Autocross Modification Guide

[AceInhole]

Hi guys. I basically just shot through a bunch of posts in this part of the forum, and remembered I had something written up somewhere. As a little bit of background info: I've been racing a 240sx in autocross since 2002. Since then, I have a few regional championships under my belt (2003, 2005, 2007), and even a Canadian class national championship (STS in 2004).

My car driven at the 2007 Devens National Tour

As of late, I've been trying to get into the National level competition, competing a lot in ProSolo and National Tour events. Unfortunately, my car was totalled this year, so it was only able to get away with winning the Dixie Tour down in GA earlier this year. Anyways, with that in mind, I have a small bit of knowledge to share, even though I may not be one of the greats (yet!).

So, this is something I made on the local forum. I hope some of you can use it as a basis or guideline at some point:

Here's the list of things that matter in order of importance for autocross: 1. Driver 2. Tires 3. Suspension 4. Weight 5. Brakes 6. Engine

1. The driver is THE most important component. In comparison to other forms of automotive racing, autocross has the highest concentration of driver input. In general, there's rarely a time when you're doing just one thing long enough to really think about it. But this is all pretty obvious....

2. Tires are the biggest "mod" in autocross. You'll spend some 90% or more of the time turning. Your grip level is dependant on your car's connection to the road, and the 4 thiings that make this connection are your tires. Tires tires tires. Can't say it enough.

3. Your tires will only work if they're on the ground. Your tires won't stay on the ground with crap for suspension. 90% of JDM coilovers = crap for suspension. If you can't afford Motons, Ohlins, or other high end shocks, buy Konis or Bilsteins. I could get into digressive force curves and how 90% of JDM dampers are linear, and the other 10% are for the most part simply poorly valved, but hey, if you buy crap, don't cry to me when the guys on real dampers are killing you in nearly stock 1989 Honda Civics.

4. You're going to turn a lot. You're going to be turning in different directions. This means you have to accelerate that mass of steel called your car laterally, all over the place. As we all know, F = MA. so, if your grip limit is lateral force, for the same force, M1*A1 = M2*A2. If M1 > M2, A2 > A1. If you can't multiply and divide, the lighter car accelerates more. Duh. Well, the unfortunate thing: you can't take out a lot of stuff in the majority of classes for autocross. What this tells you: don't add unnecessary stuff, like unneeded chassis bracing.

5. Brakes. You turn like 90% of the time, right? Well, of that 10%, you'll be braking and accelerating. Since autocrosses rarely go beyond 60 secs, you don't need big brakes, or powerful brakes. You DO need predictable brakes. So, for the most part, get good brake pads, and use good brake fluid. Other than that, don't go big brakes unless it's for a reason other than autocross, or unless they're lighter (like aluminum calipers).

6. Engine can probably be swapped with brakes, but really, no point in going fast if you can't stop. If you're doing everything right above this number, then you'll be spending a lot of time at a pretty constant speed. For a lot of people, having lots of power simply screws everything else up, anyways (overspeeding the car and getting off line, lacking control in turns, etc). In the end, you DO need power, but it's useless if you're only using it to get off line and drive longer distances through awkward lines that pinch long corners.

Those general rules down: You only need to: do general maintenance, replace worn items, and have fun till you figure out what class you can live with!

[nissanman04]

Good info...

I'm planning on building a SM s14 for next year...what tire sizes are stuffed under those fenders? 315 rears?

[AceInhole]

The tires are 285/30/R18 up front and 315/35/R18 in back. The torque to spin everything comes from a built KA24DE with a GT2871r. I can stay in 2nd from 20mph to 72mph with pretty much the same amount of power throughout

[Type X(J)]

Great guide! I agree with just doing the regular maintenance on your car and keeping modifications to a minimum (tires and brakes). Decreasing weight would be a better mod.

[n00b240]

Wow, I watched the video from another post and assumed you were running stock, wondering when the hell are you going to get out of second gear. Awesome guide. Thanks!

[crackler]

That sums the up probably 90% of what you need to know starting out.

Good Job.

I still have a lot to learn. . .

Did you wreck your car in a rain storm and your co-driver was with you? I think I heard about a 240 getting totaled on the way to an event this year. . .Can't rember all the details though. . .That sucks either way.
Modified by cracker at 8:52 PM 10/21/2007

[AceInhole]

Did you wreck your car in a rain storm and your co-driver was with you? I think I heard about a 240 getting totaled on the way to an event this year. . .Can't rember all the details though. . .That sucks either way.Modified by cracker at 8:52 PM 10/21/2007

I was with my co-driver on the NY Thruway when we were sideswiped and t-boned by a drunk driver. It pretty much ruined our ProSolo season.

[n00b240]

And T-boned? I take it insurance wont cover that. Sorry to hear that, I was looking forward to your updates at the track. Hope you get er up soon

[crackler]

I was with my co-driver on the NY Thruway when we were sideswiped and t-boned by a drunk driver. It pretty much ruined our ProSolo season.

You got hit twice by one drunk driver???

Are you biulding a new car?

[AceInhole]

And T-boned? I take it insurance wont cover that. Sorry to hear that, I was looking forward to your updates at the track. Hope you get er up soon

His insurance already paid out, although I had to do some arguing when their first offer was appalling. $17k worth of receipts does wonders.

What happened was: as we were passing he merged into us and "pitted" us into a spin. As we were spinning, he sped up into the side of my car (T-boned), and we luckily were dragged to the side of the road. There was enough time between the hits for me to shout a few directions at my co-driver (too used to instructing at autocrosses I guess) and notice a few oncoming cars. Once we were at the side of the road the other guy tumbled out of his car and almost fell on top of mine.

I've already bought a "new" car, although it was a real POS to start with. Here's a pic of it next to the crashed S14, with my G35c in the background.Pretty much restoring it from the ground up, but it's gonna require a a lot of work as the guy I bought it from left it sitting at a light house (next to the ocean) for a few months

[Slappy]

AceInhole - great post and congrats on being multi-time champion!!

[AceInhole]

AceInhole - great post and congrats on being multi-time champion!!

Only regionally.... and one Canadian class. I'm working on the SCCA National one, though

[crackler]

Just wondering, how much rear camber is on the old car, or did the DD realign it for you?

[Slappy]

Only regionally.... and one Canadian class. I'm working on the SCCA National one, though

Thats still the yellow jacket right? Wear it proudly.

[AceInhole]

Just wondering, how much rear camber is on the old car, or did the DD realign it for you?

That side is where the car was t-boned, so the camber/ toe is the drunk driver's work. It used to sit at about 2.5deg, but I reduced it to 1deg static with the new car since I have a slightly better camber curve in back.

[crackler]

I hear people talk about camber curve.I know what static camber is, is camber curve equivelent to "dynamic" camber, due to roll/suspension loading/ unloading, or something else, and how do you know what the camber curve is for a givem car? and will diffrent sway bars/shocks/springs/bushings effect it? how do you tell if it has positivley(SP) or negitvely(SP)?

[crackler]

I found THISBut how do you measure or know what it is on a given car at a given amount suspension travel.

[AceInhole]

The easiest way to physically measure camber gain is to unbolt the shock/spring and simply jack the wheel up, simulating "roll". You can measure your camber at different points, although really you can assume gain/ loss is linear and approximate it from a nominal (normal ride height) and max (as far as it'll go) position.

The front is a bit more difficult to physically measure, since the strut is incorporated into the suspension geometry. Really you calculate either end with simple geometry, although in front it seems as though caster is the easier way to utilize dynamic camber to your advantage, anyways.

[crackler]

The miata has shocks/springs front and rear, no struts. I might have to check this out over winter.How do you measure you camber? I don't have a camber gauge. Also do you know of a shade tree way to check toe? I have some shocks I am going to sway, and I want to make sure my toe doesn't change.

How does the caster affect it.I think I have 4 - 4.5 caster and 1.5 -camber.I have seen on the miata boards, guys saying to give up camber for cast to gain a better camber curve, but I never really understood why. I have about as much of both as I could get.

[AceInhole]

The miata has shocks/springs front and rear, no struts. I might have to check this out over winter.How do you measure you camber? I don't have a camber gauge. Also do you know of a shade tree way to check toe? I have some shocks I am going to sway, and I want to make sure my toe doesn't change.

I measure camber with a level and a contractor's protractor. I've been thinking about giving the digital levels a shot. They claim an accuracy of 0.1deg. For toe, you can try the string method, which a lot of club racers use. It's described pretty well here (as well as a decent means of making a camber gauge):http://www.quadesl.com/miata_alignment.html

Quote »How does the caster affect it.I think I have 4 - 4.5 caster and 1.5 -camber.I have seen on the miata boards, guys saying to give up camber for cast to gain a better camber curve, but I never really understood why. I have about as much of both as I could get.[/quote]Caster is basically the angle of the axis that the wheels steer about. The more "tilted" it is, the more your camber will change when the wheels are turned. Basically, you could potentially have it so that when your wheels are straight, you have zero camber. Then, as you turn your wheels, camber will go up (in relation to the amount of caster).

[crackler]

Thanks!