The Long and Short of Downforce

[BTRobertson]

If anything in here is incorrect, please let me know and don't flame. I'm learning a lot about this stuff and at the very least figured it would help to get all this info into one spot so we can discuss it, etc. Enjoy!

Thanks to HighRev1 (props buddy), I've been doing a TON of research on suspension, handling, tire traction, and all other manner of characteristics that affect the way we drive our 8th gen Honda's. I came across many, many articles and one common thread ran through each of them: downforce, mechanical or otherwise (like aerodynamic, for you spoiler lovers out there). How does it affect tire traction and go counter to the assumption that "more weight = more traction"?

So, I figured I would give you all some of knowledge acquired not only through driving, but through expanded study of the physics of downforce. After all, when we enter a corner in our cars, mechanical and aerodynamic downforce on the tire contact patch is applied. HR1 made reference in his sticky "Suspensions" that when you increase the load, you decrease traction. I thought "What? How can that be?" But, you'll all come to learn why it's true and why the most grip is realized when your car is sitting at rest in a parking lot - a FLAT parking lot, of course. Or, I would also argue (I could be wrong here) that the car could be moving, but along a perfectly flat surface with no rises or falls - in this situation, the load on the tires would be equal and also have the most grip, but it's the same concept. You couldn't park your car on a hill and have the most grip...we're talking equal distribution of weight.

First, I'll define Downforce, the subject of conversation here. Downforce is the vertical loading on your tires, whether that be mechanical loading (the weight of the car due to gravity, essentially) or aerodynamic loading (the downforce applied when air causes your car to apply vertical load onto the tires using carefully placed spoilers or other aerodynamic features).

One demonstration of mechanical downforce is to drag your pencil eraser lightly across your paper. Pretty easy, right? Now press down on it from above and drag it - not so easy anymore. More vertical load, more friction. But hold it, before you start to yell at HR1 for his alluding to "more weight/downforce in a corner, less traction", read on...

In our experiences on the road, lighter cars tend to handle better than heavy cars. Yep, think about it. Why is it that a heavy car has much more of an issue handling hard cornering well where a lighter car almost always handles corners better - again, variables are present, but generally speaking this is the case. Why do our 8th gen FG2/FA5s rip corners where most stock Mustang GTs wipe out or worse around the same bend at the same speed with larger tires? Yes, suspensions and drivers play a role, but I'm being general here.

When you increase weight on tires, you increase your traction potential. While this is a true statement, there's another part to this: you also increase the amount of WORK the tire must do when weight increases on them. When a heavy car corners, even one with a great suspension and all the lovely hardware in tow, the work required by the tire's contact patch to keep this car on the road is incredibly high. In fact, if you were to sketch the relationship between the increase in traction versus the increase in work given the increase in weight on a graph, you would see that work load increases at a faster rate than traction gains do.

Click here to see a great article on weight transfer formulas, graphs, graphics, and other great info on the topic: Weight transfer by Circle Track Magazine

Lateral Load:

Even in our lighter cars, this principle applies. Every car, regardless of its weight, will encounter lateral load on the tires in a corner. A larger car usually has a wider, larger tire, but it's only to compensate for the increased work load on the contact patch. Increase the size of the contact patch, you increase the Coefficient of Friction. Notice our car comes stock with 215 width tires? That's the size deemed fine to handle the stock lateral load given the weight of our cars. Note: a 4,500 pound VW Toureg comes with 245 or wider tires, I believe. Why the wide tire? Should be obvious by now. 4,500 pounds of lateral load is far harder for the tire to keep traction with than 2,900 pounds of Honda. You couldn't put 215 tires on a Toureg and have it be stable whatsoever; you can put wider tires on, as long as they fit the body of the car, of course, but smaller, you never would.

This is why HR1 makes note about using 225 width on our cars folks!!! We're stuck with the same lateral load due to the weight of our cars, but INCREASE the contact patch of the rubber and the traction ratio INCREASES versus increase in work given the mechanical weight of the car! There's also an argument that wider tires reduce gas mileage...again, should be obvious to you by now that this is true. You're increasing friction! More friction = more work to overcome the Coefficient of Friction, since the force required to overcome friction increases as friction increases. The force required here is your engine...I digress.

Another question: ever wonder why your car (or any car) feels "mushier" when you have more people in it? Increase the weight means increasing the lateral load, means gaining traction at a SLOWER rate than the work load increases on the contact patch of the tire.

This is why HR1 says that if you can reduce body roll, you decrease the rate at which weight is applied to the contact patch. Looking at our graph again, if you decrease the rate at which weight is applied, you are decreasing the rate at which lateral load is applied and therefore increasing the traction.

How do aerodynamics come into play now?

A rear spoiler wing increases the downforce on the rear tires at higher speeds. Front spoilers do this for front tires. So how does this help with cornering since you're increasing the weight on the tires? You're not increasing the lateral load on the tire patch because you're not adding mass to the tires, you're adding vertical downforce. Therein lies the greatest benefit of aerodynamic improvements: the weight that's loaded on the tires can't be translated into lateral load because the lateral load is in direct relation to the weight of the vehicle.

This means increased traction WITHOUT increased work.

Weight reduction:

Ever wonder why some people are driven mad with the concept of lightening their cars? Guys, fact is, every small amount helps, even if only slightly. If you decrease the weight - do I have to go over this again? - of the car, you're decreasing the overall mechanical downforce on the tire's contact patch. You're also allowing for the force required to overcome the COF - THE ENGINE - to work LESS. Weight reduction is another factor when looking for a better-handling car, but again, it has to be done smartly, not hastily. Balance is the key.

Slip Angle/Slippage:

I learned quite a bit about slip angle as well. Maximum traction potential is reached when slippage occurs in a small amount. But, braking, accelerating, and cornering are all translated differently for slippage because the Coefficient of Friction (COF; i.e. grip) are affected by all of these. For example, if you brake too much in a corner, you're using more grip for braking than cornering and you'll lose traction. See how the driver affects your car more than anything else in most cases? As we've said before, you could have all the latest and greatest hardware and tires on the planet, but if you turn sharply in a corner or brake too hard/too little, you can and will experience harsh affects in your spirited driving.

The bottom line is that there are critical choices with a) larger tires (more is usually better, but not always), b) the COF provided by the compounds found in the tire, and c) the amount of downforce applied to the tires.

As with anything, more is NOT always better, but it's critical to understand WHY more is not always better. It's also important to know what kind of driver you are - smoother drivers have smoother handling cars, period. If you enter a corner and brake suddenly or jerk the steering wheel too hard, you'll lose traction guaranteed even with a $5,000 suspension system with 16-way adjustable dampers and 22-way adjustable this and tires made of the stickiest rubber known to man.

[Highrev1]

Couple of things to add....Lemme get them together on WORD first so I can spell check

[Highrev1]

[redline8500]

awesome btr awesome...

[CanyonDriver]

Great write up

[Superjudge]

Very good post, made for some good reading and new info on my part.

[BTRobertson]

Thanks all, I'm glad it's helping. There's so much more about slip angle, too; lots of math involved, so figured it would be too boring. It's very interesting, though, because slip angle is directly involved with how perpendicular forces affect cornering and all this stuff. Amazing. Never thought I would enjoy physics so much.

[Superjudge]

My buddy races an E36 M3 and we eat drink and sleep this stuff and everything suspension related on track weekends, its really amazing to see what can be learned by just the size of a tire contact patch and suspension perches.

[BTRobertson]

Yeah, and it's no wonder there's so many choices and variations of setups to try to maximize those four small patches. There must be literally hundreds of things to take into consideration, that to simplify it the way I have in the past is to discredit the complexity of the systems involved with traction. I figured I could slap on a rear sway, camber kit, and some new tires and go have at it. Sure, I could do it a little bit, but there are so many factors that it would take a ton more experience to get my car's suspension tuned 100%...it's amazing what some reading will do for someone.

[Superjudge]

Originally Posted by BTRobertson

Yeah, and it's no wonder there's so many choices and variations of setups to try to maximize those four small patches. There must be literally hundreds of things to take into consideration, that to simplify it the way I have in the past is to discredit the complexity of the systems involved with traction. I figured I could slap on a rear sway, camber kit, and some new tires and go have at it. Sure, I could do it a little bit, but there are so many factors that it would take a ton more experience to get my car's suspension tuned 100%...it's amazing what some reading will do for someone.

I figure the best track/street setup without a budget would be coilovers, full bracing, camber plates and sticky tires on light wheels. I had a nearly perfect setup on a fomer car, an MR2 spyder and tuned correctly, it was amazing on a road course. Of course in daily driving, especially in NJ with the bad roads, it was not optimal. I would go with basic bracing, shocks with progressive springs and some sticky tires on a basic FWD setup such as the civic with struts.

The use of struts by honda really surprises me in this day and age. I'm purchasing an EX tonight and by the test drive which was fairly long, I really couldn't feel any immediate differences between the strut and independent suspensions of the other cars I drove.

Anyway, not to drift off topic, downforce and suspension tuning are both a science and methodical processes which do not call for a bleacher seat wing on a FWD chassis lol.

cheers.

[20strong]

I am happy with 235s on my Si and I havent noticed any REAL decrees in Gas milage. Just for referance.

Nice write up some valuable info here.

[BTRobertson]

Originally Posted by Superjudge

Anyway, not to drift off topic, downforce and suspension tuning are both a science and methodical processes which do not call for a bleacher seat wing on a FWD chassis lol.

I was actually going to say that. Those huge wings don't help you at all for daily driving. I know there's an outlandish styling desire for most people who have those wings, but functionally they don't help much more than the stock wing. There's a whole thing on aerodynamics that needs put in here too because there are some wings like that which actually work against proper spoilage of air for vertical downforce.

I think the best one is the Mugen/Type R ones...they're not stupid high but are more functional because they work with the rest of the under- and side-spoilers. Aerodynamics is - once again - about balance and how the parts work in concert with one another. Just slapping a huge wing on doesn't mean you'll get positive downforce to your rear tires.

[s0opa_Si]

thanks! i'm going to print and subsribe this! i like learning about suspension!

[Superjudge]

Correct sir. My friend is a club rally driver and in the cases of the high wings on the STI and EVO, the center of gravity and pivotal points of handling are improved on the car, keeping it level around turns and improving the car's ability to 'slide' on its center axis with both front and rear differentials engaged.

I have been around amateur motorsports for almost 8 years now, from club road racing to rally and autocross, and what is dubbed as 'entire' car setup wins races and stages, not overall horsepower in some cases, escpecially in the autocross realm.

[BTRobertson]

Sweet, then you can also help me out if I get something wrong. My abilities are less in the practical application of this stuff and more in the written word portion - I know how to take information and communicate it back, and don't mind writing long-winded posts. I'm a novelist on the side, so that's probably why. I wish I had more track experience, but maybe one day that will come. For now, I can only expound theory to prod discussion between people WITH experience (like yourself, HR1, etc.).

[Superjudge]

well thanks for the credit, but I'm not fully exprerienced with suspension engineering, I'm more familiar with it from having different setups on a few different cars and changing setting or parts and then in some cases changing them again. I have played around with hobby cars such as MKII and MKIII VW's, my girlfriend's stage II MKV GTi, a former MR2 spyder and even a Volvo 240 lol. I'm a big fan of grassroots racing and HPDE's.

I am buying the civic EX for daily driving although I know I will add a thicker progress rear sway and maybe a shock/spring package down the road as soon as I see what works best here on rough roads. I had the progress bar on my former mazda 3 and it worked effortlessly. I never removed it from the stiffer setting either even with snow tires, but due to the poor quality of the car, never got to play with the suspension or bolt ons.

I like to contribute the little I know though lol. There must be people here who know more than me, I wonder if anyone here has experience in the NASA racing Honda challenge? That person would be a bank of knowledge on the subject.

[BTRobertson]

Let me venture a guess at what HPDE means: High Performance Domestic Engines? I'm probably so far off. LOL

But yeah, I think there are folks here with racing knowledge, but Honda Challenge experience, not sure on that one. They'd be PEPPERED with questions, if these thread are any indication. For crying out loud, there must be several header threads posted each week asking the same damn questions. LOL

[Superjudge]

haha yeah I've seen all the exhaust, intake and header topics here and the many on most other forums.

HPDE= High Performance Driving Experience. It's a track day and classroom education in your street driven car for beginners. There are instructors who guide you on cornering and driving skills and it's the entrace way to competitive motorsports. There are schools all over the US who offer them and I have taken a few classes in past cars. Deciding on the level of class, you can pass people and be more competitive while driving on a real road course. At one point, I wanted to get my amateur liscence through a school.