Very long answer to 4the4's question & why MMM's struggl

MMM Fans > General Discussions > Very long answer to 4the4's question

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gofour Unregistered User (8/8/02 12:37:25 am)	Very long answer to 4the4's question As simply as I can explain it. The race car should be viewed as a flat table with four legs (one at each corner). Now imagine a drop of water on the tables surface. If you cut off one of the legs of the table then the water will run to that corner. Weight is the same. If you lower a corner of the racecar then you will place more weight on that tire. This is done by compressing the spring with a jacking bolt at that corner of the racecar. This is the concept of (ride height/corner weight). Now remember that a racecar undergoes multiple forces when in motion in a corner. One force - centrifigal - is acting on the racecar so as to push it from the inside of the track outward toward the wall (tie a weight to a string and swing it - the weight will pull outward), a second force is the straight line momentum of the car itself from the rear wheels pushing it from behind. The result of these two forces is called the resultant vector. If you can imagine a large box sitting on the floor and one person pushing it from behind and another person pushing it from the left side the resultant vector would be forward and to the right. Same for a racecar - the natural motion of the car in a turn would be for the right front corner to head toward the wall. This is the natural resultant vector. Controlling these two concepts used to make up the foundation of nearly 70% of a race cars handling. The trick was to make the race car turn against the resultant vector using weight distribution. Think about that box again - how would the angle the box moved be changed if someone was pushing down from the top? What would happen if someone pushed down hard on the right rear corner of the box while it was being pushed from the rear and the left side? (It would be pushed clockwise in a big circle) What would happen if someone pushed down hard on the left rear corner of the box instead? (It would turn counter clockwise in a big circle) As you can see the amount of pressure applied to each corner of the box could be adjusted to make the resultant vector change. This is what is done with ride height (by compressing the spring) to get the car to turn. Try it with a box and the folks at home and you will see what I mean. Now lets short-cut this discussion to say that through trial and error it has been determined that pushing down on the left rear makes the car turn best. For years that was the secret to handling - get the most left rear weight possible. Then it was discovered that this could be overdone and teams started to learn that if they compressed the oppisite corner of the car they could control how much the car turned. This is the concept of crossweight also known as wedge. If you weigh the car on scales placed under each tire and add the weights together you determine the total weight of the car. Now if you compress the left rear spring you will add weight to that corner and make the car want to turn but if you compress it too much it will turn too much (oversteer also called loose). To compensate for this you compress the opposite corner (right front) more or less until you can controll the amount the car turns. Add the weight of these two corners together then divide by the total weight of the car and you will get your cross weight percentage. The cars used to use between 51% and 56% crossweight (or wedge) depending on how tight the radius of the turns were. During the race the crew would either compress or decompress one of these springs to add or reduce the cross weight in the car (more if the car was loose less if the car was tight). This seems all so simple until you put the car in motion and it begins to experience weight transfer. Racers soon noticed that the weight would move around on the cars when they were in a turn going fast. They do all sorts of things like weight naturally moves from the left to the right of the car (you probably experience it as body roll on the family car) in a hard turn. It also moves from the rear to the front when entering a turn under braking and back to the rear again when exiting a turn under accelleration. Soon it was discovered that a stiffer spring in the right front would resist this weight transfer and help keep the weight on the left side where it was best. Stiffer springs in the front would resist weight transfer from the front to the rear, and so on. Through trial and error the best set-up for each track was determined. Best springs, wedge%, front/rear% etc.. The Wood brothers went to Indy to pit Jim Clark in the 60's. They noticed that the Indy cars were using tires of a larger circumference on the right side than the left. Now imagine an ice cream cone setting on its side on your table at home if you give it a push it will naturally turn in a circle. Thats what larger outside tires did for the race car. The difference in circumference between the inside and ouside tire is called stagger. Stagger changed the game. This allowed the car to turn easier which allowed more speed to be carried through the turns and now the cars would generate enough centrifigal force that the cars were going beyond the traction limits of the tires. The amount of front or rear weight would be adjusted to place more on the front or rear to help put more on the tires that were braking traction first. (different today as you will see later). People became creative. Cars were constructed with large amounts of left side weight to help them turn, bodies were lowered to keep the center of gravity down to reduce body roll, rear axles were shifted so that the outside tires had a longer wheelbase than the inside tires making the rear end push the car in a counterclockwise direction, etc..etc..etc.. These things are still done today but are regulated by the rulebook to keep things within reason and the racing more even. Then came a huge revalation. Weight transfer was best controlled by the shocks. The teams started using shock dynos to measure the weight transfer charateristics without having to go through so much trial and error on track testing. The first folks to master the shocks had a big advantage on the rest of the field. Tires changed from bias ply to radials. The radials didn't change circumference much with pressure changes which removed the adjustability from stagger. Teams started to concentrate more and more on adjustments like camber to help cars turn and brake bias came into play. Many of the simple concepts I discussed earlier did not work in the same ways any more. The results of the old methods didn't yield the same results. One such example is the question you posed regarding front weight. Used to be that adding front weight would help keep the front tires from sliding across the race track and lower the traction limit of the rear loosing the car. Suddenly more front weight caused the car to get tighter instead. Cars had become so good at turning especially with the addition of power steering and much improved sidewall construction the extra weight caused a condition called slip angle. You see this when you reach in your car from the outside and turn the wheel while at a stop. You will notice that the wheel (rim) will turn a few degrees before the tire starts to rotate.The side wall is flexing. The more front weight you add to the race car the more the tire's sidewall will flex before it turns. This means the driver has to turn the wheel further and further to make the car turn before it actually does. The racecar driver feels this as a push or understeer. Things like slip angle (sidewall flex) can not be explained using the simple model I began this discussion with. It is a variable called dynamic state physics. The simple model most people use when trying to understand how a race car works (the one I started this loooong explination with) is called static state physics. Many many teams at the local track are still using static state methods and they walk around the pits confused and are usually easy to beat. I am totally confident that our boys at MMM have an excellent understanding of dynamic state physics as it trelates to the racecar. All of what I have mentioned so far is largely mechanical. Racing today is no longer ruled by the mechanical. Todays game must incorporate the aerodynamic world. Unfortunately many of the dynamic state mechanics that I have described do not work they way they used to in the new aero age. Many Winston Cup teams are now using 600lb springs in the right rear and it is the heaviest spring in the car. This is contrary to anything we have discovered using static or dynamic reasoning. Guys like Sterling Marlin even admit that they do not fully understand why things work the way they do now but you can bet there is a team of engineers that do on his team. I full expect that MMM must make the transition from dynamic mechanical thinking to the new world of aero in order to become competive again. This is the reasoning behind my constant call for R&D efforts, increased engineering involvement, and much much more data acquisition. There are things going on now that are simply not understood and the old methods (hard work or not) will no longer work. Sorry for the length of this but hopefully my one or two sentence comments will make more sense to all of you in the future.
gofour Unregistered User (8/8/02 12:48:03 am)	Oh and... Oh and another thing. My spelling doesn't count after midnight.
bobbyfan55 Registered User Posts: 15 (8/8/02 8:00:55 am)	Re: Oh and... gofour, Super info here. Thanks. The examples are great and exactly what a non-techie like me need to understand the concepts we hear about all the time. This has helped me understand even more the challange all the teams and drivers are up against in the sport today. Thanks again. I'm even going to print this out for future reference. Ruth
ZNAKOMI Administrator Posts: 1553 (8/8/02 8:11:07 am)	Re: Oh and... Thanks for the GREAT post gofour. Not only a great synopsis (I'll read it a couple more times), but a great HISTORY lesson too! One thing that surprised me...air pressure. Is the PRIMARY reason for air pressure to change the diameter? Diameter has more effect than anything else? Awesome post, many thanks for taking the time!
VT1 Registered User Posts: 55 (8/8/02 8:19:41 am)	Great info! Good stuff there Gofour..........I've printed this out also. Thanks.
4the4 Registered User Posts: 542 (8/8/02 8:23:37 am)	Re: Oh and... Gofour, awsome post buddy! I'm still struggling with understanding the dynamic state physics. so I have no idea whats up with the areo stuff. I learned something with that post. IT must be hard to find people that understand this What other series would use it? BTW if you caught the featherlite mod tour race on speed channel from new hampshire the other night the guy that won it is my next door neighbor. Leaty. 4the4
Smilin4Mike Registered User Posts: 31 (8/8/02 8:38:29 am)	Re: Very long answer to 4the4's question & why MMM's str Hey, could you send that to Larry? Thanks for posting that. One of the reasons I like coming here - I really learn a lot about racing, and all the particulars of it.
gofour Unregistered User (8/8/02 12:28:04 pm)	Air pressure Zank, I am assuming you are talking about air pressure in the tires. When the cars used bias ply tires the circumfrence of the tire could be changed a great deal by adjusting air pressure. This was how you could achieve anywhere from 1/16' stagger to up to 2'' depending on how much help the car needed to turn. When the radial tire was introduced this was no longer the case. As you know a radial tire is constructed with a very strong belt around the circumfrence of the tire. Now additional air pressure does very little to change the circumfrence of the tire. Hence, the role of air pressure has changed. There are two types of weight on a racecar. Sprung and unsprung. Sprung weight is anything on the car that is above or supported by the springs. Unsprung is obviously any weight that is below or not supported by the springs. As you can imagine the sprung weight moves around on the car during weight transfer. The unsprung weight is fixed and does not move. Therefore changes in unsprung weight make for a much more predictable results. The simple concepts of static dynamics are 80% to 90% correct with unsprung weight. Air pressure in the tires is now used to effect unsprung weight. Lets think of a radial tire as a spring. The more air in the tire the stiffer it becomes, the stiffer it becomes the more it resists weight and the more it places on the other tires. If the right rear air pressure is increased then this tire will have less unsprung weight placed on it and will break traction more easily. This is done to help a car that is too tight (understeering). Air pressure removed from that tire will add weight and therefore keep the tire from slipping as much this is done to help the car that is too loose (oversteering). Our old friend slip angle plays the largest huge role here as well. The more air pressure in a tire then the stiffer the sidewall. The stiffer the sidewall the lower the slip angle which makes the tire more responsive to turning. Some prefer to think of it this way - the more air in the tire the less the tire can be compressed and the smaller the contact patch. The less air the more the tire can compress and the greater the contact patch. More contact patch less tire slide, less contact patch more tire slide. Often we hear about small air pressure changes the crew makes during the race. These changes can be small and still be effective because each 1lb of air pressure changes the tires spring rate by close to 10lbs. Now 10lbs does not sound like much until you consider the fact that the unsprung weight of the car is low. Lets say that only 15% of a car is unsprung weight. A 3400lb car * 15% = 510lbs unsprung weight. Now a 1lb change in a tire equals 10lbs. Thus a 1 lb air pressure change would yield 10/510 or nearly a 2% difference in unsprung weight weight distribution. Add the effect on slip angle where a 10lb change in air pressure is very substantial - add or subtract 10lbs on your car at home and see for yourself (Don't drive it like this; but turn the wheel while it is stopped and watch the difference) and the results can be dramatic. This is the most predictable change and is usually the first adjustment made before the larger things are tried. Air pressure is only a tuning device but it is quite effective. They usually start with a change in the right rear and then go from there.
gofour Unregistered User (8/8/02 9:40:28 pm)	Leaty 4the4, Are you talking about Jan Leaty? I remember him from more than 20 years ago. He was just starting out in the strictly stock division at Spencer and I was impressed with his focus then. He hadn't started driving Modifieds yet but had one under construction. I think that Troyer noticed his focus as well. Last I knew he was going to drive a car for some outfit that was going to be the factory team for Troyer using Maynards number 6. Is that what he won in?
ZNAKOMI Administrator Posts: 1555 (8/9/02 7:54:16 am)	Re: Leaty Thanks gofour. So I gather air pressure pushing unsprung weight off a tire, or onto it, is like a predictable "global" correction over what is happening with the sprung weight. I'm guessing also that it takes on, or pushes off, the sprung weight as well, but not so easy to calculate the sprung weight effect due to the motion of the sprung weight. Ya know, I never considered air pressure doing something to the weight distribution, I always thought it was only acting on the contact patch. Great information gofour, thanks for taking the time. Any other info you may think we're missing...we probably are! I'm guessing that the slip angle induced by lower pressure is what creates "sawing" on the wheel?
gofour Unregistered User (8/9/02 11:38:20 am)	air pressure Zank, You have it down. You are correct on all counts.
ZNAKOMI Administrator Posts: 1556 (8/9/02 5:36:43 pm)	Re: Very long answer to 4the4's question & why MMM's str gopher, If ya wouldn't mind continuing this very interesting crash course... What about the case today at the glen (in Don's post), where theyre in practice (and can screw with anything), and they're tight in, loose off... Does this imply a kind of balance, (as opposed to tight or loose all the way in and out)? Hense the shock work to tweek the timing of the weight arriving front, then rear? Does this mean that they also like the left to right weight distribution as dictated by tire wear, that's why they are doing it with shocks, not stiffer left springs? Does tire wear dictate how much weight ya gotta carry overall on the left side...and right springs are set from there? Any comments appriciated, ZANK (It's capital letters, caps lock snuck on when I registered)
gofour Unregistered User (8/9/02 6:19:12 pm)	gofour Gopher - LOL! - Point taken

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