Starting to get the hang of controlling the back end-

[Vic]

I've had mine for almost 10,000 miles now.
Early on, I learned how to control the spin during the launch. But thats just straight line stuff- Ok, well, almost straight line stuff....

Now I am getting bigger cayunes, and starting to find the limit in curves, when under a constant power, with a steady input to the gas pedal. I was always a little timid to test this limit while under power, because torque oversteer is legendary, as we all know... And then there are those decreasing radius turns to deal with. Better not romp, unless you are real familiar with the "track".

The last couple of times I experienced this, when the back end started to go out I could not "see" it as much as I could "feel" it. My instinct was to let off the gas pedal gently. (There was no possibility of turning wider, or letting it drift outward)

I have always heard on this board that there is no advance notice that the back end is going out. Maybe Corvette owners were saying this, I don't remember.

But my impression is not that negative. I found that I COULD feel the back end going out, as oversteer began, and also I found that I could control it, by feathering the throttle. Sounds like I am so "cocky-dent", doesn't it? No, not really. The first time, it was actually a little unerving, and for a moment my heart lept just a little. So I'm just an average joe, ok?

My Corvette was much easier to drive at the limit. (Becasue the limit was so much lower!) Ha-ha. Just kidding.

Seriously though, my 89 Vette, (with 100 extra ponies), tended to understeer in the most gradual, friendly manner. It gave you all kinds of advance notice to make corrections.

I have heard here that 34 psi front, and 32 rear will reduce the oversteer, and maybe induce an easier-to-control understeer?

Anyone else wanna share their high speed driving impressions, or techniques?

(Reposted from a less appropriate forum)

 

[SoCal Craig]

Vic, I can see you really want to go to the track bad

At speed on smooth pavement the Viper handles quite well (at least my Gen 2 does). It understeers under power and oversteers upon lift-throttle (all cars do). In slower turns you can steer with power oversteer if necessary. I think what makes it difficult is that the Viper has such high limits (big power, big grip) as compared to many other cars. This makes approaching the limits more of a challenge.

Never "romp" on the throttle. Try to be smooth on and off. Remember that you have 4 small contact patches and it's all about weight transfer and being smmmmooooooooothhhhhh. Otherwise it is spiiiiiiinnnnnnnn.

More front tire pressure (34/32) would reduce understeer, which is good because the stock Viper understeers a bit too much. Same as my Porsche and BMW.

 

[Vic]

Well, now I'm confused, cause there are those who say it tends to oversteer. One guy even got some new toe links or something in the back, to reduce the oversteer.

Instead of hosting a battle of opinions, I'll just take everyone's points under consideration, and try it for myself on the track.

Oh yeah, huh, I forgot that lift-throttle can induce oversteer. Smoothness rules the day.

About tire pressures, I thought a higher pressure on the front would reduce the grip, and cause the front end to understeer. You have stated it the other way around. Is there something I am missing here?

From my small amount of personal experience I can draw from, it seems that higher pressures induce slippage, lower pressures increase grip. Consider when you are launching the car. If you have all your tires pumped up to the maximum shown on the sidewall, you will easily spin the tread right off them, and not get a good launch. Its a little like ice skating. Setting them lower, like 32, gives a much better launch.

So, when turning at high speed, wouldn't a relatively higher front tire pressure allow the front end to slide before the rear, thereby inducing understeer?

And let me define what I mean by understeer. I mean it as a "push", as they say in NASCAR, which means that the front wheels are sliding so much, that even though they are turned, the front end of the car does not go exactly in the direction the steering wheel is pointed. I'm sure this is what you mean. So then, if relatively higher pressures cause a tire to slip more, how could more front pressure reduce oversteer?

<snip>
More front tire pressure (34/32) would reduce understeer, which is good because the stock Viper understeers a bit too much. Same as my Porsche and BMW.
<unsnip>

I don't understand how that can be true. Am I missing something?

 

[SoCal Craig]

Most street cars are setup to understeer (some more then others) as it is safer in the eyes of the manufacturer. Take your car on a skidpad and you'll see what I mean.

Most racers dislike it when their car understeers or "pushes" too much as it makes it difficult to control the direction of the car with the throttle. Most want their car to understeer slightly at intial turn in and after it takes a set, yet they want it to gradually transition to oversteer upon lifting from the throttle.

Remember it's all about weight transfer front to back and side to side. On the throttle more weight is on the rear tires (meaning more grip) and the car understeers (assuming you didn't mash the throttle and spin the rears). Lift-throttle or trail brake and you transfer weight to the fronts thereby increasing their grip while lightening the rear grip. On most cars this transitions to oversteer (again some more than others). A skidpad is the best place to learn this.

Gradual and smooth transitions are desirable versus being abrubt. Some cars (and drivers) are good at this and some not. 911's are famous for their snap oversteer if you lift or brake at the limit in a turn. I remember a POC instructor writing on the classroom chalkboard two large words "DON'T LIFT." The same creed applies to most other cars as well.

I'm not a suspension or tire engineer, but the way I understand the tire pressure thing is as follows:

More pressure increases the sidewall stiffness and reduces flex. This reduces a thing called "slip angle" which as I understand it is the angle between the direction the tire is pointed and the actual direction the tire takes due to sidewall flex under load. Imagine if you had very little air in the tire. As you made a turn the side G's would make the sidewalls flex alot. The wheel/tire would be pointed where you want to go, but as the tire makes the turn the tire "crabwalks" sideways because of the sidewall flex. The greater the sidewall flex the greater the "crabwalk" or slip-angle as it may be. Therefore, the tire is actually carving a wider arc then it would if it followed the angle it is actually pointed.

Have our friend John Dearing (SspeedRacer) e-mail you his photo from Willow Springs (Turn 4 I think). It's amazing how much his front sidewalls are flexing in that photo. Almost looks like they are comming off the rim!

I assume there probably is a limit to how much you can control understeer/oversteer with tire pressure. If you go way overboard on pressure I believe you take the tire out of it's optimum parameters to do it's job properly.

There's much more that can be said about this subject and I'm sure I'm not the one who knows this stuff best. I suggest you read some books, or search the internet, or ask someone else smarter than I.

Good luck

 

[Vic]

Dayyyuuuuum! I had no idea there was so much to it! Seems like every subject has a depth to it, at its extremes.

Thank you very much for taking the time to relate this stuff to me. I appreciate the fact that you took my question seriously.

I'm gonna save your post for further consideration.

Wow....

 

[treynor]

Craig is correct, it's pretty involved. To add a further wrinkle, tire pressure affects the temperature your tire achieves at a given level of use. A higher pressure results in lower tread temperatures, and typically less traction assuming the tire is below its optimum temperature.

 

[Vic]

Hmmm... Thats heavy..

 

[SoCal Rebell]

Ok Vic, time to put some of this theory into practice. Myself, John Dearing, JoeB01 and I think a couple of other Vipers are running Willow Springs on Friday July 12th, for info and sign up go to www.speedventures.net Take a sick day come out to the track only $120 for the day and learn!!!!!!

 

[Vic]

Right on, Ron.

My responsibilities change from hour to hour, so its hard to schedule.

But if nothing really pressing has come up, I can get away.

I look forward to the day when my business runs itself, until then I am always "on duty".

No rest for the wicked!

 

[SoCal Craig]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by SoCal Rebell:
Ok Vic, time to put some of this theory into practice. Myself, John Dearing, JoeB01 and I think a couple of other Vipers are running Willow Springs on Friday July 12th, for info and sign up go to www.speedventures.net Take a sick day come out to the track only $120 for the day and learn!!!!!!

<HR></BLOCKQUOTE>

Ron, how come you didn't invite meeee??? Am I getting too fast for you

Seriously, I'd like to go, but I gotta check with my work schedule and family (Sister and kids in town).