Roll Center

[BrianD]

Does anyone else use or have information relating to using an underground roll center? Our 2004 car used a roll center that was around an inch underground, but no one can really tell a difference in driving that car versus others.

[BrianD]

Does anyone else use or have information relating to using an underground roll center? Our 2004 car used a roll center that was around an inch underground, but no one can really tell a difference in driving that car versus others.

[clausen]

Hi Brian,

There isn't any dramatic difference between the behaviour of a car with the roll centre above or below the ground, its just a matter of the roll moment getting longer the further you go down. It's not like all of a sudden the laws of physics change.
What does happen is that as the roll centre passes through the ground plane, its migrates wildly laterlly during roll. This is a reason to try and make sure that the roll centre doesnt go through the ground, or get close to, during the suspension travel.
I'm also pretty sure that for a given roll scenario, the roll centre will migrate laterally during roll in one direction if its above the ground, and the other direction if its below the ground. So if you're keen on playing with the effects of lateral migration, this might be a way to acheive different effects.
We did some pretty intresting experiements when i was at a test day with a touring car team and we did some laps with the static roll centre towards one side of the car, and then towards the other side ofthe car. This was done by assymetrically setting up the wishbones. I cant remember the details, but the differences were really noticable when we got up to around 100mm lateral offset. I recall that more bad effects than good effects come out of it though, which strengthens my belief that good geometry minimises lateral RC migration.

Hope that helps

[Kevin Hayward]

Just a little point on lateral migration of the RC. You can get it to go to any side you want wether the RC is above or below the ground.

Play around with a simple four bar linkage program (assuming double a-arms) and you'll be able to see which parameters of the suspension achieve this.

Cheers,

Kev

[jack]

Roll center above ground:
-less roll
-more jacking
-ride hight increases
-loads outside tire
-unloads inside tire
-improves turn in response

Roll center under ground:
-anti jacking
-ride hight decreases
-loads inside tire
-unloads outside tire
-"sluggish" trun in response

...IMO you cant say one or the other is better, especially without considering the rest of the suspension kinematics, which is why you probably cant tell the difference between the two cars. roll center is just one variable in the equation. the trick is just to know why you did it http://fsae.com/groupee_common/emoticons/icon_wink.gif

i kant spell...

[Big Bird]

Just a minor correction if I may offer it. A roll centre above ground loads the inside wheel and unloads the outside wheel for the first instant of the corner only. The weight then transfers to the outside wheel as the car rolls. The final loadings on the wheels in a corner (assuming corner is long enough for the car to reach a settled, steady state condition) depends on lateral acceleration, track and centre of gravity height - not RC height.

What the roll centre does is set the point about which the car rotates. The inertial lateral force acts through the centre of gravity of the car, a point obviously above the ground. The roll moment causing roll is the distance from the CoG to the RC, multiplied by the lateral force. Therefore greater distance between the two, the greater the roll moment. However the roll inertia increases with the square of the distance between the two. Therefore the lower the RC, the slower the roll acceleration and therefore the longer it takes for the car to get to steady state.

If you are racing around ovals with long corners, there is no problem if it takes a while for the car to settle - might make the car a little less nervous for the driver. However if you want quick response, as in getting through slaloms, a higher roll centre might help. You have to figure out how much weight jacking you can put up with...

Hope I haven't bored you.

Cheers,

[Dr Claw]

so the slower roll acceleration is cause for jack's ""sluggish" turn in response" charachteristic right? because it takes longer to settle?

can someone also lay to rest the "more normal load on a tire" myth/rumor? I hear this is why underground roll centers arent very practical at (higher speeds) because it wears your tires out faster. but at lower, FSAE speeds, you just heat your tires up faster...correct?

[Charlie]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Dr Claw:
can someone also lay to rest the "more normal load on a tire" myth/rumor? I hear this is why underground roll centers arent very practical at (higher speeds) because it wears your tires out faster. but at lower, FSAE speeds, you just heat your tires up faster...correct? <HR></BLOCKQUOTE>

I guess you are going to have to clarify this 'rumor.' http://fsae.com/groupee_common/emoticons/icon_confused.gif Not sure what you are asking. I'm not quite sure how roll center height alone could wear your tires faster, but perhaps I'm missing something.

One thing I wanted to add, is 'sluggish' has a real negative connotation, and 'responsive' has a positive one. But you could just as easily substitute 'stable' for 'sluggish' and 'twitchy' for 'responsive'.

In the past FSAE auto-x events nobody cared about stability and responsiveness was key. One reason for the trend toward the minimum wheelbase. But the recent courses, especially as bumpy as the '04 track was, seem to favor stablity more than in the past. Its not a terrible word anymore. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[jack]

dr claw, i think what you are getting at is the instant center heights, relative to the ground. if your IC is below the ground, it will cause the tire to scrub in bump, and if both of your IC's are below the ground, so is your RC, so i guess i can see how that connection can be made. check out Fig. 17.10 in the milliken book for a less confusing explination.

charlie, i agree that "sluggish" may not be the correct term for that characteristic. thats just the word claude used. as i said before, personally i dont think there is a right way to do any of this stuff, just have a purpose behind your setup. see i mean??

[Charlie]

I think 'sluggish' is a proper term, and its used often I just think its negative connatation is a bit decieving. It wasn't directed at you jack, see I mean? http://fsae.com/groupee_common/emoticons/icon_wink.gif

[clausen]

Cheers Kevin,

I must admit to not having done enough such experiementation myself.

[Denny Trimble]

Scrub happens more as the IC is above or below ground, and closer to the tire (shorter VSAL).

[Alan]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Big Bird:
The final loadings on the wheels in a corner (assuming corner is long enough for the car to reach a settled, steady state condition) depends on lateral acceleration, track and centre of gravity height - not RC height.
<HR></BLOCKQUOTE>

Don't the final loadings depend on among other things the relative roll stiffness on that end of the car and the roll center location at that end? I would agree that the total load transferred is independent of the roll center height but how much transferring each end does is not. Or am I just misunderstanding what you are saying?

[Big Bird]

Apologies Alan, I was just trying to explain that the TOTAL overall transfer to the outside wheels is independant of RC height. RC height affects the initial transfer and then the time taken for final transfer figure to be achieved.

Of course how this is distributed between front and rear wheels relates to front to rear stiffness, therefore affecting oversteer/understeer characteristics, blah blah blah. The trouble with vehicle dynamics is that if you try to explain one thing then you open the floodgates to all this other detail - resulting in another stupidly long post. And if you ask anyone in my team or on these forums, I already have a reputation for waffling on!

Cheers all

[BrianD]

Our IC's were underground along with the RC. So, the IC's are what make the tires run at a higher temperature? The main reason behind going underground was to get more heat to the tires, so they ran within the optimal operating temperature.

[Storbeck]

For the heat in the tires I don't think it matters whether they're above or below ground, it's the distance from the ground surface, and the length of the IC's that makes the difference.

Picture the motion of you're tire as it goes up and down, the more the contact patch moves side to side as it goes up and down, the more heat you get.

[IsheeM]

Storbeck,
If I am correct what you are describing is tire scrub, which is bad. The more the tire scrubs the longer it takes to realize the full cornering potential of the tire, and it also increases tire wear, no?

[Storbeck]

As I understant it what you say is correct, the heat comes from scrub, friction as the contact patch slides across the road, there are a bunch of negative things that come from it, but you will get more heat.

I'm sure someone will correct me if I'm wrong.

[Vector006]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by IsheeM:
Storbeck,
If I am correct what you are describing is tire scrub, which is bad. The more the tire scrubs the longer it takes to realize the full cornering potential of the tire, and it also increases tire wear, no? <HR></BLOCKQUOTE>

Claude mentioned that it is a rumour made by tire companies that tire scrub is bad...becuase they dont want their tires to wear out before warauntee. For our little cars, scrub is very good since it is difficult to get the tire temps up to op-temp. This scrubbing action occurs at about 10Hz, depending on track surface ofcourse. From what i noticed, there were very few cars that came off the track with blistered tires this year, it either means their driver wasnt pushing the car hard enough, or the tires weren't getting hot enough.
In reply to IsheeM...im not sure what you mean by realizing the full cornering potential of the tire. We are typically talking about 2mm of scrub, im not sure if this would be a concern.

[jack]

...don't forget that scrub also changes the slip angle of the tire, too.