Front/rear track width effects

[rpmag]

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]

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]

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]

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]

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]

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]

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]

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]

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]

"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]

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]

"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]

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]

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]

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]

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]

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]

Sorry, I lost my finger, Ill be back...
Goran Malmberg

 

[hemipanter]

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]

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.