Autocross theory and wheelbase

A short wheelbase will make a car very maneuverable and capable. But you better have the hands to catch it near the limit. 

The difference between a STS civic and CRX is night and day from everything I was told. The little extra wheelbase of the civic made it much more predictable.. But autox generally rewards hero runs and so the CRX remains very difficult to beat. 

In reply to Adrian_Thompson :

The way I heard Porsche’s history of development with the 911 is either “The ultimate triumph of development over design” or “You can’t turn a pig into a thoroughbred racehorse, but with enough development you can make a berkeleying fast pig!”

Remember, the longer the wheelbase, the bigger the difference between the line the inside front cuts and the inside rear that follows (Think about a schoolbus navigating a tight corner).  

ProDarwin said:

Remember, the longer the wheelbase, the bigger the difference between the line the inside front cuts and the inside rear that follows (Think about a schoolbus navigating a tight corner).  

This is likely the biggest concern for longer wheelbase on an autocross course.  Heck, even on the street there are times where that effect of the extra 9.5" of wheelbase in the E38 vs the Jeep is noticeable.  

Here's another thing to consider. Autocrossing has one very specific element that other forms of racing do not: cones. You can't knock them over, so, do front and rear overhangs play a part as well?  

The M3 does well, and look at the distance from front wheel to front bumper... not much. 

I assume that makes it better than this:

An interesting article on overhangs, the worst might surprise you. 

In reply to pinchvalve :

Interesting but I assume front overhang is more important than rear.  

Agreed, front overhang is likely a bigger issue than rear for autocross.  And that's shown in a lot of performance cars focusing more on short front overhang than rear (also effects other things, of course).  

The more opposite lock is involved, the more overhangs matter.  So overhangs should really only make a significant difference when your run has transitioned from autoxing to drifting. In such situations, front overhang matters more at apex while rear overhang matters more at corner exit.

In reply to pinchvalve :

Link is interesting.

As to your comparison of the M3 with the Mark 5 for Autocross, perhaps there are additional factors than overhang 

I ran our 72" D-sports Racer at a couple of autocrosses and the only detriment I fond of a wide car was the slaloms. Even with that that car, versus my F500, was so much more stable. In the F500 you indeed need to be very careful about how rapidly you move your hands. If not you'll snap spin the car on the 2nd or 3rd cone of said slalom.

As for the wheelbase; on the F500 if you get the car on opposite lock and you have to unwind the steering wheel back past 9-3 to around 8-2 or 10-4 your pretty much done. with that said I have no clue if this is a product of the short wheelbase or narrow track.

In reply to Gingerbeardman :

I don't think that he's saying you can't cut as close to a cone in a wide car, just that the centerline of the car will be further from the cone by nature of the car being wider. So if the centerline has to be further from the cone to not hit it, then the car has to travel a greater distance, especially in a slalom. 

So on an NB miata, the track width is roughly 56", so the center line can be 28" plus half a tire from a cone. A C6 corvette has a 62" trackwidth, so the center can be 31" plus half a tire from a cone. Using stock tire sizes, the center of the miata will be 31.8" from the cone, and the center of the vette will be 35.8" from the cone. So over a 6 cone slalom, the vette will travel an extra 4 feet just due to the added width. 

I realize that between these two cars, that won't matter basically at all, but in equal cars, that could be a multiple place difference.

Also, vettes can't be remotely reasonably compared to a miata, even in autocross. They both handle beautifully, but a vette can run a huge amount more tire and has more than double the power-weight of a miata. What do you mean by "Corvette has the highest rankings in autocross finishes across almost EVERY category?" Vettes aren't in every class so that can't be possible.

frenchyd said:.

We know that a tear drop has the least aerodynamic drag.  While wings, and spoilers add significant drag.  

Sort of

It’s more accurate to say that a tear drop has the least drag for a body of revolution in the Reynolds Number range that a drop of water (0.5mm) falling at 20mph at results in.

sleepyhead the buffalo said:

frenchyd said:.

We know that a tear drop has the least aerodynamic drag.  While wings, and spoilers add significant drag.  

Sort of

It’s more accurate to say that a tear drop has the least drag for a body of revolution in the Reynolds Number range that a drop of water (0.5mm) falling at 20mph at results in.

LOL

More importantly, while wings and spoilers typically add drag, they also generally improve lap times. So perhaps that's not the best example to support lower drag designs for autox cars

In reply to Driven5 :

Do spoilers make a fast car faster? Or by the time you get fast enough the additions of spoilers don’t hurt?  

A maximum width front spoiler makes the front wider  and more likely to tip a cone than the more rounded front end will.  Yes you can drive just a little bit wider of the cone to avoid hitting the cone.  However, by definition that means you are driving further. 

One  thing I’ve never seen is back to back use of spoilers and splitters and no spoiler and splitter. 

To show that the added weight, drag, and width actually yields a faster time 

ProDarwin said:

Remember, the longer the wheelbase, the bigger the difference between the line the inside front cuts and the inside rear that follows (Think about a schoolbus navigating a tight corner).  

That also depends on what slip angle the rear tires are working at.

On loose surfaces, the rear tires' slip angle may be such that you have to worry more about whacking the exit cone.

On asphalt, it often seems like the rears more or less follow in the same tracks as the front.

Gingerbeardman said:

In reply to Knurled. :

I call BS on this. Unless you can't see where the car is going (some cars have ridiculous overhangs, poor sightlines, misplaced fender ridges, etc.) you can cut EXACTLY as close with a Miata as you can a Corvette.

Miata might be the answer, but Corvette has the highest rankings in autocross finishes across almost EVERY category.

You could drive an H1 on every cone too, but the narrower the vehicle, the straighter the course is.  The center of mass can take a tighter line because it can be 30 inches away from the cone instead of 34.

In reply to frenchyd :

I saw something like that at this years ice races. There is a class that run the overhead wing.   Most are old sprint cars. Think WOO.

 Anyway, a friend has a state of the art car he built.  tube frame, rear mid engine, four wheel independent suspension.  Then he slaps one of those huge wings.  In a race he caught a snow bank, flipping it on it's side crumpling the wing.  He removed it and ran the next two races and did really well.   He said after that he was surprised.. Me, "What does that tell you ?"    Next week he was back with another big wing.

Racers don't like to get out of their comfort zone.    If they think it works, it does.

Regardless of the downforce effect, or arguably lack there of for the majority of drivers, at autox speeds: Reducing drag won't help any more than increasing drag hurts... And that has been pretty well established to not be the case.

I've seen upwards of 85 mph plus on the F500 but obviously it's not for very long, being an open wheel car there is obviously tons of drag. The shifter karts run about 1-3 seconds faster than my car and I'm about 1 second faster than the next fastest cars. I'd say drag is meaningless at autocross except if my car has less drag than another car in my class I'd say there would be small gains to be had.

iceracer said:

In reply to frenchyd :

I saw something like that at this years ice races. There is a class that run the overhead wing.   Most are old sprint cars. Think WOO.

 Anyway, a friend has a state of the art car he built.  tube frame, rear mid engine, four wheel independent suspension.  Then he slaps one of those huge wings.  In a race he caught a snow bank, flipping it on it's side crumpling the wing.  He removed it and ran the next two races and did really well.   He said after that he was surprised.. Me, "What does that tell you ?"    Next week he was back with another big wing.

Racers don't like to get out of their comfort zone.    If they think it works, it does.

Plus few if any amateur racers will do all the work required to make splitters and wings/spoilers and then take them off to see if they actually help or hurt.  

The above kart is, by far, the fastest autocross vehicle I've ever driven. I adore it as a machine as well as an experience. As a data point, it would suggest that wheelbase should be shortened as much as possible with nearly no theoretical limit.

HOWEVER, if you actually drive one through a slalom, you quickly discover that theory is less than perfect. Yes, you approach and dispatch slaloms at tremendous speed. You also simply cannot maintain anything close to an ideal line because the kart won't turn until you lift the inside tire and it takes quite a bit of steering input to do so. So, you drive rather violently and hope not to hit any cones.

EVERYTHING is a compromise. A nearly endless array of variables influence every single factor in car (and kart) design. As usual, the answer is 'it depends,' but if you look at the purpose built cars that show up to nationals, I would bet nearly all of them have made an attempt to keep their wheelbase as short as possible within class/budget/talent.

Interesting the disdain for splitters- they are as much a drag reduction item as they are a downforce generator.  The underside of the car is the dirtiest, and the splitter lowers the amount of air that spills under the car.  It also happens to increase downforce on the front.  karts don't have as much of a problem due to them being so low.

I totally understand questioning wings and whatnot- poorly done aero devices can hurt more than the help, for sure.  But those tend the be wings that are in bad places to be ineffective for what they are supposed to do.

To me, what would be interesting is having an effective splitter and then adding just enough rear wing to balance the car.

In reply to white_fly :

The kart's on/off handling could have more to do with not having a rear diff than having a very short wheelbase. An FSAE car's wheelbase is almost as short, but it has a rear diff, and they don't have the same problem.

In reply to white_fly :

That would seem to reinforce the idea that the more square the wheelbase/track ratio is, the more important the differential (or lack thereof) is for handling.

I think people put way too much faith into "optimal" wheelbase and track ratios. You can make either extreme work well. I personally think it's very, very low on the significance scale. Heck you could even look at the Nissan deltawing with essentially zero front track width and it still works great. I think the only really good rule of thumb for autocross is to make your car as small, light, and powerful as possible and cram the most tire you can get underneath it. You could very well say that if you aren't nearly unweighting both inside tires while cornering then you're car is wider than it needs to be, at least for autocross.