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rpmag (Automotive) (OP)
21 Sep 06 7:57
I have accumulated some basic data on sporting car suspension. All are RWD with front or mid-engines.
Approximately 70% have a wider rear track and the remainder wider or equal front track. Switch to a racecar list and the percentage is reversed to 70% wider front track.
This racecar % could be accounted for by the need to accomodate rear rim width's, but is there another reason why there is such a predominance of wider rear track sporting road cars?
GregLocock (Automotive)
21 Sep 06 8:09
Wow. That's not what I'd expected.

OK first you have to consider the articulation angle of the halfshafts.

Next, you need to make sure that the driven wheels stay on the road.

If push comes to shove I'd rather have a wider front track, but that is a rather idealistic.

Cheers

Greg Locock

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

Aday (Automotive)
21 Sep 06 14:30
Often this difference can be attributed to two things:

1.  Styling thinks the car will look better with a wider rear track

2.  Suspension geometry or tire size changes in the front or rear suspension for packaging, performance, etc. later in design cycle(or if the vehicle is an iteration of a pre-existing architecture) may change the track width.  It is unlikely changes in the opposite axle will be made at this point just to make the front and rear track the same.
rpmag (Automotive) (OP)
21 Sep 06 18:45
Greg, I have considered the driveshaft angle and for my applicatio I may need to rotate the gearbox relative tot he engine. I also had planned on a wider front track, but how much wider would really make a difference?
Aday, thanks you, I had not considered the changes over the design cycle, I will follow this up.
GregLocock (Automotive)
21 Sep 06 19:42
Oops. Just had a reboot.

Overall I think you would be happier with the maximum track you can get at the front, and I think practically you will tend to be happier with the maximum at the rear as well.

This will minimise steady state load transfer at all lateral gs. this is probably more important than linear range u/s

The advantage in linear range understeer from having a wider front track are small, and easily got by bigger a/r bar or bump steer or compliance steer or roll heights etc etc etc.

Cheers

Greg Locock

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

hemipanter (Automotive)
22 Sep 06 8:38
What do we think about the steady state cornering situation
using different Tw front-rear? Lets say the starting point is an equal Tw and neutral steer car.
Goran Malmberg
BillyShope (Automotive)
22 Sep 06 14:48
When it comes to track, the old hotrodding adage seems to fit: If some's good, more's better!

Having said that, I recognize that there are exceptions. Anyone who's been around karts recognizes that the front track is usually much less than the rear, simply due to static weight distribution. This might also play a part in the statistics cited.
hemipanter (Automotive)
22 Sep 06 15:52
I am not to familialar with carts, but if they got a flexing frame they might react in a different manner than a regular race car.
Goran Malmberg
GregLocock (Automotive)
22 Sep 06 21:34
If some's good, more's better!

Yup.

Goran, I never think about go-karts, they confuse me too much!

What do you mean by a neutral car? One on the verge of oversteer?

Take a car with 50/50 weight distribution, equal roll stiffness each end, same tires etc etc in a steady state corner (no wonder vehicle dynamics is confusing, we regard continual acceleration as steady state)

Now decrease the rear track. This forces more load transfer onto the front axle, causing more understeer. Does it get any more complex than that?

Cheers

Greg Locock

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

hemipanter (Automotive)
23 Sep 06 4:25
"Now decrease the rear track. This forces more load transfer onto the front axle, causing more understeer. Does it get any more complex than that?"

Greg, no it does not.
Cheers
Goran
rpmag (Automotive) (OP)
23 Sep 06 5:19
well rear track for my purposes is 1570mm and I had pencilled in 1630mm for the front, but nothing is fixed of course. The final numbers will be dependant on rim dimensions/offsets available as well as other factors.
BillyShope (Automotive)
23 Sep 06 7:19
"Now decrease the rear track. This forces more load transfer onto the front axle, causing more understeer."

I'm afraid the effect is exactly the opposite. As the rear track is decreased, the load difference between inside and outside rear tires is increased for the same couple. The maximum lateral force capacity of a tire pair is achieved when the tires are equally loaded. So, when the rear track is decreased, the lateral force capacity of the rear tire pair is decreased, and the result will be more OVERsteer.
hemipanter (Automotive)
23 Sep 06 8:22
Billy, we must consider the chassis to be a rigid body. For a single axle the Wt=(cgh*g*w)/Tw, but when the two axles are working together the chassis will roll the same degree for both axles. Therefore, if we narrow the Tw for one axle to become close to ONE wheel, the other axle which obviously has a wider track will take ALL of the Wt.

Regards
Goran Malmberg
BillyShope (Automotive)
23 Sep 06 8:37
Goran, check out the equations at the top of page 683 in "Race Car Vehicle Dynamics." Isolating the rear track variable (which Greg was doing in his example) results in an increase in rear tire loading disparity as the rear track decreases. If this track reduction is carried to the extreme, as you suggest, the value of KsubR cannot be maintained (no room for springs), but, again, if you isolate the single variable of rear track, a reduction in rear track will always contribute to oversteer.
hemipanter (Automotive)
23 Sep 06 8:55
I have an example. The wheels has an wheelrate in relation to the chassis. As fare as Wt is concerned we could as well replace the wheels whith one spring at each corner of the chassis and put one cornerweight scale under each spring. Now, move two of the springs, representative for one axle, closer together. Lift 100p of one side of the car and put it down on the other side and you will read the difference, rised load transfer, att the other axle.

Cheers
Goran Malmberg
BillyShope (Automotive)
23 Sep 06 11:39
The equation, to which I referred, has the track in the denominator on one sie of the equal sign and the loading disparity in the numerator on the other. So,if we are indeed isolating the effect of track change (and assuming no error can be found in the derivation or limitations in its application), I consider the matter settled. The direct result of a decrease in rear track is an increase in oversteer.
hemipanter (Automotive)
23 Sep 06 18:09
Billy,
Greg was the one claiming that narrower rear track is causing more understeer. What I say is only what happen to the load transfer in such case. I did the  described experiment last spring using my own car and wheelspacers to change Tw. I used 100 kg of weight of ballast so the car weighted in at 1325 kg. With a 1510 front and 1580mm rear track the corner weight was 265-265 front and 408-408 rear. When lifting over 50 kg to the other side the number get 259-271 and 264-451.
hemipanter (Automotive)
23 Sep 06 18:11
Sorry, I lost my finger, Ill be back...
Goran Malmberg
hemipanter (Automotive)
23 Sep 06 18:42
Billy,
Greg was the one claiming that narrower rear track is causing more under steer. What I say is only what happens to the load transfer in such case. I did the described experiment last spring using my own car and wheel spacers to change Tw. I used 100 kg of weight of ballast so the car weighted in at 1325 kg. With a 1510 front and 1580mm rear track the corner weight was 255-255 front and 408-408 rear.
When lifting over 50 kg to the other side the number get 349-261 and 362-453. Using the spacers for a 1610 front track gave the following numbers, 248,5-261,5 front and 365-450kg rear. So, there was near to zero difference at the front axle and the rear get more equal loading.

This was not to serious experiment so it might be error somewhere.
Cheers
Goran Malmberg
BillyShope (Automotive)
23 Sep 06 19:34
Goran, you can calculate this effect and eliminate measurement errors. Just determine the moments created by the loads about an arbitrary axis parallel to the long axis of the car. You know the sum of these moments must remain constant (or the car would be rolling over.) You know, also, that the sum of the loads at the front (or rear) must remain constant. So, you've got two equations and two unknowns.
BillyShope (Automotive)
23 Sep 06 19:37
All right, it's 4 equations and 4 unknowns. Anyway, you can solve it.
GregLocock (Automotive)
23 Sep 06 20:23
"Take a car with 50/50 weight distribution, equal roll stiffness each end, same tires etc etc in a steady state corner (no wonder vehicle dynamics is confusing, we regard continual acceleration as steady state)

Now decrease the rear track. This forces more load transfer onto the front axle, causing more understeer. Does it get any more complex than that?"

Well, no one seems to agree, but Car Sim behaves exactly as I'd expect, more understeer with a narrow rear track.

Look at the plots, top link in the gallery at http://www.geocities.com/greglocock/
or this link might work
http://www.geocities.com/greglocock/gallery/track_us.jpg






Cheers

Greg Locock

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

hemipanter (Automotive)
24 Sep 06 7:18
Billy, thanks, no calculate problem. Sorry to say that I lend out my "Race Car Vehicle Dynamics" and newer get it back, strange...

First, as we see there are different opinions about the effect of track width. I know it usually are, that’s the reason I brought the question up in the beginning (n:5) of this tread.

I have been looking around the www several times about this issue. And there are a lot sites claiming wider track front as a cure for under steer. However, these sites use a larger "how to fix the balance" sheet that seem to emanate from the same source.

Here is a cut from a SAE paper that at least is on of its own.

"When selecting the track width, the front and rear track widths do not necessarily have to be the same. For example, track width is typically wider in the front for a rear wheel drive race car. This design concept is used to increase rear traction during corner exit by reducing the amount of body roll resisted by the rear tires relative to the front tires [4]. Based on the corner speeds and horsepower to weight ratio of FSAE cars, this concept should be considered by the designer".

Greg, I don’t say I don’t agree with you concerning the "wider front track understeer theory". In fact I was inclined to think so myself, but the doors are still open.
I have also performed a number of scale model experiments,
showing the same as my full scale test already mentioned.
When it comes to driving the car some inch wider front track is hard to detect and takes consistent repeated driving on the skidpad. Because of a surgery I have not been able to drive my car until now this summer, so I am in lack of driving evidence.

I may mention that my connection at Ohlins schocks is of the same opinion as we been discussing this issue before.

Based on my experiment I made my own Tw calculation formula for an excel sheet, very much like the one for the wheel rate I sent you. Dont know if I dare to show it now....

Regards
Goran Malmberg


BillyShope (Automotive)
24 Sep 06 7:48
So, Greg, are you going to break the bad news to the people who developed Car Sim? Obviously, there's an error in their code. The equations in RCVD are straightforward and easily derived.

Goran, I'm sorry that I dismissed your example so readily. In looking at the numbers more closely, I see that you meant 249-261 and not 349-261. I then realized the differences are just measurement error. And, of course, this is reasonable. You could push the left front 5 meters to the left and the right front 5 meters to the right and the loads must remain unchanged.
BillyShope (Automotive)
24 Sep 06 9:04
Goran, to combine a couple of American sayings, when I shoot from the hip, I end up with my foot in my mouth.

As the front track goes to infinity, the front tire loadings would become equal. As the front track goes to zero, the larger load would go to infinity and the smaller to zero.

So, it is a matter of 2 equations and 2 unknowns. In this case, the left front should have gone to 249.4 and the right front to 260.6 kg.
BillyShope (Automotive)
24 Sep 06 9:10
At my age, I'm beginning to wonder if I'll get this right before I'm gone.

What I meant was "the larger load would go to positive infinity and the smaller load to negative infinity."

There! I hope that's it.
hemipanter (Automotive)
24 Sep 06 11:37
Agreed, I am 63....

Goran Malmberg
GregLocock (Automotive)
24 Sep 06 19:25
Billy, Um, well you probably know Tom Gillespie. I don't!
CarSim is his baby.

I've just reclaimed RCVD - I think you are focussing too much on the rear axle equation, you then need to consider how it affects the front axle as well (because the total roll stiffness decreases). I'm also a bit leary of using the simplified equations, p 682 is much better.

Incidentally it is worth pointing out before anyone else does that I just used the generic car model in CarSim as a baseline, which is not 50/50 weight balance etc etc.

Cheers

Greg Locock

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

BillyShope (Automotive)
24 Sep 06 22:00
I'm probably one contact away from Tom. (Instead of the average of 4 which is claimed for any two people.)

I thought the idea was to isolate the effect of track width. If so, we can't change total roll stiffness. If all of the other parameters remain constant (using either set of equations), the results are obvious. I see no reason why KsubR, of necessity, should be a function of track.

So, if you have the time, please rerun the comparison, being careful to vary only the track. I would be very surprised if the software gave an answer different from the equations. But, if it does, Tom Gillespie has a problem and I'll see if my friend can contact him.

GregLocock (Automotive)
25 Sep 06 5:12
OK, done, same link as before. You're right, when I upped the rear sta bar to bring the total roll rate back to the baseline value, there was no longer an increase in load transfer across the front axle, but (obviously) it did increase at the rear, so the oversteer contribution from the rear increased.

So back on track... (sorry)

Cheers

Greg Locock

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

hemipanter (Automotive)
25 Sep 06 5:37
Talk about being old and getting things straight. First I wrote 349 in place of 249, and then I discovered I had another
Rc number for the rear axle in my excel sheet where I saved my testing numbers. Anyway, this did not affected the compareable numbers to much in that the rear axle wheel did get a more even load (less load transfer) and the front axle stayed the same.

Let us say we have a car that weight 1000kg, 40-60% f-r
weight distribution. Tw 1600-1600mm, Wb 2500mm, Cgh 400mm,
Zero Rch, wheel rate 10kg/mm all 4 wheels and 1g side
acceleration. Then Wt=1000*1*400/1600=250kg distributed 125 each axle, creating a tire load of 75-325 front and 175-425kg rear.

Now, raise the front axles Tw to 2000mm, what would the 4
numbers look like? Anyone that have a program, this would be an esy task.

It really should feel nice to come to an agreement here since I feel a little confused.

Cheers
Goran Malmberg
hemipanter (Automotive)
25 Sep 06 5:44
I should add, with 2000mm front track, still use the same 10kg/mm wheelrate.
Goran
BillyShope (Automotive)
25 Sep 06 8:39
Goran, now that Greg and I have overcome our misunderstanding (a misunderstanding which would have been resolved within 15 seconds if we had been close enough to see each other wave his arms, instead of halfway around the globe), we would agree that your problem can be solved either by recourse to the software or the equations.

I would love to have the software, but as I now don't have an employer to buy such things for me, I must resort to my books. (I'm tiring of retirement and would love to go back to work, but nobody hires septegenarians for engineering positions. Just when you get halfway decent at doing your job, you have to retire.) So, as you're near my age, you might consider purchasing "Race Car Vehicle Dynamics" by Doug and Bill Milliken. There are other books, but I've taught from this one and am familiar with it.
GregLocock (Automotive)
25 Sep 06 8:45
Billy

Here's the ejicational version of CarSim.

The license is pretty agreeable.

http://www2.ing.unipi.it/~d5973/meccveic.htm

Cheers

Greg Locock

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

BillyShope (Automotive)
25 Sep 06 11:23
Thanks for your good intentions, Greg (though I thought I'd have to stop and learn Italian before I could go any further), but the software evidently doesn't support WindowsME, which I happen to be stuck with. (Can't even install Microsoft Office.)

Actually, I do have the Millikens' "Program Suite," so I'm not totally without computer assistance.
hemipanter (Automotive)
25 Sep 06 12:46
I am not going to bore you out, just one more Q.
Greg said...

"Take a car with 50/50 weight distribution, equal roll stiffness each end, same tires etc etc in a steady state corner (no wonder vehicle dynamics is confusing, we regard continual acceleration as steady state)
Now decrease the rear track. This forces more load transfer onto the front axle, causing more understeer. Does it get any more complex than that?"

Then, if we want the roll stiffness back as stated here...

"OK, done, same link as before. You're right, when I upped the rear sta bar to bring the total roll rate back to the baseline value, there was no longer an increase in load transfer across the front axle, but (obviously) it did increase at the rear, so the oversteer contribution from the rear increased."

Does this mean (bringning the roll rate back to the baseline value) that if we reduce the rear axle to half the Tw we have to increase the wheelrate by 4?

Regards
Goran Malmberg



BillyShope (Automotive)
25 Sep 06 14:59
Not at all. The roll stiffness due to springs is unaffected by track. Similarly, you would think that the roll stiffness due to a sway bar would also be unaffected by track. After all, we're talking about torque per unit of angle, so, if the sway bar mounts are unaffected by the track change, there should be no change in the sway bar's effect. But (and this is why I don't like using spreadsheets when the original equations are available), those who put together the software packages are inclined to assume that a proportional change in sway bar mounting locations will accompany a change in track. (I notice the Milliken software does the same thing.) So, it was necessary for Greg to go back and fiddle with the sway bar in order to maintain the same roll stiffness.
hemipanter (Automotive)
25 Sep 06 16:59
Great,thanks!
Cheers
Goran Malmberg
BillyShope (Automotive)
25 Sep 06 17:17
One more comment: The roll stiffness due to the tire rate changes with track and will, in itself, necessitate a small change in sway bar stiffness. I don't know about Greg's software, but this does not seem to account for all the change indicated in the Milliken software.
GregLocock (Automotive)
25 Sep 06 17:47
I didn't work out the change in roll stiffness, just fiddled with the a/r bar until I got the right overall roll rate. Sorry, it's a bad habit analytically, it's just that in ADAMS I always end up fine tuning by iteration.

Cheers

Greg Locock

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

hemipanter (Automotive)
25 Sep 06 18:13
I did checked mu tire spring rate up using the cornering weight scales. You might allready know some numbers but here is mine anyway. 40 kg/mm, at 1,5 bar. Dimension 15 wheels x 13 wide x 24,5 diameter Avon slicks.
Goran Malmberg
BillyShope (Automotive)
25 Sep 06 21:00
That's reasonable. I usually use 1500 pounds/inch (about 30 kg/mm) in my calculations, but, for my purposes, I could probably be just as happy using your number. Unless you're a working engineer, tire data is not that easy to come by.
GregLocock (Automotive)
25 Sep 06 21:46
That's very stiff, almost twice what a typical production tire is at that pressure. Rule of thumb: pressure is about 80% of the spring rate at 2 bar, the rest is sidewalls.

Cheers

Greg Locock

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

hemipanter (Automotive)
10 Oct 06 3:17
I checked the roll stiffness out using my ½ scale model. The A-arm is parallel and equal length in order to lessen Wr changes. The chassis has a bearing at ground level to create a fixed Rc so that a force applied only create a rotating moment to the chassis.

With a Tw of 600 mm we will now apply a force to the chassis at Cgh distance.  The chassis  roll get 9,5 dgr. Using the same force but a Tw of 800 mm the chassis will now roll 7,0 dgr.

As the suspension system here is a parallelogram the Wr in both the 600 and 800 cases remains the same. During heave there will therefore be no difference in deflection.

Goran Malmberg
rpmag (Automotive) (OP)
27 Dec 06 7:58
an update for those who may be interested. Going through my database of front/rear track differences in sportscars a new fact has come to the fore. When I compared F/R track in cars released in the last 4 years the ratio has changed from that previously noted. It is now 3:1 wider at the front.
Wider at front:
Noble m15 89mm wider at front
Ferrari F430 53mm wider at the front
Lamb Gallardo 30mm wider at the front
Porsche Cayman 42mm wider at the rear

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