Autocross theory and wheelbase

In reply to Knurled and GameboyRMH :

You're absolutely correct that the handling of the kart is influenced tremendously by the solid axle. That is one of the variables that is fixed by the rules in my case.

The intent of my post was to say that while a short wheelbase does tend to work well in an autocross scenario, it is merely one of many variables that need to be accounted for. It's all about finding a package that works well as a whole.

freetors said:

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.

Excellent point regarding the Nissan  Deltawing.  “We all KNOW  a wider track will corner faster than a narrow track”.  

So my question is do splitters and spoilers help or hurt in autocross?  At the average cornering speed of ? 35 mph?!!!! 

Does the weight,drag, and width of a splitter/spoiler help or hurt?   Does it help or hurt enough to offset its cost and development time?  Is there someplace else where a similar investment of time and money make a similar or even greater improvement in lap times?  More horsepower? Better balance?  A more optimized suspension geometry? 

Maybe psychological ? It looks racy, it’s very visible so it must be faster?  

Knurled. said:

echoechoecho said:

width has way more effect on autocross course than wheelbase, since shorter wheelbase cars tend to be narrower also they tend to be faster. The slalom equation only ask for width and grip. simply put a smaller car runs a shorter course.

slalom equation:

(π/8)√(W+1/G)= Time in seconds between cones

*W is width(feet) of vehicle, G is total lateral grip(Gs) 

But I was told online that the extra width of a BR-Z relative to, say, a Miata was irrelevant.

 

(I don't like wide cars for this reason.  You can't cut as close)

Well to put a too fine a point on it, you can cut just as close to the cone with a wide car as a narrow car, but a wide car has to travel further to do so than a narrow car.  

freetors said:

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.

The Deltawing is a little misleading WRT making absolute comments about track width- it was super narrow at the front, and max to the rules in the rear.  The problem of tipping moment vs track width is very subdued for that- but if you go to the extreme narrow for track width- you get a motorcycle, which does not corner all that well.

The track width vs. CG height, and how that loads the tires is probably the most important thing.  As that's pretty much where you need to start when designing the suspension and how it's supposed to travel given the expected side loads.

       " if your not  Unloading the inside wheels "

   I Know my Experience is mostly Round Track, My Inside rear was as Hot As the front Tires ( Both )  So I will Try to Keep all 4 working,  with a big block in a Vette chassis the weight will like the wider Track at 67 inches  , thats outside to Outside, but the Relitive short length at 96 inches should help in tighter Turns, I know the LeMans C4 was lengthend to around 101 because of speed,   and I may take off a Couple more down to 94-92.

So it seems to me that WB Length could have more impact than width in A/C.    Way back when I remember the figure of 70% width to Length , Like an Aircraft Balance number

freetors said:

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.

You could also say that your tires don't have enough grip.

The debate about autocross aero is a silly one. Yes, it needs to be implemented properly, but even at low speeds it can make a very large difference.

Aero can definitely help at autocross speeds.  You just end up with comically large wings, etc. that would be massively too much at higher speeds.  

rslifkin said:

Aero can definitely help at autocross speeds.  You just end up with comically large wings, etc. that would be massively too much at higher speeds.  

Exactly- if aero is ineffective at autocross speeds, then drag is also insignificant as well (which is the counter claim to aero bits).

Especially since spoilers and splitters on the front are more drag reduction devices than downforce devices- the drag and underflow they reduce also reduces a lot of lift- and reducing lift is also known as increasing downforce....

Also, most classes have a minimum weight.  So, you can have it be ‘dead weight’ ballast, or active performance adding downforce.

back to the case at hand... longer wheelbase, and wider track can generally mean more “volume” with which to create downforce.

white_fly said:

The debate about autocross aero is a silly one. Yes, it needs to be implemented properly, but even at low speeds it can make a very large difference.

Think about where aero helps in autocross. It doesn’t help when you’re going straight, but only when you are going around the cones.  Especially cones that change direction dramatically.  However those typically are slow speed    30mph?  

How wide are the wings that lift an airplane off the ground at 30 mph?   Would those same wings lift the plane off the ground if they were a little over 4 feet wide?  

In reply to frenchyd :

If you really think that AM car corners all corners at 30mph, I have a GRM Challenge V12 Jag for you in my garage.

Just because you are convinced that aero isn't effective for autocrosses does not make it right.  I very much doubt that you are an aero engineer with any expertise.  

In reply to sleepyhead the buffalo :

Longer wider also means travel further. 

In a race where shorter narrower doesn’t have to travel as far. 

alfadriver said:

In reply to frenchyd :

If you really think that AM car corners all corners at 30mph, I have a GRM Challenge V12 Jag for you in my garage.

Just because you are convinced that aero isn't effective for autocrosses does not make it right.  I very much doubt that you are an aero engineer with any expertise.  

So what speed are you going around the cones that change direction?  

I know there is a massive difference between courses.  I’ve been on tracks where you never get out of first gear and tracks where you could almost shift into high gear. 

At some point it stops being an autocross and becomes Solo 1. 

Yes on those high speed events aero makes a difference. 

You are right . My only formal training in aerodynamics came when the USNavy taught me to fly. I really learned a lot about low speed aerodynamics practicing stalls. What would you like to know? 

Frenchyd in my F-mod / F500 car hairpins are taken at 25-30 mph but the bulk of the corners are between 50-70mph. Comparing the on board footage from A-mod to what I see, the difference is a solid 5-10 mph more. Also keep in mind very few A-mod competitors run on small lots at local events. 

I've seen video of the A-mod pictured above doing 75 mph in slaloms and 70 mph in 180 degree turns.  If you can get a Piper Cub to take off at 25 mph I'm willing to be a car with that much swept area on the wings is generating several hundred of pounds (or more) of downforce at 50 mph. Also note the aforementioned car is pulling near 3Gs in corners and almost 4Gs on the brakes............on an autocross course.

I remember reading back in the late 70's / early 80's that the small front air dam and little rear spoiler on Trans-Ams & Z28s generated a couple hundred pounds of down force.

As there are no long straights at autocross the drag penalty isn't anywhere near what it would be on a road course.

In reply to frenchyd :

Seems to me that your low speed aero experience was not exactly applicable for 20-75 mph vehicles that interact with the ground.  If you were right, then the top autocross cars would quickly figure it out and remove them. Instead, many get solid engineering advice from real professionals, and have working aero hardware. 

I don’t know why I respond to your posts. Sorry, everyone. 

In reply to alfadriver :

I’ll grant you that racy wings, splitters,  and  spoilers add a serious look to a car.  Yes at 50-70 mph they are doing something. At 25-30?  

My question is the time and  money spent, could it be used elsewhere?  Would added power, better gearing, reduced unsprung weight, whatever yield the same or better improvement?   

Or is the psychological impact too much eye candy to ignore?   

They'll do a lot less at 25 - 30 than at higher speeds, but if they're big enough, they'll do something.  And in some cases, they're not there for the slow spots on the course, but to keep the car planted in the faster slaloms with a very loose suspension setup to help it rotate in the tight / slow spots. 

frenchyd said:

alfadriver said:

In reply to frenchyd :

If you really think that AM car corners all corners at 30mph, I have a GRM Challenge V12 Jag for you in my garage.

Just because you are convinced that aero isn't effective for autocrosses does not make it right.  I very much doubt that you are an aero engineer with any expertise.  

So what speed are you going around the cones that change direction?  

I know there is a massive difference between courses.  I’ve been on tracks where you never get out of first gear and tracks where you could almost shift into high gear. 

At some point it stops being an autocross and becomes Solo 1. 

Yes on those high speed events aero makes a difference. 

You are right . My only formal training in aerodynamics came when the USNavy taught me to fly. I really learned a lot about low speed aerodynamics practicing stalls. What would you like to know? 

Training in how to fly a plane does not equal a degree in aerodynamics, they taught you enough to manage the plane.

Single element wings vs the multi-element wings shown in the picture.  Think extending flaps vs having them closed.

Each element adds more square footage to the wing area.

The high angles of attack are obvious.

The multi-element wings above have been tested and they have been proven to work at Solo II speeds.

From my own experiences, I used to race 1/10 RC road cars.  The rear wings were mounted to the rear suspension.  Tires were soft/sticky foam rubber.  Bodies were floppy plastic in fun designs.  The Renault Group C body was one of my favorites along with a Shelby Can-Am body.  I've had several races where the rear wing came loose or was knocked off.  It made a noticeable difference in how the car handled.  This was a few pounds of carbon fiber, battery, motor and electronics running at about 20mph at the most.  The little single element wing made it a decent handling vehicle, once gone it became a spin master and you had to slow down quite a lot more for the corners and you couldn't get on the throttle as early.

If you want to continue the discussion, please go start a thread in the Aero forum and the professionals can continue to prove you wrong.

Lets get back to the subject of the original post.

frenchyd said:

In reply to alfadriver :

I’ll grant you that racy wings, splitters,  and  spoilers add a serious look to a car.  Yes at 50-70 mph they are doing something. At 25-30?  

My question is the time and  money spent, could it be used elsewhere?  Would added power, better gearing, reduced unsprung weight, whatever yield the same or better improvement?   

Or is the psychological impact too much eye candy to ignore?   

Have you ever actually watched any of the Pro Solo racing?  Do you truly understand how fast and advanced these cars can be?

Tom1200 said:

Frenchyd in my F-mod / F500 car hairpins are taken at 25-30 mph but the bulk of the corners are between 50-70mph. Comparing the on board footage from A-mod to what I see, the difference is a solid 5-10 mph more. Also keep in mind very few A-mod competitors run on small lots at local events. 

I've seen video of the A-mod pictured above doing 75 mph in slaloms and 70 mph in 180 degree turns.  If you can get a Piper Cub to take off at 25 mph I'm willing to be a car with that much swept area on the wings is generating several hundred of pounds (or more) of downforce at 50 mph. Also note the aforementioned car is pulling near 3Gs in corners and almost 4Gs on the brakes............on an autocross course.

I remember reading back in the late 70's / early 80's that the small front air dam and little rear spoiler on Trans-Ams & Z28s generated a couple hundred pounds of down force.

As there are no long straights at autocross the drag penalty isn't anywhere near what it would be on a road course.

Yes at 70 mph there is downforce generated. At 25-30 ?  

My first Autocross was a tight parking lot with my main competition Sir Sterling Moss driving the freshly restored by the factory  ( and updated) DBR2 Aston Martin. Yes the one that won LeMans in 1958  

His car spanked mine in power. I had overcammed it in a vain attempt to catch up to the competition. We both had the same tires ( although he put a fresh set on midweek)   And there is no way in the world I’m going to claim to be a better driver than Sir Sterling Moss   He was fast, 2nd  fastest in my group.  

I was just faster.  

The point is clearly certain aspect of cars  are required for different tracks.   If you go fast enough aero works.  But tight twisty tracks reward narrow short cars. 

alfadriver said:

In reply to frenchyd :

If you really think that AM car corners all corners at 30mph, I have a GRM Challenge V12 Jag for you in my garage.

Okay, first, I LOL'd.

Second, if that is the car I think it is, the GRM feature on it mentioned that the car was better than they were, they couldn't keep up with how quickly it could change direction.  And that they wished they had their driving ability in a 19 year old body (or something like that).  There was no such thing as "braking", it was more or less not needed, and actually detrimental because you had to go fast enough to make downforce to go fast.

Aero makes you faster in corners and slower in straights.  Aero is used because you gain more in making the slow parts less slow than you get in making the fast parts even faster.

In reply to Stefan :

And to help steer this conversation back on course, people should note that the driver in that second video has used that car to win the E-Mod (big power, limited aero, tube frame, composite body) National Championship 11 of the last 12 years, and the 12th was by his co-driver in that same car.  If a 'medium' wheelbase were an advantage over a short wheelbase, he would probably not have chosen bodywork with an 80 inch wheelbase as the starting point when originally setting out to (very successfully) build a car to win national championships with.

Someone touched on it here but if you have a short wheelbase and know you can add stability with downforce I'd imagine a short wheelbase car would be the way to go.

I dialed back my 73" wheelbase car because my son found it to edgy. I'm super comfortable with oversteer and had the car set up so it would rotate rapidly in tight hairpins. I was fine with a car that oversteered on turn in to high speed corners.  

If you're a driver who is not especially comfortable with oversteer I'd imagine a slightly longer wheelbase car would suit you better.

In reply to frenchyd :

Holy berkeley dude...

YES PROPERLY DESIGNED AERO MAKES SIGNIFICANT DOWNFORCE AT 25-30MPH!!!

Take a look at this video of aero 'working' at 5ft/sec, or about 3.5mph.