MomentumGT

305's are huge. We're contemplating 305's as well but we will need wide body for that. 255/45's and we had to bang and pull to get them not to rub. I would agree with Ace in sticking with a smaller tire, but damn 285's are still pretty big. For some reason my manhood feels challenged. LOL.

-Jon

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UNITEDMASTER

Hello gents ,yea I too think 305s are a bit much . If your dead set on going to a bigger tire, first take temps of the current set up to see if your even geting them to their optimal operating temp9 OUT/MID/INNER.305 in front is gonna have increase the scrub radius quite a bit, as stated before

Hello MOMENTUM GT- The idea you have makes sence,maybe if you get a bracket that has multiple holes in it then you can test & tune to find the desired angle of the RLCA to tune in anti squat. Or if you put the whole subframe on more of an angle. Im not too sure how it would operate because when I think anti squat I automatically invision a 3 or 4 link type suspension & we have indipendant arms. Have you considered the big bar soft spring theory(Carrol Smith). Find a spring rate that offers the desired squat characteristics(rear) VS some roll resistance ,Then use the sway bar to to ultimatly tune Roll characteristics F & R. Didnt you say your on stock sways, also make sure you sway bar is not under pre load. I cut & shortened my front end links,& used a generick energy suspension rear end link kit with the correct length bolt so that the sway bar is parralell under static load & it made a huge differance in handling characteristics( More stable under side loading) .I think this will help the rear squat cause the bar is not in twist before its even loaded. PEACE

P.S. Glad to see some real talk going on in the motorsports section. Keep it goin guys.

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AceInHole

Sorry I missed this part before: shortening the rear forward trailing arms will help (as will offset subframe bushings). On my S14 I have anti-squat pretty much dialed out.

What happens is this: as your rear suspension travels, your spindle's vertical alignment (rotation about the axle) is controlled primarily by the upper trailing arm. What I've found is that shortening the arm increases the forward rotation, which negates the natural anti-squat. This of course is a scratch pad analysis, but it seems to work in practice.

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AceInHole

What are the 255/45's on? A 17" rim? If so, the 285/30/R18's are about 1.3" shorter overall. You'll most likely gain more clearance all around (the strut being angled, will have more clearance, and your upper rail and inner tub will, too).

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KA24DESOneThree

A front airdam alone, on a NASCAR at 112mph with a 200mm ride height and a 100mm airdam, has a coefficient of downforce (CD) five times greater than stock. It also increases rear lift by almost 20%, but the total CD is about three times greater. Drag decreases by about 10%.

The addition of a 100mm splitter to this airdam netted another 10% increase in total downforce, and yet another increase in rear lift. Drag decreased across the body, wheels, and airdam but increased in the underfloor area. However, pitch sensitivity is increased, and a car at my car's stiffness level may very well experience it.

Adding a diffuser to the airdam/splitter combo yielded another 3.9% increase in overall downforce but a 1.4% increase in drag, again mostly from the dirty underfloor area.

What about canards/dive plates? They're generally regraded as not being terribly efficient but can have secondary effects like the generation of a vortex down the sides of the car and reducing air spillover under the car, leading to more efficient downforce production from the undertray.

That's all I'm going to write now because I want to go work on my car, and I take none of the credit for the tests done or for the gist of the paraphrased paragraphs. All credit is to Simon McBeath and all information can be found in Competition Car Aerodynamics.

I'll start writing about rear wings and whatnot probably tonight or tomorrow. I figure if I get this out there, maybe someone else can shed some more light.

Omarius Maximus

Out of curiosity, why is pre loading the bar bad? I figure it would tighten up the swaybar's response a bit. It would also prevent the swaybar from holding the inside wheel up mid corner, thereby maximising the contact patch.

As far as front grip goes....how do you guys tune for the dynamic camber curve of a machperson strut suspension? I sold my 240, but on my GTI, I had to throw on custom spindles to move the lower control arm ends down, so that they are parallel to the ground. The roll center moves up above ground, and the dynamic camber curve looks much better this way. Instead of really fat tires, maybe someone could mess around with balljoint spacers on a 240.

Keep in mind that the 997 GT2 uses 235s up front, which I suppose has gotta mean something..

nevaland9

ive seen Sashas car in person and have seen it perform. Its amazing, only making 320rwhp he just got 1st in the canadian touring car series against factory race E46 M3s, Z06's, a 3 rotor Rx-7 and much more. He told me he feels the flat bottom took at least 2-4 seconds off his lap time around Mosport.

Hopefuly this winter ill be doing the same to my S13

Flat Bottom FTW

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AceInHole

There is no way to preload a bar (other than inducing bushing bind) in a manner that will be equal left and right in the same direction.


I sacrificed static camber for a huge caster angle, which won't work well on a GTI since it is FWD. Other than that, I attempted to locate my normal suspension travel within the better portion of the camber curve, which on struts, isn't very much.

SPLParts and Battle Version sell lower control arms with longer ball joints. A spacer setup on a 240sx isn't very practical, as the ball joint runs directly through the spindle. You'd need 2 adapters bolted together, one for the spindle, and one for the ball joint, in order to create an effective spacer.

It basically means the weight bias on the 997 GT2 is towards the rear. The load demand on the front tires for rear engine cars isn't very high.

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KA24DESOneThree

Word Ace. Hell, the older Turbos were running 205/55s in the front and 225/50s out back. Just because a car comes with a certain tire from the factory doesn't mean it's the best setup. Also, I would guess that the GT2 will come from the factory with P-Zero Corsas and I would also guess that running 235s will allow the car to understeer more, making it safer for the morons who insist on buying a GT2 despite the fact they can't drive for shit. Oh, and the wheelwells just aren't that big on Porsches.

Balljoint spacers won't just magically make more grip than a wider tire. I have a fairly decent camber curve because I adjusted the angle of my LCAs, but it's still not optimal. I just run lots of camber.

S14SwimShark105

So has anyone ever thought about, what if you have a sunroof car and you decide to lift it upwards, not open it completely but have it so it's angled up facing the wind, how would that affect the car? good downforce? haha

And Sasha's car only has 320whp? I thought it had a lot more haha thats pretty crazy, Do you know how much his car weights? Does he have the B-magic hood? size tires he's running? haha

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Omarius Maximus

You can have lots of tire, but when 1/4 of it isn't connected to the road, it will be equal to or worse than having a smaller tire with proper suspension geometry.

If the angle of the lcas isn't optimal, I suggest maybe hiking the front end of the car up about an inch if you have coils. Then test to see if the front end feels more planted.

MomentumGT

Its on a 17" rim. We wanted to go with the 285's on the 18' but the cost of 2 or 3 sets of rims plus tires were out of our budget, so we stuck with the 17's. We also didn't want the stacks of RA1's that we purchased for the season to go to waste. haha. So we make due and try to find more mechanical grip elsewhere.

-Jon

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MomentumGT

The problem I foresee of angling the subframe would be, I'm guestimating here, that the drive shaft would also be placed in an angle and may cause unwanted vibrations. The problem with just swapping the s15 sub is that the subframe mounting points are off by about 10mm, so custom offsetted solid subframe bushings are in order. Some guys who have done this swap just shove the sub on and squish the bushings, but we've had experience with rotted out bushings where the subframe was moving while on the track and that was no fun. lol.

-Jon

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frosti108

sand bags out back and lead in the front frame rails

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Wiisass

Sorry, I missed a lot, this might be a little long.

Jon, you need a lighter race car. I'm still kind of surprised that a 10/8 setup was bottoming out. But if it happens, it happens. I'm glad the stiffer springs are working for you.

As for changing the rear geometry, this is something I have thought about. What I was thinking was leveling out the lower control arm and possibly messing with the mounting points for the upper arms. There's a lot to consider when you do something like this though, so be careful. And make sure to check that there is no bushing bind through the usable range of suspension travel after modifying anything. The amount of anti-squat that the suspension will see depends on the angles of the arms and where they intersect. It's the same thing as roll center, it's just looking at the car from the side rather than the front. It does the same thing. It determines the amount of load transfer that goes through the suspension members and the amount that goes through the shock/spring. I would do some measuring first and see where things lie before changing too much. Obviously, there is a good amount of anti-squat already designed into the suspension. You could get rid of all of it by making the upper and lower arms parallel to each other. But this is probably something that we should start another thread about.

Do the scotch tape and wool testing before you do too much. There's a lot you can learn from that. If you're totally set on doing this aero stuff, I would work a lot on reducing drag. So do a full underbody, seal everything you can, treat air like a fluid, you want to know where it's going and that it can't go where you don't want it.

There are also other ways you can measure the effects of an aero package. If you can find a long smooth road and a calm day, you get an estimate of the max downforce the car will produce by looking at shock travel. But search around, and I'm sure there's some good ideas in the books you just got, but there's a good amount of testing you can do without having to spend a ton of money. And coming up with a good plan is the only way to do this effectively in terms of both cost and performance.


Front/rear spring rate is going to depend on a lot of things. What you're really interested in the front/rear natural frequency ratio which is determined by wheel rate and sprung mass. At least on these cars, the front will be higher than the rear, but by how much is going to depend on the actual weight difference front to rear and also what size sway bars. Everything is a compromise and everything relates to everything else, so it's hard to say there should be a given ratio of front spring rate to rear spring rate. And it also depends on how you want the car to handle. If you want to make it a little more understeer biased, you'll make the front stiffer, and vice versa for oversteer. And then when you throw aero into it, things start to get real messy.

So you biased the lateral load transfer to the front with the removal of the rear sway bar and then decreased the amount of lateral load transfer at the front with the wheel spacers. With that much power, you should look into getting some more tire under there. Otherwise you're going to have to keep pussyfooting around on the road course. There's probably a lot of other changes you could make as well, but again, that's another thread.

Good point about measuring tire temps, it is important to know and a valuable tuning tool. 305s are going to take a lot to fit and it may not even be worth running such a big tire. You need to be pretty serious if you're going to fit 305s. As for scrub radius, I really don't think it's as big an issue as people seem to think. Scrub radius is really only an issue under hard braking on rough surfaces, it can give weird feedback at the steering wheel and the car can dart around a little. But this isn't a FWD car, so scrub radius doesn't have nearly as much effect on the suspension. A higher scrub radius can create some higher loads in the suspension members under braking though, but they should be cancelled out because they should be even on both sides, at least for straight line braking. Trail braking throws a lot of other stuff into the mix and scrub radius is just one of the factors.

As for Jon's idea, fabbing a bracket with multiple holes could be a good way to test, but I still think a lot of measuring and running numbers is the best way to get in the ball park and then maybe some fine tuning afterwards. As for "small spring, big bar," I don't believe Carroll Smith advocated that idea. I know he mentioned it and said it was common for a lot of people to run, but I don't think he ever said he actually liked it. He did say to run the softest springs that you can get away with and then balance out the lateral load transfer with the sway bars. This doesn't mean to run soft springs and really stiff bars. Personally, I like to run the spring rate that you need, and then use as small a sway bar as possible just so you can adjust oversteer/understeer.

And like Aceinhole said after you, there is no way to preload a sway bar that would actually be good for handling. Unless you're only turning in one direction. Because the bar has to twist clockwise to turn one way and then to turn the other way it has to pass the neutral position and then rotate counterclockwise.

I don't think just shortening the rear trailing arm is going to get rid of any of the anti-squat. I do see what you're saying about the way the wheel travels under load, but I think there is a lot more to do to really get rid of the anti-squat designed into the suspenion. I thought we had the discussion about shortening that arm somewhere else, didn't we?

See all the other responses about what preloading a sway bar just doesn't work.

Depending on what spring rates you're running and how low the car is and how much wheel travel you're going to see, you can determine what amount of camber you need to run to keep things balanced when cornering. Usually it ends up being a lot of camber. But like Aceinhole said, you can get away with a little less static camber depending on your caster. As you turn the wheel a lot of things happen, depending on your caster angle, the outside wheel will gain camber with steer angle, but due to kingpin angle, the outside wheel will lose camber due to steer angle. And then depending on wheel travel and suspension, when cornering, the outside wheel will usually lose camber relative to the road because the body will roll more than the wheel will gain camber, but sometime with a MacP strut setup, the wheel will be gaining camber already and it will amplify the effects of the roll camber gain. So it's kind of a mess.

As far as tire size, that also depends on a lot of things. Suspension geometry, spring rates, vehicle mass, etc, so it's hard to compare one car, which is totally different in pretty much everything, to another car. It's like comparing spring rates or sway bar size or anything like that.

I don't know about the spl lowers, but the Battle Version ones are the same as the TC sportline ones that I've messed around with a little and they have a spacer, but it doesn't look like it lowers the arm very much versus stock. The pivot for the stock ball joint is down there already and the bearing in the TC/BV arms doesn't seem to be much lower in height from the spindle versus the OE ball joint.

It depends when that part of the tire isn't connected to the road. In some cases, you need to run a lot of camber to get the wheel in optimal position for cornering, so when running straight, your contact patch is lessened. Which isn't good for braking, but it's another one of the necessary compromises.

The driveshaft could be a problem. As well as if you're just rotating the subframe you aren't going to be fixing much of the problem. If I remember right, the line connecting the two upper mounting points slopes downward and the line connecting the lower mounting points slopes upward. So by rotating the subframe, you're going to be changing it, but it's definitely not the way you want to do it. And I had thought that the S15 subframe was a direct swap. If you need to do offset bushings and all that stupid stuff, it might be better to just figure out what was changed and modify it on the subframe you had. But like I said before, we should start another thread on this and maybe we can come up with some good ideas.

Alright, well I think that's it, sorry for it being so long.

Tim

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racepar1

Under 90mph any aerodynamic downforce created will only have a mild affect on the car, trust me my dad ran several formula atlantic cars over a 10 year time period and we did TONS of aero testing. That's not to say that it will do nothing at all, it will just be minor. If you really want to improve the aerodynamic performance at those speeds add some underplating and diffusers to reduce drag.

UNITEDMASTER

Hello,hello. As far as the preload thing goes I wasnt saying to preload the bar,just to make sure there is not any twist /bind it it before its under load.Kinda my idea of preloding, but not the true definition of the term in this case. Good points carry on,carry on!!!!!

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g6civcx

If you're really serious about it, you can build a scale model of your car and experiment in a small wind tunnel. It's not the real deal but it's much much cheaper than a full scale wind tunnel.

But the small tunnel is still expensive for me so I don't know what to say.

Wiisass

There's a million better ways to spend his time and money and probably get more out of it than making a scale model of his car and running it in a wind tunnel. He would be better off, making a CFD model and using that as a tool to analyze the aerodynamic performance of his car before messing with wind tunnel testing. There's also tons of on the track and less sophisticated testing that he can do that will give him more gains/time or per dollar than wind tunnel testing would. It's just not a good solution for someone on a budget especially when there are so many other, better options out there.

Tim

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KA24DESOneThree

Actually, this could be a great way to fab up an undertray without having much downtime on the car. Make the undertray for the scale model then just multiply the dimensions by whatever the scale is.

It'll save cardboard boxes from becoming patterns, at least, and if it's not perfect I can always adjust.

PoorMans180SX

http://www.voltex.ne.jp/english/witem/witem.html

Just a random plug here, but Voltex wings are designed and tested with a wind tunnel. In case you're still looking for a wing.

I think I'm going to have to read through this thread a couple times.

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Wiisass

But wouldn't that be taking one step forward, then one step back, then another step forward. You need to measure the car to make the scale model, so you would have all the measurements needed for the undertray already. So then if you're making the scale model, and then measuring that for the undertray, you're just measuring the same thing you already did with the scale factor you're using. Unless you found a matchbox 240sx that you're going to use, I don't see it as being very productive.

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KA24DESOneThree

I'm still looking for a wing, but those prices are just plain nucking futs.

I was talking about buying an off-the-shelf 1/24 Silvia plastic model, Tim.

racepar1

You are gunna need front downforce as well before you just throw a wing on anyways or else you will have high speed understeer. I think you should concentrate on the undertrays, diffusers, and splitters first and then go from there. If you do too much at once you won't know what works and what doesn't.

slappythehomelessclown

Not gonna work on a street car.

racepar1

Trust me man there is really nothing "street" left on this car.

mifesto

it simply sounds like he needs/wants more front grip in the slower corners, so any time or aero i dont believe would make a significant difference. i say just alil more front camber with overall R-compounds i think would be suffice. maybe slightly stiffer rear sway bars too.....

INeedNewTires

This is going to sound pretty ridiculous to some of you guys, but i know its worked in the past.... I cant remember exactly who did it, but someone put carpet underneath their car, and it was so effective they banned it the next year completely. I think i recall it was a lower powered/lower speed class and just some dude had this idea to completely eliminate the air moving under the car, so thats what he did. He basically attached a piece of carpet, cut in the shape of an undertray, to the front of the front bumper where the front lip would attach, and then let it hang down and drag the ground. Obviously it needed to be clear of the wheels and suspension components up front. Then he attached another piece in the center body of the car, behind the front wheels, let it drag the ground, and the same for the rear. Now i dont know exactly what would be involved in this but from what i recall it did amazingly for such a simple/cheap mod. Of course it would wear out quicker than an aluminum undertray but at its cost, it could be replaced every race with minimal costs.... An added benefit is that the carpet could catch 95% of the fluids that may drip on the track, from engine, cooling system, transmission, etc. Although it may have spewed carpet fibers all over the track, they are a hundred times less detrimental to traction than oils/fluids are...

Seems like that would be the cheapest, most effective "aero" mod he could do at the speeds he is talking about, which is the topic of this thread started by the OP.(low speed aero on a budget) I know all you guys are telling him buy this, change that, but he was asking about aero in low speed, so thats why i posted this.... again i know its ridiculous but no flaming please..........

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a_ahmed

Heh I get this gut feeling that heat + carpet will most likely = fire and destruction lmao, just a thought, WEIRRRDDD.