Hankook Ventus C71 DOT racing tires/suspension settings/tire temps

[timcking]

Here's an FYI for anyone considering using the new 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. The only downside I've experienced is I have to vacuum out the tub after each drive. Oh well.

Tim

[bolus]

Thanks, looks like it comes in 275/40ZR-17 for those of us with the wider rears.

[MadMaxAtom]

Imagine how sticky they would be up around 160-180 degrees! The curse of the Atom is not being able to load a DOT tire enough to really get it in the tires sweet spot,especially on cool days at autocross. They just don't get enough time/load to heat up adequately. Occasionally,I would do a back to back run,and that run would always be my quickest time and sometimes FTD!

"Focusing on smoother transitions/braking seems to greatly reduce this concern."

It sounds like you are becoming comfortable with the Atom. Good!

"The only downside I've experienced is I have to vacuum out the tub after each drive."

After a while,you will just pick out the big rocks and dead things....

"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."

I concur. Driving with wider tires all around is actually much less twitchy/nervous feeling than narrow tires (relative diameters being close)...and I was just as surprised.

Atoms like wide sticky hot tires..

[Karl]

Have you driven on A6s? If so, how do these Hankooks compare?

[timcking]

Have you driven on A6s? If so, how do these Hankooks compare?

I can't help you there.

Tim

[Richard Buckingham]

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.

Looking at tire pressures, it looks like these are pretty close to optimum.

I'm very new to this kind of thing, it's fascinating to me and I'd like to learn more... so I have lots of these kinds of questions:

• What guidelines can I follow to know what indicates a 'good' temperature spread across all the tires and how to interpret good/acceptable/poor setup from that info?
• How do I know to add more or less toe or camber and by how much on each corner?
• What determines ideal tire pressures?

Is there a good reference document somewhere or other calculations you use personally to determine what adjustments to make?

I have an SRA which I've run for ~600 track miles so far at various tracks and various weather conditions (from 65F to 110F!) I still have the factory Hoosier R6 on there and I've never once adjusted toe or camber settings from the factory defaults (though I have switched to softer springs) I always run at 13/17 PSI and have never tried adjusting away from that either. Prior to the SRA I tracked stock street cars only so playing with my setup was never a consideration for me.

I'm also still learning to drive the car to it's max too so I'm not looking for faster times necessarily and I find it difficult to draw conclusions about setup based purely on feel (I don't have enough hours behind the wheel to do that) I just want to learn more about perfecting setup based on the track layout and conditions and whatever other info I can gather. FYI: I am considering the Hankook soft compounds as a replacement tire too.

[timcking]

Richard, I think you're right to focus on getting familiar with the car and not complicate it with making adjustments at this point.

That said though, there's a couple of adjustments based on my experience that you might want to consider. First, when I received my 2009 Atom 3, the OEM setting of the brake-bias was so far off it really needed to be adjusted. I had to adjust the brake-balance bar a full 2-1/2 turns to get it right. And, with good tires, it really makes a difference. Note, this won't really change the handling, only improve the braking so that the rear wheels contribute fully to stopping the car. On mine, it was a huge improvement to utilize the full braking potential of the car.

Another adjustment to consider is to corner-balance your car. The Atom is such a light car combined with the asymmetrical driver position, it would be useful. This is fairly simple and to learn more just Google 'corner-balance' and you'll find everything you need to know. You'll need a way to measure the weight at each wheel. I used a heavy-duty bathroom scale and blocks the same thickness of the scale for the other three wheels. You'll also need to place 'ballast' of the same weight as the driver in the driver's seat. You'll be adjusting the shock spring perches to achieve the desired wheel weights (note this also adjusts the ride height, so you'll have to measure this as well).

Here's some answers to your questions. If you use the internet, you can find a lot of technical discussions on these topics.

1. The objective with optimizing cornering is to maximize the full use of the tires across the full tread width. To do this, the goal is to equalize the tire temperatures across the tread so that you are getting full use of the tires.

2. Adjusting the camber varies the tire temperatures across the tread. The camber adjusts the angle of the tire on the road. By optimizing this, you can equalize the tire temperatures as best as possible.

3. The tire pressure is another adjustment that influences tire temperatures. This adjusts the tire's center temperature relative to the outside/inside temps.

For example, if you have three tire temperatures across the tire (outside/center/inside) the goal is to have them all equal. This is difficult to achieve, but the closer you can get them, the better. Now, if you have the outside and inside close to equal, and the center is different, that's a pressure issue. Normally increasing pressure will raise the tire's center temperature, and, conversely lowering the pressure will lower the temp.

However, if you have three temperatures that all are different, the center should be the midpoint of the two others (outside/inside). If it's not close, it's a pressure issue. Camber is the adjustment for equalizing outside/inside temperatures.

Of course, there are other adjustments with the toe-in front and rear, along with the shocks, and with many cars caster and anti-roll bars.

I've found the recommendations in the manual are excellent nominal settings. Other than the brake-bias and corner-balance adjustments, I'd concentrate on driving and gaining experience before getting into the fine tuning. Once you're ready to go farther, a good starting point would be to confirm the settings are those specified in the manual. Then you could begin to build a database where you log all the tire temps, pressures, settings, and track conditions. Prior to that, you really don't want to start messing with things other than the two I mentioned.

Hope that helped.

Tim

I'm very new to this kind of thing, it's fascinating to me and I'd like to learn more... so I have lots of these kinds of questions:

• What guidelines can I follow to know what indicates a 'good' temperature spread across all the tires and how to interpret good/acceptable/poor setup from that info?
• How do I know to add more or less toe or camber and by how much on each corner?
• What determines ideal tire pressures?

Is there a good reference document somewhere or other calculations you use personally to determine what adjustments to make?

I have an SRA which I've run for ~600 track miles so far at various tracks and various weather conditions (from 65F to 110F!) I still have the factory Hoosier R6 on there and I've never once adjusted toe or camber settings from the factory defaults (though I have switched to softer springs) I always run at 13/17 PSI and have never tried adjusting away from that either. Prior to the SRA I tracked stock street cars only so playing with my setup was never a consideration for me.

I'm also still learning to drive the car to it's max too so I'm not looking for faster times necessarily and I find it difficult to draw conclusions about setup based purely on feel (I don't have enough hours behind the wheel to do that) I just want to learn more about perfecting setup based on the track layout and conditions and whatever other info I can gather. FYI: I am considering the Hankook soft compounds as a replacement tire too.