Remedying understeer

[romaier]

Hi all,

I've spent much time perusing this forum but never really posted. Now, however, I have a question regarding an understeering problem.

I'm designing the front suspension of an off-road Baja SAE vehicle (in conjunction with the rear end team) for a design project at Uni. The lastest generations have had a nasty understeer problem, particularly at turn in. Some of the guys believe that by adding more roll in the front relative to the rear by means of a lower roll center will help this.

I, however, am not sold on this. I am a novice though, but I have been reading up on books such as "Race Car Vehicle Dynamics" by Miliken & Miliken in order to get a better feel for this. Am I misunderstanding something here? The old car has a wheel base of 64" with the new model shortened to 61". Also, the track is 58" being narrowed to 56". What would you all recommend as a remedy for this understeer problem?

Thanks

 

[Tyler33]

Wait a minute, wouldn't lowering the front roll center cause the front to roll more (assuming its below the CG)?

 

[GregLocock]

yes. That's why you should raise it.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Tyler33]

We want more weight on the outside front tire so why would we want it to roll less up front? It seems that we want the front to roll more and the rear to roll less so weight is transferred forward and toward the outside tire.

Our problem has been that the front tires are not biting enough because there has not been enough force on them.

 

[patprimmer]

The forces creating the roll act through the CG no matter what else you do.

It is the forces that resist roll that act through the suspension.

By having more roll resistance at the front and less at the rear, the front outside wheel takes more of the load.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NormPeterson]

"We want more weight on the outside front tire so why would we want it to roll less up front?"

Roll is the observable result of lateral load transfer onto the elastic suspension elements (springs, sta-bars, tires, sometimes shocks/dampers), not the cause of it.

In situations like this where the load has more than one path (the two suspensions & their tires), it tends to go where the greater stiffness is, in proportion to that stiffness vs the stiffness at the other end. Raising the front RC effectively stiffens the front without affecting the rear . . .


Norm

 

[bentwings1]

I think that an adjustable anti roll bar would be good on the front only and adjusting the suspension so that as the front suspension/frame encounters roll the outside wheel negative camber increases and the inside wheel goes toward positive camber. Say 3-5 degrees. We have noticed that the front tires definitely roll in or out on the rims on the entry to the corners. We only run about 10 pounds so it is no wonder. You should see them at 100 mph. We have not added the roll bar yet but we are considering it next summer.

Our buggy has so much power coming out of the turns that it nearly carries the tires so it is important to get the line set up early in the corner. Basically just steer with the throttle. If you make a mistake you are off the trail in an instant....ask the 6 inch tree I hit at 60+.

99 Dodge CTD dually.

 

[patprimmer]

When I was involved in racing a class one buggy, we ran about 40 PSI tyre pressure despite what the tyre sponsor said.

Those that took his advice of 18 to 20 psi had recurring tyre and rim problems as well as slower lap times.

There is a lot more to cornering off road than tyre temperature and co-efficient of friction.

You can dampen the rebound of a spring, but how do you dampen the rebound of a tyre sidewall.

Also if you hit an embankment hard with one wheel as you are turning tyre compliance has a lot more effect than does the tyre rubber being at optimum temperature.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tyler33]

Ok so I guess we had that one a little backwards. Intuitively, it seems that if your front outside spring is compressed further, there is more load on that wheel.

Most of my experience says that the rear of the car should have high roll stiffness and the front low: Many teams run a rear sway bar and I've never seen a team with one on the front, teams with solid rear axles and swingarm supension (like sport atv's) generally turn well, and our car seemed to turn better with softer springs on the front.

 

[JHarshbarger]

Baja SAE is pretty much different than any other type of racing. It's different from typical off-road racing because the vehicles are very underpowered and typically can not break the rear tires in a corner from torque alone. The lack in power also makes these cars dive significantly over jumps.

What most teams do to allow the rear to slide and take the corners faster is to overload the rear axle to point that it breaks loose. With a high rear roll stiffness, the rear tires are overloaded and with a little bit of throttle they break loose. Our team runs a swing arm (very high roll stiffness), some teams with an independent rear run ARBs, and some teams just try to through off the rear traction with high tire pressures and positive camber.

If you start messing with spring rates, make sure you consider jumps. A soft front spring rate can cause the car to dive over a jump even more than most of the Bajas already do. In order to your can jumping decently, you'll need to look into increasing the relative roll stiffness of the rear apart from the springs.

_ _
Joel Harshbarger
USF Motorsports