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Dear Dave - Readers Letters

Geek-At-Large

Dave Coleman
Nov 1, 2007
0711_sccp_01_z+dear_dave+dave Photo 1/1   |   Dear Dave - Readers Letters

Direct 'Dear Dave' tech letters to dave@eyesoreracing.com. Coleman will share mind-numbing details, earth-shattering revelations, and technical nerdisms in this space each month.

Q. College transportation or rally car?
I don't have a car, or even a learner's permit, but I'm lucky enough to drive my parents' car once in a while. I recently turned 16. When I go to college, my parents are promising to get me a new car. I'm looking at the VW Rabbit and Jetta. I'm trying to stick with something practical, but my dream car is a Toyota Supra and I want to build it for drift.

I know the VWs are both front-wheel drive and they can be built as rally cars. I already know all the engine stuff I want to do when I make my purchase, but I don't know how I should tune the suspension for rally.

Would the set-up be similar to a road racing car, as I've seen with Project Corolla and Project Time Attack Miata, or is it more similar to drifting? Also, what kind of tires should I use: soft, medium soft, harder compound? And what tire pressures? I've been thinking about this for a long time, because in rally racing you can both grip corners and drift through them on slippery surfaces. One last thing I need help with is what type of LSD I should use for rally.

I am trying to prove to my friend that front-wheel-drive cars can do just as good in rally as the all-wheel-drive ones.
Dexter Alleyne
Bend, OR

A. Is it just me or is the room spinning?
I'm not sure where the Supra fits into the picture, but rallying your daily driver is a bad idea. You will hit a tree some day. After your first tree hit, it's always more enjoyable to still have a working car the next day. With that out of the way, I'll do my best with your questions.

Although there are occasional tarmac stages, US rallies are almost exclusively run on gravel, so suspension set-up is nothing at all like drift or track set-ups. The rough surface requires a high ride height and soft springs. Also, suspension articulation over uneven surfaces is hugely important, so anti-roll bars are either eliminated altogether, or a smallish one is used on the non-driven wheels, just to trim the handling balance.

Not only does shock valving need to be different, the shocks have to dissipate a lot more heat and withstand vicious loads. Dampers, camber plates, bushings and control arms that can withstand seasons of road racing won't last one 15-minute rally stage.

Tires? That's easy. You need rally tires. Naturally, there are different types for every conceivable surface. Even tarmac rally tires are vastly stronger than road racing tires seemingly engineered for the same surface. Though tarmac stages and race tracks are both on asphalt, road racers seldom jump, ditch hook, run over fist-sized rocks at 100mph or slide through three consecutive corners with all four tires spinning. Kinda makes you wonder why they bother with road racing, doesn't it?

Just like road racing, though, poor guys like you and I can get away with running cheap used tires cast off by the well-funded teams, so don't despair. At least not about the tires.

Unless you're rich, there are only three kinds of tires to worry about. On gravel, run the hardest compound gravel tire you can find so they'll last longer. In snow, get street snow tires, put inner tubes in them, and pump up the tire pressure to the max allowed on the sidewall. In the rare instance of a set of tarmac stages that don't lead directly into gravel stages, you'll need actual tarmac rally tires. I've never been in a rally with enough tarmac to justify buying tires.

You didn't think to ask about brakes-you should have. Since gravel has less grip (therefore less weight transfer), the front-biased braking of street cars makes no sense. Gravel braking performance gets much better by simply removing the proportioning valve that reduces rear brake force on a stock braking system.

Differentials in rally depend on driving style as anything else. Foot-to-the-floor drivers who hurl themselves into corners and work out the details later often find a welded diff perfectly suitable. The more subtle, who feel for grip through the whole corner and make tiny adjustments on the fly, complain that locked diffs give either full oversteer or full understeer, with no room for finesse in between. We prefer a looser diff, either a helical or a relatively loose clutch diff, or even an open diff on a rear-driver. Only the clutch diff or the welded diff, though, will get you out of a ditch if you have one wheel in the air.

As for the bet with your friend whether front-wheel-drive cars can be just as fast as all-wheel-drive ones, it all comes down to the word can. Every experienced front-drive rallyist can look back and find a few all-wheel-drive cars behind him, so yes, they can be just as fast. In all but the most unusual situations, though, it's an all-wheel-drive car that ultimately wins the rally.

Front-drivers can't make use of much more than 200hp. Anything more just goes into churning gravel. At that power level, equally prepared front-drive and all-wheel-drive cars will be very close, with an all-wheel-drive car allowing the driver to get on the gas just a bit earlier in each corner.

A well-prepped all-wheel-drive car, though, can make use of up to 400hp (though most classes restrict them to less), so given the freedom to break the 200hp barrier and given equal drivers, the all-wheel-drive car will win every time.

The advantage of all-wheel drive doesn't make it the right choice for most, though. Front-drive rally cars are much cheaper to race and, since they never run in the same class as all-wheel-drive cars, there's just as much opportunity for victory. Starting with the 2007 season, more prize money is available in front-drive than in the all-wheel-drive classes, thanks to the privately funded MaxAttack! Championship (www.max-attack.com).

Feed your need for knowledge at www.rally-america.com, www.specialstage.com, and www.rallyanarchy.com. Then, when you realize you can't afford to rally, go do some rallycross instead. Everything I said about breaking stuff and hitting trees doesn't apply in rallycross. With shorter abuse sessions, smoother surfaces, and almost no trees, even stock cars can do rallycross. Just a suggestion: Avis is your friend.

Q. Marketing vs. Physics
I've heard that wheels with incorrect offset increase steering kickback, torque steer and tramlining. Note the following, taken from your Evo vs. STI project:
"Counting on Prodrive's Subaru experience, we assumed the offset would be close to the stock 53mm, but these were 43mm. This is what people in the wheel business call an 'aggressive fit.' The wheels are pushed so far to the outside, they barely clear the fenders. This looks good, but die-hard perfectionist tech geeks like us call this kind of fitment 'doesn't fit.' Changing the offset by 10mm usually screws up steering feel and... because of the 43mm offset, the car now has noticeable torque steer."

If the 43mm offset used on the Prodrive wheels (versus the stock 53mm) made the wheels stick out by 10mm, didn't it act like a 10mm wheel spacer? My mind is saying: "wider is better for handling and these Prodrive wheels would be doing the same thing." Please educate me and tell me where my thinking is flawed.Samuel MathewPontiac, MI

A. The flaw in your thinking is where you take a marketing slogan and use it as an engineering principle. You're absolutely right that 10mm less offset is the same as a 10mm spacer, but who (other than the guy selling you the spacer) says the spacer will automatically make your car handle better?

The relationship between the tire centerline and the steering axis was pretty well worked out by the factory and changing it can have complex consequences. In simple, two-dimensional terms, the point where the steering axis hits the ground (The Dave Point-and don't you forget it) is effectively a pivot point. A front driver pulls the car forward with the tires, and does so, effectively, at the center of the tire's contact patch. The distance between this center and The Dave Point (a distance called the scrub radius) is effectively the lever arm with which the tire tries to steer the wheel around The Dave Point, yanking the steering wheel out of your hands.

Engineers balance this force with various other wheel-yanking forces coming from, say, the axles, and various complex three-dimensional suspension realities. Then deal with them through steering geometry, power steering valving, and strategic use of friction. Mess with this lever arm and you'll feel it in the wheel.

Does a wider track make a car handle better in spite of this? Depends on how you define handle. In terms of pure cornering grip, maybe. But if you consider the car's willingness to go where it's pointed on uneven surfaces, or how much you have to fight the wheel when hitting the gas or brakes (torque steer happens under braking too), then the answer isn't so clear.

None of this is a condemnation of wheel spacers, just the way they're marketed and used. If predictable, easy-to-control handling means more than looks, try to keep the offset within 5mm of stock. If the wheels you want are only available in a higher offset, use a high-quality spacer (preferably a billet, hubcentric one like those made by Eibach and H&R) to get the wheel where it belongs. If you use a bolt-through spacer (as opposed to the thicker ones that have their own studs) make sure the lug nuts engage the studs by at least the width of the stud. On a 12mm x 1.5 lug nut, that means at least eight turns of thread engagement; on a 12mm x 1.25, at least 9.5 turns.

By Dave Coleman
94 Articles

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