Noticable wheel lift during corner exit/acceleration

[Anvit Garg]

Hello:

I have built my pretend FSAE vehicle in CarSim, and am noticing that the inside rear tire and sometimes the front tire lifts off during corner exit. The data shows that the vehicle is recovering from braking, so it still has a negative (small) longitudinal acceleration, and a very high lateral acceleration.

The vehicle was design for approximately 1.6 deg/g with front and rear ARB, and the front frequency is 1.99 Hz and the rear is 1.90 Hz.

The front auxiliary moment is only 1.67 N-m/deg and the rear is a 22 N-m/degree.

I first suspected that the rear roll bar was too stiff, and thus was removing all of the inner wheel's vertical load and transferring it to the front outside wheel. As a result, the inside front wheel followed suit and transferred its vertical load to the rear outside wheel.

So I tried first increasing the front ARB, then decreasing the rear, and several combinations of the two, and I had minimal to no reduction in the inside rear and some of the inside outer wheel from lifting.

Next I suspected that the front springs were too soft, so I decreased the rear, still nothing, then I swapped the front and rear (stiffer on front, and softer on rear), still nothing.

I then returned the rear springs to 225 lbf/in, and then increased the front spring rate to 330 lbf/in from 200 lbf/in. I was then able to see a significant decrease in wheel lift.

Can someone please suggest what other causes that I can not understand?

I am suspecting the rear roll center to be the cause of the problem as well. The front roll center is 1.9 inches off the ground at design height and the rear is approximately the same. I believe in real life this is very rare, and it is probably suggested to have a higher rear roll center height.

[Anvit Garg]

Hello:

I have built my pretend FSAE vehicle in CarSim, and am noticing that the inside rear tire and sometimes the front tire lifts off during corner exit. The data shows that the vehicle is recovering from braking, so it still has a negative (small) longitudinal acceleration, and a very high lateral acceleration.

The vehicle was design for approximately 1.6 deg/g with front and rear ARB, and the front frequency is 1.99 Hz and the rear is 1.90 Hz.

The front auxiliary moment is only 1.67 N-m/deg and the rear is a 22 N-m/degree.

I first suspected that the rear roll bar was too stiff, and thus was removing all of the inner wheel's vertical load and transferring it to the front outside wheel. As a result, the inside front wheel followed suit and transferred its vertical load to the rear outside wheel.

So I tried first increasing the front ARB, then decreasing the rear, and several combinations of the two, and I had minimal to no reduction in the inside rear and some of the inside outer wheel from lifting.

Next I suspected that the front springs were too soft, so I decreased the rear, still nothing, then I swapped the front and rear (stiffer on front, and softer on rear), still nothing.

I then returned the rear springs to 225 lbf/in, and then increased the front spring rate to 330 lbf/in from 200 lbf/in. I was then able to see a significant decrease in wheel lift.

Can someone please suggest what other causes that I can not understand?

I am suspecting the rear roll center to be the cause of the problem as well. The front roll center is 1.9 inches off the ground at design height and the rear is approximately the same. I believe in real life this is very rare, and it is probably suggested to have a higher rear roll center height.

[Mikey Antonakakis]

So I'm not a suspension guy, but even though it's not an easy fix in real life, maybe your center of mass is too high... or wheelbase too narrow. Fixing either one will fix your wheel lift issue. How many lateral G's is your car pulling in the sim when the wheel(s) lift?

[Jevon]

Not much to add, but have you tried increasing both the front and rear spring rates? It looks like your front and rear ride frequencies are rather low. Ours are around 3Hz and so are a lot of other teams I have seen. Whats your vehicle weight, distribution, and your resulting wheel rates?

- Jevon

[Anvit Garg]

The car is pulling approximately 1.3 lateral g's

We have assumed a 10 inch cg with instant centers located 5 inches above the ground. I have the front RC at 1.9 inches above ground with 1.5 in suspension travel.

I am right now re-doing the rear to have a RC 2.89 inches above ground with the same 5 in instant center height.

I will definitely look into increasing the ride frequency.

The vehicle's mass distribution is assumed to be 50 50 (i know its unrealistic) and we are saying 600 lbf with a 150 lbf driver.

Also I have assumed each corner's sprung mass to be 125 lbf.

Front ride rate: 50.6 lbf/in
Rear ride rate: 46.1 lbf/in

Our ride rates are weird because the motion ratio of the rear is less than the front. I am fixing this to have nearly equal or greater in the rear. As a result the springs in the rear are 220 lbf/in and the front are 200 lbf/in.

I am pretty sure this is a major cause of the problem.

[Mikey Antonakakis]

Try something closer to 40/60 weight balance, towards the rear, if it's easy to change in your sim.

[Silente]

Hi,

i have to say i don't know exactly how carsim works. the reason of a wheel lift, on real cars and also in adams/car could be because, when extending, your damper reaches its maximum lenght. maybe when braking or rolling your damper works in this condition...

[exFSAE]

Changing wheel rates to go with a higher natural frequency shouldn't do anything. Why would it? 3+Hz natural frequency is also getting into the un-necessarily high side for these cars... 50 lb/in wheel rate may be a tick on the low side but the only concern there is the car bottoming under braking.

Your ride rates don't look weird, nor do your springrates.

Anyway-

Lifting wheels at 1.3G.. that's pretty low lateral acceleration. Not really sure how your car is recovering from braking on corner EXIT. But, these would cause a wheel to lift:

1. Too much lateral load transfer. Either CG is too high or track is too narrow. 10" CG is reasonable. What is your track width? If it's somewhere near 50" you shouldn't have this problem.

2. Diabolical amounts of rebound damping somewhere. Car goes into roll and the dampers "hold" the tire up in the air. I believe for these type cars, damping rate at the wheel of 5-8 lb*s/in is appropriate?

3. Running out of droop travel, as Silente suggests. Car goes into roll, damper wants to extend but runs out of travel and can't put the wheel back down. Believe this can be solved with a shorter free length of the spring, in relation to the max length of the damper.

Those are the 3 that come to my mind anyway.

Edit - 4: Massive friction in your suspension and/or dampers (though I don't think CarSim would have this modeled) also preventing the wheel from returning.

It would be worth validating your CarSim model as well. Put it into a constant 1.3G skidpad turn. What does it claim the wheel loads will be? Then compare that to simple hand calc, what should the wheel loads be? Are they the same?

[Anvit Garg]

Silente: That seems to have partially fixed the problem! I ended up disabling the dampers all together.

Unfortunately I am still getting wheel lift, but not as severe. I hope it is because of my error in inputting variables or defining parameters in carsim, and not the design.

I will work on it some more once I get some rest. Thank you for the input and please let me know if anything else comes to mind.

[Steve O]

Anvit, #3 from exFSAE is probably hitting the nail on the head, something is moving too far! Some portion of the car is either moving too much or you didn't set up your suspension to allow for enough droop based on the rest of the car movement in which case, it goes back to my first point that some other point of the car is moving too much relative to the wheel that is lifting. Your solution: make the lifting wheel allow for this movement, or stop the movement. As exFSAE suggests, try shortening your free length, try increasing your MR to use up more of your available travel. And my final note is to think about this: On our real car, we blew out our front left shock on a corner (it was a Risse shock so no one should be surprised) and during a right hand corner, the right rear lifted until we fixed the problem. Our middle of competition solution was very stiff springs in front because we did not have replacement shocks. The rear wheel was not designed with enough droop available for the front shocks to bottom out as they were.

[Charlie]

You see FSAE cars do this in real life too, so I don't think it's necessarily an inherent issue wiht your model.

That doesn't mean it should be something you want to happen...

[exFSAE]

True, FSAE cars do 3-wheel, but at 1.3G seems low.

[Silente]

Question to all of you: Is 1.6 deg/g a common sense roll gradient for Fsae?

We are using different values for our car and also in formula cars i have seen lower roll gradients...

[exFSAE]

There's nothing inherently wrong with it.

[John Fratello]

One thing to try is stiffening up the whole car. If it is a lack in available droop travel you should immediately see it.

[Mark TMV]

Just thought I'd add a picture to this thread...
http://i44.tinypic.com/hti9af.jpg
During endurance @ VIR

[Rex]

+1 for insufficient droop - have seen this happen on a couple of the cars I have worked on, and it's typically droop limit on the shocks. My car still lifts the inside rear a bit for this reason. I'd rule out droop limit before messing with stiffnesses - would be a shame to compromise in other areas (i.e. f/r roll stiffnesses, RC height, etc) just because your shocks weren't long enough or you were riding too low in the travel on them.

[scott_rfr]

Mark TMV what part of the course was this photo taken?

This car has sufficient droop travel, probably more or equal to most others SAE cars.

Scott

[Charlie]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by scott_rfr:
Mark TMV what part of the course was this photo taken?

This car has sufficient droop travel, probably more or equal to most others SAE cars.

Scott </div></BLOCKQUOTE>

I'm curious how you know it is equal or better than most cars? What is sufficent droop and how did you decide on it?

[Mark TMV]

Scott, I took this picture at the very top of the course at the exit from a chicane after the passing zone. The lift was happening every lap on acceleration out of that chicane. I'm not questioning the car's kinematics or droop travel, I'm sure the camber of the road and driver's enthusiasm on the throttle may have something to do with it. I just thought it was cool how this was the only car doing that.

[scott_rfr]

This car has 1.5" of droop travel. Never said this was better rather that the amount is probably equal to or the same as that of most fsae cars. This was also the first time we have ever run a course like this. Droop travel was based on previous car's, data aq, as well as basic hand calculations.

Mark, thanks I was just wondering because right now our data system dosnt have a built in gps so it is kind of hard to correlate where exactly something is happening on a course this large. So thank you for the picture and information.

Scott

[Rex]

Would be interesting to see a pic of the rear end of this same car at the same point on track, just to see if the outside rear looks massively compressed.

Of course this discussion is so situational that it's hard to pinpoint without seeing a model or a specific car. 1.5" seems like a reasonable amount of droop travel, but if the car had 3+" of compression travel and one stiff ARB, seems like an inside wheel could/would still come up right?

I seem to recall a discussion some years back where a team was lifting a wheel and it ended up being due to massive caster on the front wheels (go-kart effect). Anvit you might check that out too. Sometimes it's a combination of factors that manifest into a single problem...

[scott_rfr]

Car has a total of 3" of travel and no arb's hooked up at the time. Suspension sits half way into the damper.

To anyone that was taking pictures in that part of the track was any other car doing this at all?

Scott

[Anvit Garg]

sorry exFSAE, I missed your post. I am going to get some sleep and then re-read everything and reply.


In case I have the model set up incorrectly, I have proposed that we subject the vehicle to the K&C test to determine an overall mechanical grip for various setups and then create a contour color plot using a finite difference method.

EDIT: I just read your post Matt, I did increase the model to have approximately 10-11 degrees of caster so I can get the ideal camber into the turn from the front tires. I hear the average for FSAE starts around 6-7 degrees.

[Anvit Garg]

exFSAE: I am going to perform those calcs you suggested for skid pad, that seems like a good way to start troubleshooting.

If the forces are okay, which they seem to be behaving appropriately (to my limited knowledge), then I will go back to suspecting insufficient travel.

I am not even sure if carsim has an option to limit the suspension travel. I need to read the help file to see where it specifies the shock/spring length

I had disabled the shock, and I saw a noticeable reduction in wheel travel, it is possible carsim assumes a standard damping if I do not specify it. I will use the values you have suggested.


I really appreciate everyone's replies, and I will give them a try tomorrow. I will report back.

Also I recall in one competition picture posting that the same phenemonen of rear wheel lift was noted. Ill see if I can find that.


Also my track is 48" in the front, 46" in the rear, +/- 1.5" travel in the front, +/- 2" in the rear.

[Anvit Garg]

The wheel lift resembles this picture:
http://www.wwufsae.com/Waltman/Pictures/Hosted/Competition/2008_FSAE_West/Autocross/Autocross_Web_Gallery/images/IMG_2934_resized.jpg

That is more extreme, and the front wants to lift but doesnt yet.

And the exact caster I selected was 9.33 positive, I was tempted for more, but I kept it at that for now.


Here is a CarSim plot of the initial wheel hop:
http://img.photobucket.com/albums/v412/daconfusion/before.jpg

Then this is quoted from my report:
"Figure 7-5 shows that the vehicle was experiencing 1.33g laterally, and -0.095g longitudinally while traveling at 37 km/hr. The data showed that the vehicle was exiting a corner, and accelerating while experiencing lateral forces near the maximum capacity of the vehicle. Team 2 suspected that the significantly stiffer rear anti-roll bar was causing excessive load to be transferred from the inside rear wheel to the outside front wheel, which when caused the inside front wheel to transfer significant weight to the right rear wheel. To test the hypothesis, the vehicle's rear anti-roll bar's stiffness was reduced by 32%. Vertical force plots showed that the vehicle exhibited no change in the undesirable hopping effect.
Next, the vehicle's front natural frequency was increased from 1.99 Hz to 2.5 Hz, which resulted in an increased spring rate of approximately 330 lbf/in from the original 200 lbf/in. Figure 7-6 shows the results."

http://img.photobucket.com/albums/v412/daconfusion/after.jpg

A significant increase in front spring rate caused the inside wheels to not lift off the ground. Unfortunately, the forces were seen to still approach zero. In addition, the increased spring rate was deemed impractical for an initial design. Before proceeding to a kinematic redesign, a new roll gradient was proposed: 0.9 deg/g instead of the initial 1.6 deg/g. The initial vehicle set up, with a 1.99 Hz front and 1.90 Hz rear natural frequency showed no improvement with an increased roll gradient. The stiffened front suspension along with the new roll gradient also showed no improvement over the initial improvements the stiffer front springs provided. Lastly, the rear springs were softened. As a result additional tire lift was noticed on other wheels as seen in figure 7-7 below.

http://img.photobucket.com/albums/v412/daconfusion/last.jpg

Concluding that the behavior could not be addressed by changes in set up, a redesign of the rear suspension in hopes of changing the roll center heights and camber behavior was performed next.

[Anvit Garg]

So I then decided to increase the roll center height in hopes of minimizing the distance from RC to CG, but inadvertently increasing the distance from the ground.

As a result the rear jacking increased and the overall effect was undesirable. I then proceeded to return the rear CG back to 1.9 inches, and then adjusted the arm lengths of the rear to try and reduce the roll.

I was able to conclude that without adding rear steering, I can not cause the rear wheels to roll into a turn, UNLESS i make the roll center go underground. Are there any other possibilities?

I am still working on your other suggestions, will get back to you asap!