Lifting an Inside Front Wheel

[johnc]

From the latest Mark Ortiz newsletter:

 

IS IT BAD TO LIFT A WHEEL?

 

After watching the new NASCAR Car Of Tomorrow (COT) run on some short tracks and then at the Sonoma road race it certainly appears that the NASCAR teams have some real development to do. The Sonoma road race certainly illustrated what I would consider a real problem with the COT and that is the very radical lifting of the inside front wheel during heavy cornering.

 

The three wheeling was not incurred by going over curbs, it was the classic three wheeling very similar to a dirt car with lots of cross weight and plenty of traction and torque. The lifting of the inside wheel was especially evident in the 8-9 turn complex where the car was carrying good speed and especially coming out of turn nine, which is a flat sweeping left hander, many cars would carry the inside front, left wheel, to the point that they had to brake for turn 10. Some cars would carry the wheel 4-6 inches off the ground. Of course once they braked for 10 the left wheel, which is now going to be the outside wheel in 10, would regain contact with the track and of course it is not turning so there would be a pretty good "unsettling" of the car before it was set into turn 10. I think that the drivers are using third gear for this series of turns so there is pretty good torque available as they exit turn 9. Having run sports cars at Sonoma back in the '80s, I remember that we did not do anything related to side to side weight distribution and we ran our cars with pretty much 50/50 side to side weight. Although Sonoma is a clockwise track and is predominantly right hand turns we found that trying to bias weight to the right side of the car would adversely affect the car in the left corners and we ran the "carousel" at that time which was a very long, high speed, down hill sweeping left turn onto a good straight so the right side weight was not the thing to have in this corner. NASCAR does not run the carousel so it is highly possible that right hand weight would be a advantage in the track configuration that NASCAR runs. I am not sure if it is legal to run lots of inside weight on the COT.

 

At the short tracks that they have been running these cars on, you can also see the cars lift the inside front wheel as the power is rolled on exiting a turn, which I am sure is very much assisted by cross weight jacking trying to keep the inside rear planted for a good drive off the corner, but at Sonoma the cars could and would carry the inside wheel in both right and left hand corners.

 

I have always believed that poor chassis stiffness in torsion is a major contributor to "three wheeling" and looking at the COT chassis it certainly doesn't appear to be well braced for torsional loads. What are your thoughts on the COT as to its attribute of going onto three wheels while corning. Causes, and possible fixes???

 

 

It is not necessarily bad for a car to lift the inside undriven wheel.

 

Ideally, we would like to use all four tires equally at all times. This would involve not only having all four wheels driven, but also having the c.g. at ground level. That's impossible, of course.

 

With only two wheels driven, and the inevitability of lateral load transfer, we have to make some compromises. The nature of these compromises varies depending on the rules, the track, and our overall design and setup strategy.

 

If the rules impose the same tire size limit on all four wheels, cornering speed is moderate, and aerodynamic lift or downforce is negligible, we get best steady-state cornering with around 50% static rear weight, and similar overall roll resistance at both ends of the car. In this situation, we should not lift a wheel. To win a moderate-speed skidpad competition, we might want such a setup.

 

However, a road race is not a skidpad test. On most road courses, most of the turns are of modest duration, and are separated by significant straightaways. In this situation, when cars are nearly equal in power, the race is won on the straights, but the straights are won in the turns. That is, it becomes very important to have good turn entry and exit speed, and late braking points. The brakes also have to last through the race, which is a big factor in Cup cars on a road course. To make the car brake as well as possible, and put power down as well as possible, we need as much static rear percentage as possible. This costs us some steady-state lateral acceleration, but it gains us longitudinal acceleration, both forward and rearward. It also saves the front brakes, which are normally the ones that give out first, because optimum rearward acceleration will be achieved with less front brake bias than we would otherwise use.

 

To make a tail-heavy car corner neutrally with equal-size tires at both ends, the front end has to absorb the greater part of the lateral load transfer. When such a car is accelerating hard both laterally and forward, it my very well lift the inside front wheel. This does not necessarily mean the setup is bad. It means that some lateral acceleration has been sacrificed to increase forward acceleration.

 

The COT has front and rear clips that are designed to be more deformable in a crash than was the case in the old cars. That probably does cost some torsional rigidity as well. However, if anything, low torsional rigidity makes a car less prone to wheel lifting, at least for a given suspension setup. This is largely academic, because with a less rigid frame we will normally run more front roll resistance; we will have to, to get the same wheel loads. If we are comparing flexible versus stiff frames, with equal dynamic wheel loads, there will be little or no difference in tendency to lift a wheel.

 

One thing that does affect the tendency to lift a wheel is the c.g. height, and here there is a substantial difference between the old car and the COT. I hear that the COT has a center of gravity fully two inches higher than the old car. I have a hard time understanding where that much difference could come from, but certainly the roof is higher by that much, and I understand the frame rails are at least somewhat higher.

 

Once the front end reaches 100% lateral load transfer, any further increase in roll moment can only be reacted at the rear. Consequently, the car has much less angular roll resistance beyond the point of wheel lift, and any further increase in lateral acceleration produces a relatively large increase in roll angle, with a correspondingly greater amount of daylight visible under the inside front tire. It often becomes quite difficult to carry the wheel just a little.

 

I understand that at Sonoma, a car spends about three times as many seconds accelerating rightward as it does accelerating leftward. That would mean a right-heavy weight bias would be advantageous. However, I doubt that it's legal.

 

It's not easy for people not on a team to get NASCAR rule books. They are normally only sent out to people getting a NASCAR license, and the rules are subject to revision and interpretation in mid-season. I do know a person who works on a Busch team that started out as a Cup team, and he has the Cup rules as they existed at the beginning of the season. At that time, there was a minimum left side weight for the old cars on clockwise road courses, but none for the COT's. As of the start of the season, the old cars had the same minimum weight for the left side on road courses as for the right side on ovals: 1625 lb., without driver, out of a minimum total of 3400, without driver. COT's have to have 1650 right side out of 3400, and there is no rule specified for road courses.

 

Evidently, the plan at that point was to have the old cars run the road races, and that got changed. With 200 lb. of driver weight, distributed 50 lb. right/150 left, an old car would have 49.3% left for a road course. If the COT is required to have 1650 left for a road course, it would be exactly 50% left with driver, based on the same assumption for driver weight. Actually, the driver sits slightly closer to center in the COT, but there wouldn't be any possibility to make the car markedly right-heavy.

 

One thing that would make the car carry the inside front wheel more readily, and higher, in left turns is torque wedge: the effect of driveshaft torque on the chassis. This tries to roll the car to the right, unload the left front and right rear, and load the right front and left rear.

 

I found the above to be true when tuning the suspension on my 240Z:

 

[DHale_510]

Two thoughtds here;

There will always be one corner that unloads first. With a rear wheel drive car you want it to be a front. Never seen a well set up 510 or Z do it any other way.

Sears Point has 7 major right hand turns, and 2 to 4 depending upon the NASCARNESS of the weekend. It is common to set the carwith a staggered crossweight bias to give rights an advantage over lefts. This is with spring jacking, not weight redistribution. You bet the NASCAR guys understand weight jacking. Lifting the inside wheel is a consequence.

[proxlamus©]

i love watchn FWD cars make a turn.. the inside REAR wheel lifts off. makes me giggle

[260DET]

My summary of that newsletter would be - if getting rear wheel traction is more important than mid corner speed, then having a suspension setup that results in the occasional lifting of an inside front is the price to be paid.

 

Yeh?

 

Note the term 'optimum rearward acceleration' used in the newsletter

[johnc]

I guess it comes down to power and/or aero.

 

A Spec Racer Ford with all of 110hp should have all 4 wheels on the ground. A 240Z with 350hp can lift an inside front wheel and not have it affect lap times. If your car uses a lot of aero then lifting any wheel or having large changes in pitch angle is bad.

[tube80z]

A Spec Racer Ford with all of 110hp should have all 4 wheels on the ground. A 240Z with 350hp can lift an inside front wheel and not have it affect lap times. If your car uses a lot of aero then lifting any wheel or having large changes in pitch angle is bad.

 

This will be an interesting test. My old car had about 200 WHP and used to lift the inside tire similar to the picture you posted John. When I started running with reduced droop and a lot stiffer springs it no longer did this.

 

The video I saw of the COT on a road course was amazing at how high and long it lifted the front wheels. I wonder if the rules for the road course specify spring rates. To me it looks like the car is way too soft.

 

Cary

[DHale_510]

When I finally got my 510 to stop lifting the inside front wheel first, it lifted BOTH inside wheels at the same time. That was not better. Of course, a 510 has the aerodynamics of a brick, so I know nothing of that downforce stuff, but a Z has about the same drag number and probably more lift.....

[WizardBlack]

i love watchn FWD cars make a turn.. the inside REAR wheel lifts off. makes me giggle

 

AWD cars do, too. But then I am sure with all the wierd drivetrain setups on AWD some do it like RWD as well.

[S30 SPL]

My WRX lifts the inside rear at entry and inside front at exit...looks funny.

[katman]

I got news for you, it looks funny standing still.

 

Sorry, couldn't resist. Spent a track day with a pack of WRX's just recently, from a "pretty much stock" one with 385hp to a One Lap of America competitor with over 600. Awesome cars.

[S30 SPL]

Wagons rule...

Especially on 235mm R comps.

[WizardBlack]

Yep, Subies are cool but they still get eaten up by EVO's.

Owned both and tracked both.

That is, until they break. LOL.

 

BACK on track: Isn't a wheel lift a sign of slightly too soft suspension for the compound and the specific track?

 

Mark

[S30 SPL]

Inside front wheel lift on the exit of a turn could be a number of different things...for me, I have found this to be caused by

1. too soft of rear sway bar for the spring rates used

2. too stiff of front sway bar for the spring rates used

3. insufficient droop (downward) suspension travel

4. excessive chassis flex (monster torque, yeah right)

 

remedies that i found to work:

1. turn slower (not going to happen)

2. find a different line so the car is more level at exit

3. stiffer rear sway

4. softer front sway

5. drift the exit so the nose is planted and the car is pointed the other direction

 

as much as the EVO is a nice car, the WRX is the only one that comes in a wagon...its much more fun to eat an EVO in a wagon.

[260DET]

Pic of another wheel lifter. This Z is set up soft to run as a targa car, springs are in the low 200's, ARB's are light too. Considering all that its not sitting too bad for a car that is being pushed hard, light and stiff body helps.

[WizardBlack]

as much as the EVO is a nice car, the WRX is the only one that comes in a wagon...its much more fun to eat an EVO in a wagon.

 

LOL, so true. Even worse to be in a coupe and get pounded by a wagon. I am glad they brought it back.

[m1ghtymaxXx]

Here's my bone stock zx body rolling around the track. Definately not not much contact patch on that inside front

 

 

 

Facial expressions are priceless too!

[johnc]

Stumbled across this one and thought it needed a bump.

[thehelix112]

 

Think thats the image you tried to link in the initial article John?

 

Dave

[johnc]

Yup. The car was fairly soft back then (300 lb.in. front and 325 lb. in. rear).

[260DET]

Yes it is worth a bump, needs more discussion and less digression

 

Pic of the ZX doing its wheel lifting thing but thats mostly due to the track layout there. Stiff front ARB, no rear ARB though.