Inside tires are far less important when performing this comparison for production cars, because they aren’t doing much. \u00a0See below, a pic from a private skidpad test day I had with the 240sx. \u00a0The inside rear tire has very little load on it:<\/p>\n
A similar story in the front (ignore those brackets, part of a splitter I tested later)- not much weight on the inside front. \u00a0You can see the dark band where the tire has some heat in it-<\/p>\n There’s other things we want to do with our setup – we want to make sure our outside tires are “happy” when we are producing peak lateral grip – if they are anything less, the car won’t be able to reach its cornering potential.\u00a0 This analysis assumes the cars under consideration are both equally able to make their outside tires “happy”. \u00a0There are several ways to ensure your tires are happy, and doing so is the aim of the handling and suspension tuning books. \u00a0I won’t go into any detail here to ensure your outside tires are at their best, but pictures like these, even those taken at a regular event, are a great way to validate you’ve got things working right.<\/p>\n But again, tuning is not the mission of this series. \u00a0The mission is to evaluate the skidpad potential of two cars, taking into consideration only their immutable physical characteristics in race trim – CG height, weight, wheelbase, track width, and maybe, wheel\/tire sizes. \u00a0Like the second half of our longitudinal acceleration analysis, we aren’t going to be able to come up with any hard-and-fast numbers here. \u00a0We won’t know exactly what lateral-g our car is going to pull on the skidpad, just from crunching numbers. \u00a0There is a tuning element involved as well, and we aren’t going to know what the overall fastest balance would be here either.<\/p>\n What we *can* figure out, it which car is going to have its tires in better shape at a given lateral g – just as with longitudinal acceleration, we could figure out the mu<\/em> needed to achieve different longitudinal accelerations. \u00a0Because in this case our inside tires will be doing some contribution, we won’t be coming up with a mu<\/em> – we’ll just be looking at which car has less weight on its outside tires at a given lateral acceleration.<\/p>\n I’m going to state up front how our dimensions affect skidpad performance:<\/p>\n The first two may be intuitive by now but the third may not be. \u00a0Let’s start with the easiest and most important ones first – wider track and lower CG height<\/strong>.<\/p>\n As explored previously, the amount of weight transfer our car experiences is a function of only three things – CG height, track width, and lateral acceleration. \u00a0 It doesn’t matter where your roll centers are or where they went, what angles you have on your control arms, whether you have a live axle or independent suspension, whether it is AWD, RWD, FWD; it doesn’t matter what tires you have, how far the car is leaning, what hemisphere you’re in, or what you had \u00a0for breakfast. \u00a0CG height, track width, lateral accel.<\/p>\n We know minimizing weight transfer increased overall grip for an axle, and our goal is to produce the most lateral accel we can.<\/p>\n weight transfer = (weight * lateral accel * CG height)\/(track width)<\/strong><\/em><\/p>\n So to make the weight transfer number lower, we need to shrink the numerator and increase the denominator (for those that don’t remember algrebra, numerator = the stuff on the top of the fraction, denominator = the stuff on the bottom). \u00a0That means, a decrease in CG height, or an increase in track width<\/p>\n Sidebar – J-Rho’s Ride Height Tuning Editorial: \u00a0Getting a low CG height is very important. \u00a0In my opinion, too many people sacrifice CG height in the pursuit of other things that aren’t as important. \u00a0Take “corner weighting” for example. \u00a0Let’s say you have a car that’s at its optimally low height from a bump-handling perspective…and you want to corner weight it. \u00a0Since it is already low as it can go at all four corners, your corner weighting will only do one thing to the CG – raise it! \u00a0Often times, by a lot, 1\/2″ or more. \u00a0In my experience, corner weighting just isn’t worth it if you have to raise the car higher than it would otherwise have to be.<\/p>\n How low should I go? \u00a0I see people concerned all the time – “but J-Rho, my lower control arms are no longer parallel to the ground” – so what? \u00a0What matters is that your outside tires are happy at terminal roll. \u00a0If your suspension starts to get bad geometry when really really low, that’s a bummer, but you should still be able to make the outside tires happy with enough static camber and caster. \u00a0Make it low, make it stiff, stiff enough that the bad geometry never has a chance to manifest.<\/p>\n What about bumps? \u00a0First of all, if I had a nickel for every racer who had a problem because of bumpstops, I could retire! \u00a0Throw your bump stops away (assuming doing so is legal in your class rules :)) \u00a0There are only two legitimate bump stops in my mind: 1, something in the suspension (preferably the tire) hitting the chassis, or 2, the chassis hitting the ground. \u00a0“But my shocks will bottom out” – get shorter shocks! \u00a0“But my springs will coil bind” – get shorter springs! \u00a0If you *must* have bump stops, just make sure they’re only there in the last instant before the other bad bump thing happens. \u00a0I simply can’t believe how many “racers” set up their cars, drive around slow and frustrated, only later to discover they were riding on the bumpstops the whole time, and use that as some kind of reasonable excuse. \u00a0“Found out I was on the bumpstops” – well no duh! \u00a0Did you think that 1\/4″ of travel was going to be enough? \u00a0Did you even look at the shock after you set the car on the ground? \u00a0It’s not like some bumpstop gnomes came out and put them there – YOU did, you are responsible for the mess.<\/p>\n How do I know if I’m too low? \u00a0You’re too low when your time would be faster if you were higher. \u00a0\ud83d\ude42 \u00a0That’s kinda nebulous, so think of it this way. \u00a0If the surface you raced on were perfectly smooth, you could have the car very very low. \u00a0You would have it set so that the front suspension only reached its bump point when at or near its most extreme condition, while braking and turning simultaneously. \u00a0Similar for the rear, it would only reach that point at its extreme of turning and acceleration. \u00a0You might have it set up to rub\/grind a bit even in that situation. \u00a0You have to look at the car being one of two modes – “normal” operation, and “duress” operation. \u00a0Duress would be when one or more corners of the car have gone into the bad bump state, enough that the car, at that moment, is slower than if the ride height had been set higher. \u00a0 The lower you go, the faster the car will be while not in duress, but the more often it is likely to get into duress, and the greater the severity (from a time lost perspective) of the duress. \u00a0So the answer is, it is okay to hit whatever bump condition you have, but it shouldn’t be so often, or so bad, that the car would be faster on the average for the whole lap, were it raised. \u00a0The ideal ride height (and by relation, spring rate) will depend on the surface you race on – making it important to test on the same surface or one just like it.<\/p>\n Almost everybody I know has their car too high, or at least, don’t adequately prioritize CG height. \u00a0If my competitor has better a-arm angles, better corner weights, better working geometry, but a higher CG height, he’s going to be slower than me around the corner because he’s going to have more weight on his outside tires.<\/p>\n I am an especially big believer in minimizing lateral weight transfer on heavy or undertired cars. \u00a0This picture, of a modestly prepared first-gen Camaro (again, the Camaro of Liz Miles from http:\/\/www.milesspeed.com<\/a>) next to a prepped ST Honda Civic, is part of what made me think the Camaro had a shot. \u00a0I was able to get the 240sx lower than the Civics, and believe I can do the same with the Camaro.<\/p><\/blockquote>\n Okay, enough editorializing…<\/p><\/blockquote>\n So, lowering CG height, or increasing track width, will decrease weight transfer across the car, which will increase the possible output from the tires. \u00a0These are Good Things. \u00a0Keeping more weight on our inside tires will help them do more to get the car around the corner better, will reduce the work the outside tires have to do, and especially on the driven end, will help us accelerate out of the corner and put less dependence on our limited slip.<\/p>\n In the next chapter, Transitions, we’ll find that width is our enemy, but here in Skidpad it is our friend. \u00a0On our example course we saw skidpad and transitions each comprised about 40% of our total time on course. \u00a0Some courses will have more, others less. Because width isn’t something we can generally adjust on our car, if our car is wider than the competition we’re looking for courses with lots of sweeping turns and few offsets; vice versa if our car is narrower.<\/p>\n The wheelbase<\/strong> portion of this is not so obvious. \u00a0A diagram helps and instead of spend hours in MS Paint, I’ve gone the quick and dirty way, and snapped a photo of the relevant photo from Milliken & Milliken’s Race Car Vehicle Dynamics<\/a>. \u00a0Hopefully they don’t mind me using it here – perhaps they won’t go after me for copyright violation if I can help them sell a few more copies. \u00a0The book is full of great images like this, consider picking it up!<\/p>\n Some things to take away from this drawing-<\/p>\n 1, a longer wheelbase car will require more steering angle to generate the same radius turn. \u00a0In general, we don’t want to have to turn the steering wheel any more than we have to, and autocross has plenty of small-radius turns;<\/p>\n 2, the longer the wheelbase, the larger the gap in radius between the front and rear axles, with the rear axle’s radius getting smaller and smaller as the wheelbase lengthens. \u00a0This makes the car trickier to drive “clean” and is the reason why so many drivers hit cones with the inside rear of the car – also why on the street, people run their inside rear tires up and down curbs coming in and out of driveways. \u00a0You have more vehicle to “get out of the way” before you are actually past any given element or cone.<\/p>\n Another thing –\u00a0 a longer wheelbase vehicle, given equivalent overhangs and distribution of mass along its length, will have a higher polar moment of intertia. \u00a0While a higher polar moment is good when we are concerned about rotational stability (as we might be on a high-speed track), it comes as a detriment in autocross. \u00a0Here, we need to be able to change direction quickly, and the higher our polar moment, the more yaw (rotation of the car as seen from the top) force required to achieve a given amount of rotation. \u00a0This is compounded by #1, extra steering needed for a given radius, making the long wheelbase a double-whammy in skidpad. \u00a0Remember in skidpad, we aren’t just generating lateral force – every 1\/4th of our way around, we also have to change the direction the car is heading by 90 degrees.<\/p>\n Static weight distribution<\/strong> is best if it’s somewhat near balanced front to rear. \u00a0The ideal of wide track and low CG for skidpad accomplishes one thing – it keeps the inside tires most closely loaded to the outside tires. \u00a0But what about when we twist our viewpoint 90 degrees, from a front view, to a side view, and consider the front and rear axle pairs? \u00a0Assuming equal wheel and tire sizes front to rear, we should also be trying to keep the front and rear axles as evenly loaded as possible to maximize grip – our limiting factor in skidpad performance will always be our single most heavily loaded tire.<\/p>\n Try as we might to minimize lateral load transfer, we can’t eliminate it completely. \u00a0However, unlike with longitudinal weight transfer in acceleration, with lateral load transfer, we can build-in some asymmetry in “roll rates”, to tweak the manner in which load is transferred from inside to outside.<\/p>\n For instance, if our car is naturally front-heavy (as is the case with almost every production car made), \u00a0we’re going to be worried about our outside front tire getting overloaded in our skidpad – the dreaded understeer. \u00a0We know how much total weight we are going to transfer from the inside to the outside, but we can adjust how much of that load goes across the front axle, or the rear. \u00a0In the front-heavy case, we might want to give the rear of the car more roll stiffness than the front, so that as the car builds up cornering force, more weight is transferred across the rear axle than the front. \u00a0What this does, is slow the buildup of load on the already-overloaded outside front tire, helping grip from the front axle. \u00a0That extra load instead goes to the outside rear tire, giving it an extra drop in grip efficiency, “hurting” that axle’s grip.<\/p>\n![\"\"](\"http:\/\/www.rhoadescamaro.com\/build\/wp-content\/uploads\/2011\/01\/IMG_6069.jpg\" "\\"IMG_6069\\"")
<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n\n
<\/a><\/p>\n<\/a><\/p>\n