Just purchased H&R lowering springs [from Tirerack on sale for $346. for the v6 RWD] and would like to do a little research into how lowering the center of gravity 1.2" actually affects the performance of the car [from a physics standpoint].

Does anybody know what the distance is from the center of gravity to the ground on the stock v6?

 

Here is a link on how to calculate it:

https://www.longacreracing.com/blog....Gravity-Height

 

Thank you very much for the link. That should keep me busy and out of trouble for a little while. Once I am able to calculate this value, I hope to be able to figure out how much decreasing it affects the geometry and physical forces that determines handling characteristics of our cars. Are there any engineers out there who could steer me in the right direction?

 

That’s a serious undertaking. Finite-element analysis is one approach. Maybe some sort of mechanical simulation would be another.

 

I'm not sure how you can determine how it affects the geometry without measurements of every suspension component. Not that I'm going to discourage geeking out on this sort of thing though. In reality, I expect the improvement from lowering will come from reduced CG leading to reduced lateral weight transfer, but also from the negative camber in the non-adjustable rear.

 

I hear you but I've got a lot of time on my hands [being mostly retired] and thought it might be nice to have a way to quantify the gain similar to how you might judge whether it is worth an ECU tune to increase hp. Going from 340 to 416hp [with the VAP stage 1 tune] was a no-brainer but it's much more difficult attempting to access the worthiness of lowering one's car. Other than appearance, it would be nice to say that the handling improves by x units, so I'll check into it and let anybody interested know what I find out.

 

I went over numerous formulae when I was (much) younger, before the PC came about. I'd be interested in what you come up with, although I expect there are too many variables to come up with something even a small group could agree upon. I'm also not sure what units one would use to determine handling improvement other than seconds (lap times).

Anyway, you may find some alignment data here from before and after lowering (perhaps an Unhingd post?) that might give some empirical data.

In any case, have fun!

 

Not trying to be a "know it all", but if you can get the car on a scale and measure the weight at the front tires and the rear tires, you should be able to calculate it pretty roughly. Even if you don't know the "height" of the center of gravity, you can still "assume" a height, and then change that height by the decrease in height from the modifications made, and get a percentage "improvement".

My gut is telling me that this would be a straight relationship. For forces "laterally" sliding around the track, the moment on the tires would be the height of the COG times the weight.. As stated above, you could do a finite element analysis, but you would have to know the weights of each portion of the car, the weight of the driver, the amount of fuel in the tank, etc. And all of that would change with different drivers, weights of fuel, etc.

It would therefore be better to put it as a "percentage" improvement of the original design.

One last caveat......It is a "straight" relationship UP TO THE LIMITS OF THE TIRES. The tires are always the limiting factor in cars. For example, a Dodge Viper might have great power to weight ratio, be very low, etc. The tires are never up to the task of providing traction. So, while you may have a theoretical increase for infinitely sticky tires, the reality is that you only get improvements for so much.

 

I haven't made a great deal of headway on my research as of yet but I did find a couple of references that suggested that the center of gravity [COG] is approximately at the level of the bumper. Sounds reasonable. Our front bumpers being about 12 inches off the ground and the H&R Sport Springs lowering the car 1.2 inches leaves us with a decrease in COG of 10%.

Although I live in a small Southern California town with limited opportunities to come anywhere close to reaching the car's handling limits, I can tell you [anecdotally] that lowering the car has made what seems like a significant difference [laterally].

Near my home, there is a right turn [slightly descending grade] I often make onto a four lane road where I can be reasonably aggressive. Acknowledging that the turn is a slight down slope, I was still disappointed that the car exhibited what I felt was an unacceptable degree of body roll. The other day when half-way through this turn I realized that I felt practically no body roll at all. It was amazing!

So I am now more curious than ever to attempt to quantify this change. With the improvement in appearance and an apparent significant increase in body roll resistance, this mod is proving to be a great upgrade. I am now waiting for my wheel spacers to be fabricated [15mm fronts and 17mm rears] to finish off the look and perhaps fine tune the handling a bit more, as well.

 

Note that the tires, wheels and brakes are not lower, so your COG will drop less than your number.

 

Also, the spring rate is higher so body roll will be less.

The change in geometry due to a lower static ride height is something to consider. That willaffect the "roll center," although I'm going into things I studied around 40 years ago. Since I've never designed a suspension from scratch I've been more interested in vehicle-specific changes since then.

 

Point taken. Let's call it 9%.

 

Another great point taken. Thinking about that, it's probably the most important factor because the difference was really significant. It is becoming more and more apparent that I will not be coming out of retirement anytime soon to start a second career as an engineer!

In any case, I'll just have to factor spring rate into my formula.

 

Best way would be to have taken it to the track before and after under similar environmental conditions. Handling is not just about COG - you could have springs dramatically lower COG but be severely oversprung. Body roll under cornering is not necessarily a bad thing - it may feel better when the car corners with less roll but at the limit it may not actually be better at the limit. Too many variables - including how it changes F/R grip.

In the end, for me and what I do with the car:
- I enjoy how it makes the car feel, slightly more visceral, flatter, a bit more responsive. I also modified my front alignment to be a little more aggressive than stock.
- I really enjoy how the car looks after.
- I trust reputable companies like VAP and H&R that they put work into designing springs that will perform well.

Beyond that, it boils down to empirical testing - anyone have before/after experience with lowering springs at the track?