lack of information out there about the Atom I hope that it will be helpful to others.



Event 1-3 Recap
Stock Alignment, Stock Shocks (JRIs), Stock Springs + Tenders, R888s 205/245

Starting Stock Tire Pressure: F:20 R:25
Starting JRI Shock Settings: F:50 R:30

To use all of the tire surface F:12 psi R: 20psi

Car generally has steady state understeer.
Entry understeer increases with decreases in rear shock relative to front.
Lower shock values result in a car that is slower to respond in transitions and slaloms.
Preferred shock settings as of event 3 F:68 R:35
R888’s never seem to get adequate heat to perform.
Car is all over the bump stops in cornering.

Plans for future events:
1. Add front camber, consider adding front toe out
2. Remove tender springs, adjust ride height to compensate for no tender springs
3. lower rear ride height to increase stability
4. cut or remove bumpstops

Video From Event 3:

[Richard Buckingham]

Why go with R888 other than you drive the car there and you are not allowed to change them? Are you permitted to use a tire like the Hoosier A6 (A7 now) or Hankook Z214 (C71 compound)? You'll never get enough heat in the fronts unless you can find space to do a couple of minutes of very aggressive braking prior to your run (steering left and right doesn't do it, braking does though) even the Autocross compounds take a while to heat up though. Buy some tire warmers maybe?

I've Autocrossed my Atom only twice, I have very little Autocross experience prior to that but I was the fastest car both times because the car is that good even when you're sliding around everywhere. That course looks very open compared to a lot of courses I've seen in other videos; the other place the Atom may catch you out is tight turns... the turning circle sucks, especially with understeer!

Looking forward to hearing and seeing more and let us know what class the SCCA put you in and what cars you're up against and the relative times. I think the Atom if it's classified fairly should do well (can you imagine another 2.4L 4-cylinder NA 230HP street car that's faster? Unless there's a Caterham out there you should be golden.)

[762]

The car is only on R888s because I just bought the car. Locally we have some of the fastest autocross cars in the nation (XP and DM National Champions) and arguably one of the best autocrossers in the nation (Brian Peters), the Atom will not be close to FTD until it is fully developed on A6's or A7's.

Fred Zust's XP Lotus Exige

[MadMaxAtom]

I use to autocross my Atom a lot (15-20 events per year for 3 years) until I discovered road racing. I took 6 FTDs against very tough fields.Most of those FTDs were on V710s,but 2 were on R25b Hoosiers,which I now use for road racing. Because solo is one lap, you need to get the softest tire you can as you will never build adequate heat in any tire to get it sticky (unless the course is 3 miles long) I would not recommend removing the bumpstops..and be careful you don't lower the car so much the suspension binds,especially the front. Stiffer springs and revalving is the answer. There is information out there..you just need to find it. Best of luck.

[atom ant]

Plans for future events:
1. Add front camber, consider adding front toe out
2. Remove tender springs, adjust ride height to compensate for no tender springs
3. lower rear ride height to increase stability
4. cut or remove bumpstops

No offense but I would scratch all 4 and simply change the wheels/tires and you would be able to stay with the best of them. 205/245 is not even close to ideal if you are going up against any track prepped cars. The lotus in your photo has rears that easily exceeds 300 and the front is probably 225 or more. The correct compound and front to rear tire width ratio relative to power will change your car. A6 295/225 or better yet 315/245 would turn the atom into a beast.

[762]

Bigger tires are one thing and will help the car. However, the car is all over the bump stops so stiffer springs/removing the tender springs will need to be done regardless of tires just so the car has enough shock travel.

It is possible with stiffer springs the car will not roll enough to require the camber, but once you start throwing A6's into the mix the grip will cause additional roll and most cars still require additional camber. However, the Atom is so light that it could change that strategy. Only testing and lap times against national caliber benchmarks will show what actually works.

On the upside I should have an event later this month and will get a few changes made in advance.

[Lane]

I agree with Atom Ant and I'd put tires near the top of the list. I use Hoosier A6 tires, with a 295 width in the rear which improves the car's balance and grip out back considerably. The overall difference in grip and handling over my prior tires (245 R888's in the rear) is difficult to believe.

[CosmosMpower]

I agree with Atom Ant and I'd put tires near the top of the list. I use Hoosier A6 tires, with a 295 width in the rear which improves the car's balance and grip out back considerably. The overall difference in grip and handling over my prior tires (245 R888's in the rear) is difficult to believe.

What size wheels do you use to support the 295? What size front tire do you use? Do you have to remove the fenders to run the wider wheels/tires?

[timcking]

762, I've got some experience autocrossing my Atom 3. Tires are definitely the first step. I'm using Hankook tires as described below (from an earlier post in 2012). I've got two seasons on them and they are still quite sticky. I've probably driven them about 2,500 miles since (both autocrossing and street). I'd estimate they still have 50%-75% of their original tread They don't heat cycle with the low temperatures in autocrossing. And, with such a light car, they don't suffer much wear.

The next step, based on my experience would be to properly adjust your brake bias. This is critical to maximize your braking. Mine was off considerably from the factory.

Depending on how far you want to go, you could adjust the camber and ride heights to the factory-recommended track settings. These work quite well on my car. You then could cross-balance the suspension and then work on optimizing the shock settings and tire pressures.

After this, my biggest gains have been aclimating to the Atom. Unlike all of my previous experience autocrossing, the Atom is a "point-and-shoot" car. The fastest approach is to minimize distance traveled, unlike lower-powered cars where you want to maintain your speed.

Autocrossing the Atom is definitely fun.

TIRE INFO:

Here's an FYI for anyone considering using Hankook Ventus C71 compound (soft) DOT-legal racing tires. The tire sizes I chose were: 225R45/15 front and 245R45/16 rear - basically one-size up frt/rear from the U.S. Atom 3 OEM tire sizes. I drive my Atom on the street with an occasional autoX. I've probably run four autoXs on the OEM Toyos as a baseline and have about 5,500 miles on them on the street.

With the 3/4" larger diameter Hankook rear tire, I readjusted the ride height as per the Atom 3 manual for the street settings. I also adjusted the camber to -1 degree front and -1.6 degree rear (Atom 3 track settings), with 0.0 toe-in in the front. The rear toe-in was not changed.

The change in the front toe-in was to correct an issue I had driving my car on the street with the OEM toe-out setting; where upon initiating a turn, the steering wheel would not self-center. With the 0.0 toe-in, this issue has been resolved.

With the first autoX on the new Hankooks, the improved grip was dramatic over the Toyo Proxes R888 tires. However, I wasn't sure I was getting the full potential from the tires.

As a result, this weekend I took an infrared thermometer to an autoX. The following were my tire temps for the four runs, with the three temperature readings across the left/center/right portions of each tire. The ambient air temp was 70 degrees. This was a relatively high speed course run in third gear with a center section of the course comprising of two laps (predominantly left turns). Typical car times ranged from the lower 50 secs (modified autoX special) to sedans in the upper 50 to lower 60 sec range.


TIRE TEMPS

LF: (17 psi hot - run 4) RF: (17 psi)
1. 84/98/106 1. 99/110/114
2. 89/96/105 2. 98/103/109
3. 86/94/102 3. 106/108/115
4. 100/113/122 4. 110/112/116

LR: (20.5 psi) RR: (21.0 psi)
1. 91/95/103 1. 114/120/123
2. 97/103/112 2. 102/107/112
3. 101/108/115 3. 108/111/116
4. 108/118/126 4. 117/123/129

Since most the turns on the course were to the left (particularly two large sweepers at each end of the center section of the course), you see the higher temps on the tires on the right side of the car, and, the temps on each tire being higher on the right side of each tread section.

Basically there are two primary factors in tuning the suspension to obtain equal temps across each tire. One is the camber and the other the tire pressure.

Looking at the tires on the right side, initially I thought this indicated a need for more negative camber. However, I then realized that (based on the temps) the tires on the left side were contributing almost equally to the cornering as those on the right side. This really surprised me. It's a credit to the Atom design and undoubtedly why there are no sway-bars on the car. Of course, the driver's weight on the left side helps with this as well.

What this told me, is that increasing the negative camber would improve the temperatures on one side, but it would have the opposite effect on the tires on the other side of the car. In this case, more negative camber would improve the tire temp distributions on the right side of the car. But, it would have the opposite effect on those on the left. And, based on the temps observed on the tires, both sides of the car's tires are essentially contributing an equal amount to the turning traction. Thus, that direction would result in no net gain.

Looking at tire pressures, it looks like these are pretty close to optimum. With the Toyos, I used 13 psi front/17 psi rear cold, which seemed to work well with the OEM suspension settings. I tried those initially with the Hankooks and it didn't feel right. I went to 15 psi front/20 psi rear, and the rear didn't feel quite right. For this autoX I went to 15 psi front/18 rear (cold) and it seems to be very close. It also felt better as well.

With these suspension and Hankook tire settings, the car is excellent on the street. It rides noticeably less harsh since the Hankook sidewalls seem to have slightly more give. Surprisingly, the steering with the wider tires is also less sensitive to pavement irregularities as well. I can now drive one-handed comfortably on smooth pavement.

Of course, that's not really what this tire change was all about. The C71 compound is an excellent choice with the inherently light weight of the Atom (my car is about 1,245 lbs with a 1/2 tank of fuel). The improved grip is phenomenal. I'm still exploring the full braking potential with the cars stopping ability.

I've found my Atom is well-balanced and very predictable in steady-state cornering. Additionally, it's really predictable and well controlled in accelerating out of turns. You can control the rear slip angle with the throttle pretty easily. And, it's also very stable braking. Where it's a little challenging is in quick transitions, particularly where you'd apply the brakes or let off the throttle at the same time. I think this is inherent in the 40/60 frt/rear weight distribution combined with the compression-braking of the K20 engine. Focusing on smoother transitions/braking seems to greatly reduce this concern.

I hope this is helpful for anyone considering or trying the Hankook DOT tires.

Tim

[762]

New alignment, corner balance, and removal of tender springs is finally happening this week.

The process of researching everything has netted some useful information I thought I should post:

Atom 3 Specs from Sector 111:
Toe F: 2mm in total R: 3mm in total
Ride/Height: F: 120mm R: 115
Camber: F: -1 R: -1.25

SRA Specs from Sector 111:
Toe F: 2mm in total R: 4mm in total
Ride/Height: F: 115mm R: 110
Camber: F: -2 R: -1

Front Spring on Stock JRI: HRF 180 - 60 - 050 [Free Length (mm) - Inner Diameter (mm) - Spring Rate (N/mm)]050 N/mm = 285 lbs/in

Rear Spring on Stock JRI: HRF 180 - 60 - 060 [Free Length (mm) - Inner Diameter (mm) - Spring Rate (N/mm)]
060 N/mm = 342 lbs/in

Stock Spec Race Atom Spring Rates from Sector 111: Front 400 lbs/in & Rear 500 lbs/in

Alignment on my Stock Atom:


Corner Weights:
Stock Atom w/windshield, roll bar, stock exhaust, and 17in wheels