I was once having dinner with a part owner of a fairly famous race team and we were speaking about a lots of things cars. Increasing track he said would lower the center of gravity. I also remember discussing this skeptically with Onasled. This is in fact not true and should have been really obvious. The only way to lower the CofG is lower the car and or move physical components around. Increasing track simply leveages the mass in way that allows the tires to perform more work, and, like lowering the RC, spreads this load out a bit more evenly between the inside and outside tires.

I'm sorry I wrote this so many times. It's just panfully obvious.

 

I would like to see the math on this..... I lean the other way.....

thinking c of g in small airplanes...... I need to get out my books...... but I am too lazy......maybe one of you engineers could explain this

 

It's simply really and I'm pissed I just threw out some elses info...trusting their worldy experience. The actual location requires time and en engineer, but knowing the location cannot move with altering ride height or moving physical components is a simple and true statement.

Draw a triangle with the point being the CofG Spread or narrow the base without altering the height of the point. That's the relationship.

 

I have to get going but I got out the Fred Puhn book and can not find anything exactly addressing this.....not that it isn't there... i just can't find it....... and let me be clear...... I have no clue..... and in no way am taking a position....just curious

 

Oh I understand, I was simply clarifying further. And, I obviously have no clue sometimes...I trusted the man speaking to me due to his experience, but the info never sat well.

The Punn book has lots of curious gaps in info. But I liked that book a lot...lost a while back.

 

I am not an engineer, but I fooled around with sailboat design enough to understand center of gravity. Increasing the track (moving tires or increasing their width) would not affect the height of the center of gravity above the ground. The vertical location of the center of gravity is dependent on the distribution of all the weights of various pieces in the car. If you don't move those weights up or down, you don't change the location of the center of gravity.

It is true however, that if you widen the track, the car will behave as if the center of gravity was lowered. This is because the overturning tendency is dependent on the relationship between the vertical height of the center of gravity and the horizontal distance between the center of gravity and the wheels. Imagine trying to tip over the triangles in Meb's post #3 - tall, skinny triangle, easy - wide, low triangle, hard. But the center of the triangle has not gotten closer to its base, hence CG has not lowered.

 

given that no other material is added or taken away if you widen the track.... assoc weight has to be moved.....

 

No, the weight does not move hence the triangle analogy. This is a static demonstration. When the car turns, a wider track resists weight transfer better than a narrow track.

 

If you are widening the track By definition you are increasing the distance between the outside edges of your tires....you are in essence moving weight further out. Using the same triangle model mentioned above, if you widen the base, the apex will drop. I think the point is that this drop is insignificant relative to the effect of, say moving components, or lowering the car. YOu would have to go REALLY wide to have the equivalent effect of lowering.

 

I can end this speculation with one example: Draw a triangle on a piece of paper with the point up and equal left and right sides, and draw a horizontal line through the triangle where you imagine the center of gravity is (you don't have to be correct in your guess of where to draw it). Now, take scissors and make a vertical cut, cutting the triangle in half, so you have two equal halves, a left and right. Would you agree that each of these new triangles still has a center of gravity at exactly the same level as the line that is drawn horizontally through each? Good, now move them one foot apart (increasing track). Now, if you still aren't convinced, rest a horizontally long rectangle on top of the two triangles. Put the two halves together and imagine where the center of gravity is for the entire system, then move them to the left and right edge of the rectangle and imagine where the center of gravity is for the entire system. You had better be imagining the same center, or there is no hope for you!

 

Look here:

http://www.longacreracing.com/articles/art.asp?ARTID=22

There is a wheelbase (length) component in the calculation

 

That is for finding the center of gravity, not how length nor width affect it. The length is needed because of the means used to calculate it, not because it affects where the center of gravity is.

 

Thanks

 

How about changing the tire size? Stock tires on a Mini Cooper are 175/65-15, the radius of this tire is: (175/25.4x.65+15)/2=9.739" Now if you go with a 195/50-15, the radius would be: (195/25.4x.5+15)/2=9.419" So, 9.739-9.419= 0.319 By changing to a wider/lower tire you have lowered the center of gravity by 5/16" and increased the tire patch/overall grip. Note: speedo will not be correct.

 

I hate saying this but you're right...In a 3 dimensional object of unifom mass length or width will impact where the center of mass for the obect is in space but widening the object keeping everything else constant does not change the center of gravity relative to the ground. (I should stick to chemistry )

 

moving the tires out or in per se will shift so little weigth as to be negligable. But track does affect ride height, and hence can change the Cg. But it's camber dependant. With 0 camber, move the wheels out, two things will happen. One is the wheels will move out parralel to the ground, and the other is the effective spring rate will change (a bit lower) and the car will droop a bit. If you have negative camber, moving the wheels out will drop the bottom of the strut a small amount as well. This will compound with the effective spring rate, and you'll get even lower a ride height. If you do it with posative camber (why, who knows, but it's possible) you can counter act the change in effective spring rate or even exceed it, raising the car. Depends on base spring rate and offsets.

But I have no clue what the magnatude of these effects are. They are probably less than the effective change of lever arm on roll stiffness, but that's just a wild guess.

Matt

 

Doc, I hope you meant to write that track does not affect CofG??? All else equal, moving the wheels a foot farther out left and right will not alter the CofG location. All things reality, neg camber lowers ride height as you wrote and because the motion ratio is altered the springs become less effective. About 10% less effective for a 15mm wider stance per side on a 38mm wheel offset.

I can tell you from personal experience that the difference in track just 5mm makes in handling is very, very noticable. 15mm up front and 5mm in the rear with 38mm offset wheels is huge! You need to try these.

Altering tire diameter will affect CofG hieght; I have 205/40/17s as my track setup. These offer a gearing advantage as well as a CofG advantage.

I stand by my observation and analogy; if the hight of the point of the triangle does not change with respect to base width, the CofG remains the same. Aside from the realistic subtleties Doc pointed out, and these are real, the individual who first imparted this info me was not correct, with noted exceptions pointed out by Doc.

 

I think this is where actual and effective CoG comes in. Lots of drivers I know talk about the effective CoG when discussing wheel-base and track vs the actual CoG. The effective CoG will not change drastically, but it will change with track changes. By increasing the track you are increasing the load req'd to overturn the object, same as lowering the CoG.

 

yup

 

but it's really an function of integrated mass density. To put that in english, the Cg is a function of what mass is positioned where. When the wheels move out there is no change in ride height (initially ignoring the effect on springs etc, just looking at geometry) only if camber is zero and the spindels (or whatever they're called on FWD cars) are parrallel to the ground. If not, there is a vertical offset of the rotation axis of the tire relative to the car as the wheel is moved in and out. If camber is positive, moving the wheels outward will raise the ride height of the car. If the camber is negative, the opposite will happen. This is straight geometry, and is independant of mass distribution or suspension theory. The spring effect just mess up the geometry. You can change spring rates, or pre-load to make the strut the same length as before, or maintain ride height (by increasing the length of the coil over or the pre-load or spring rate), then the Cg doesn't move relative to the ground.

What's confusing the issue here is that the Gg is pretty much in the same place in the car independant of track or camber. But the ride height changes, and there for the Cg changes. Or not, depending on the details.

I'd agree that the universal statement "increased track lowers Cg" isn't correct. But "increased track will lower the Cg via a lower ride height for a car with negative camber, all else being equal" is true absolutely. Sorry it's a mouthfull!

Matt

 

Yes, I completely agree. The notion that making track wider had a significant affect on the CofG is what I was disputing - and I ought to take this up with the fellow who got my brain on the track...no punn

Integrated mass density...or centroid mass axis...