Autocross theory and wheelbase

In reply to white_fly :

So how many autocross cars have the advantage of an F1 cars  design team?  

frenchyd said:

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?  

 

Please note how wide that car is from the cone.  

alfadriver said:

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.

You make valid points  regarding track width, vs CG height. But that is a bit of a red herring.  The simple fact is the wider the car the further the car has to travel. 

All other things being equal. 

Yes you can have poor suspension design on a wide car just as well as a narrow car.  

But if a wider car has to travel further it is at a disadvantage.  The more severe the change of direction and the more turns the more of a handicap.  

As someone else has already said the A mods don’t show up at the small tight turn events.  

Tom1200 said:

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.

I too like a tail happy car. With probably the same weight and 3 times the power you have I used an 88 inch wheelbase  to good effect. including  some 150+ mph straights. 

But items projecting forward also require you to travel further than cars with less overhang when the tail end is swinging wide.  

Is the down force generated by the projection great enough to overcome the handicap of a longer course?  

Driven5 said:

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.

It takes a lot of skill to drive a car like that.  So all of those NC's are very, very well deserved. To be able to deal with the "twitchiness" of a short WB car like that is pretty amazing.  Even with downforce...

The shortest WB car I autocrossed was 88", which was our Challenge Alfa.  And I was faster in our GTV which has a 91" WB.  But that comparison has nothing to do with the WB, so it takes a lot of development and driving skill to actually notice the WB change.  Once you are at a good point, shorter is better- as it will rotate quicker- which takes forces off of the tires to rotate the car.  

Given the current state of the art in terms of EM design, I bet a mid engined car like an early MR2 or X19 with it's original lay out would not be as fast, as they would be longer.  And many of the current designs have the lower moment of inertia benefits as mid engine cars- so that's a wash

I know the original poster can ask a topic be locked but can they ask one responder be banned from the thread?

i know I haven’t replied much but I’m loving the intellectual discussion that’s going on. I’m just a fly on the wall soaking up knowledge. Thanks. 

Stampie said:

I know the original poster can ask a topic be locked but can they ask one responder be banned from the thread?

 

i know I haven’t replied much but I’m loving the intellectual discussion that’s going on. I’m just a fly on the wall soaking up knowledge. Thanks. 

What you’re asking is that I delete every post they make in this thread?  (cause that’s the only way I know to do it.)

Also, to be clear, this question is in reference to a challenge build, right?

edit: I.e. that the question is about a car that doesn’t have to conform to ‘classist definitions’ 

I’d bust out some math, re: aero at 25mph, but I don’t think it’d end ‘our dear questioning friend’

In reply to sleepyhead the buffalo :

No let’s not do that and yes a Challenge build but I like the twist into aero. 

frenchyd said:

Please note how wide that car is from the cone.  

Please note that cone is marking the outer bound of the course, it is not a slalom cone.

frenchyd said:

In reply to white_fly :

So how many autocross cars have the advantage of an F1 cars  design team?  

That's a FSAE team. They were validating CFD, which there are many grassroots ways of doing.

To mods/others:  I would hope this thread does not get locked*, but some of these posts cluttering it can be pretty annoying.

*because I'd like to come back and have a serious discussion with actual math.  I've been trying to analyze the difference in actual distance traveled through a slalom as a function of wheelbase and its been an interesting endeavor.

 Maybe Funny But i like when A post Takes on an Exploritory vein ,Ya Know not Way Off,

  the 70% rule applies well, width to Length, working with what I Have and plans laid  My 96 inch WB chassis could stand a little out of the Length but I won't go to 90,

92 will likely be My final cut    Instant center will play a bigger part in putting down the power than C/G but on a dry sump car our rules dictate a 10" crank C/L to the Ground,with a Standard BBC Pan I think I Have 3" ground Clearance with a 12" crank C/L thats likely to go up but not lower,  the Vette has 4 link at the wheels and IRS so some adjustability needs to be Built in,

But that's straying from your Post

something else to keep in mind, iirc, there's an NHRA limit of 90" of wheelbase for engine swaps?  Feel free to correct me if I'm wrong there.

For track work, I tend to favor stuff that's 1.7x wheelbase longer than it's average track width, from stock.  Although, that hasn't been a "rigorous" study.

edit:
I'm not sure this is what you're asking.  You've got so many projects, it could be anyone of a number of things you're considering....

but, if you were wondering... yes, you should put the motor from the Q45 into the 350z

In reply to sleepyhead the buffalo :

Yes you are correct but I believe that was waved for the Challenge (maybe just above 11.5). If I was going below that I'd be concerned. My engine/trans/diff will be 77 inches from pully to rear hub center. I want everything inside the wheelbase so with a short driveshaft and radiator/fans I'll be at least 90 inches. 

Edit Close and I actually did consider that but I've now jumped way off the deep end. Not finalized so still in need to know status but I'll be able to choose what wheelbase I'll end up at. 

ProDarwin said:

*because I'd like to come back and have a serious discussion with actual math.  I've been trying to analyze the difference in actual distance traveled through a slalom as a function of wheelbase and its been an interesting endeavor.

There's a trade off between distance traveled vs. available grip and speed through the maneuver.  Narrow track width does limit the available grip- which is why cars get wide.  Which really messes up the math, as "all things being equal" just isn't true.  Of course, at some width point, you don't add enough grip to justify the increased distance traveled.

But that's where the trade offs start.

Doing the slalom math based on a fixed vehicle, then the time vs. distance equations work quite well.  But this isn't about fixing vehicle parameters, it's about how to change them to make it better.  Short wheel base makes the car rotate faster, but also makes it less stable.  Width of the track is a trade off between available grip vs. distance (and most of the time, the increased grip/cornering speed more than offsets the increased distance).  Weight changes how fast you can use the ultimate grip, CG effects how the tires/suspension can work- thus ulitmate grip, etc.

Those Mods are crazy fast,

Also throw in suspension design; if you take two cars with equal track width, one with a wide chassis & short links and one with narrow chassis & long links you're going to get noticeable different results. 

In reply to Tom1200 :

Can you expand on that because I happen to have both. I've been told the longer suspension arms are good so that camber doesn't change but is there more?

Stampie said:

In reply to Tom1200 :

Can you expand on that because I happen to have both. I've been told the longer suspension arms are good so that camber doesn't change but is there more?

It depends.  It's *possible* that the camber gain of the short vs. long arms make the wheel reaction the most efficient.  Not likely, but possible.  But for your likely constraints, Stampie, without looking at each car, the long arm suspension would be the better choice- as the camber gain vs. leaning (and the lean due to the force and CG) would mean the tires work more effectively.

That's just a guess.

I think what Tom was really saying is that "all things being equal" is almost never, ever the case.  (which is where I think frency is getting caught up)

Stampie said:

In reply to Tom1200 :

Can you expand on that because I happen to have both. I've been told the longer suspension arms are good so that camber doesn't change but is there more?

Ford tried to negate the camber issue with their pickup by using long overlapping straight axles. Which could be another approach to the issue of longer arms narrower chassis. 

While you would need to get creative I can see a wide base left side with a narrower base right side inside. Effectively gaining very long arms in a very narrow width.  

The problem as I see it is there is no perfect camber gain for every radius corner.  You still need to select a particular camber gain and work towards that.  

Stampie said:

In reply to sleepyhead the buffalo :

yes a Challenge build

If you shorten a chassis for a challenge car, any chassis, you will be my hero. 

In reply to white_fly :

Your mental image and my mental image are probably two different things. 

For the suspension arm length issue, shorter arms give more camber change, long arms give less (assuming we're talking about macstrut, swingarm, multilink, or unequal double wishbone suspension). Do you want a lot or a little? Depends on how much body roll you're designing the vehicle to have. For a race car you don't want much body roll, partly for aero reasons, partly for driver comfort, and partly because it's an easier way to keep your tires roughly perpendicular to the ground, so they have long arms to reduce camber change.

Shorter arms are pretty much only chosen as a packaging compromise, but you could design a car to work with a lot of body roll like the street model McLaren F1, it's just a lot of design work and feels funny going around corners. Interestingly you see the same design intent on the suspension on the original F1 and the FRICS used on the P1 - to have soft springs and good grip at the same time, the first with clever geometry, and the second by decoupling spring stiffness from body pitch & roll control.

^on a DWB, the length of the arm doesn't dictate camber change (rate), the length difference between upper and lower arms does.  The length of the arms would dictate track change (and resulting toe change) throughout travel.

If I remember my reading from Tune To Win it effects track, camber and roll centers. While I'll admit that I have only a rudimentary understanding of Carrol Smith's explanation of why longer links are better for a given set up but I do have practical experience.  Our D-Sports Racer started with short links and then was upgraded with longer links and the difference was night and day.  The corner speeds where higher but for me the most noticeable was the transient handling and specifically being able to really make the most out of trail braking. (Note I steer the car with the pedals) The stability during straight line threshold braking was hugely improved as well. If you have a choice go with the longer links. If you don't have a copy of Tune To Win get it.

alfadriver said:

ProDarwin said:

*because I'd like to come back and have a serious discussion with actual math.  I've been trying to analyze the difference in actual distance traveled through a slalom as a function of wheelbase and its been an interesting endeavor.

Doing the slalom math based on a fixed vehicle, then the time vs. distance equations work quite well.  But this isn't about fixing vehicle parameters, it's about how to change them to make it better.  Short wheel base makes the car rotate faster, but also makes it less stable.  Width of the track is a trade off between available grip vs. distance (and most of the time, the increased grip/cornering speed more than offsets the increased distance).  Weight changes how fast you can use the ultimate grip, CG effects how the tires/suspension can work- thus ulitmate grip, etc.

This is a purely theoretical exercise.  We can easily quantify the impact of width & grip on slalom speed.  I don't have a way of doing that with wheelbase, but I was giving it a go and its interesting.

The slalom equation posted earlier in the thread is assuming a single point moving in a sin wave offset from the cone by 1/2 vehicle width.

The turning radius of the back of the car is a function of the front wheels minus the wheelbase.  So really you have 2 sin waves going through the cones, 1 with an amplitude of 1WB less than the first, with the rear wave trailing the front.  This (rear wheel) wave is what determines how fast you can slalom.  The more you lengthen the wheelbase, the further the front wheels (and vehicle center) need to travel for a given slalom, reducing speed.  This is actually true for all elements of a course, but slaloms are the easiest to analyze mathematically.

In reply to ProDarwin :

The problem is how do you calculate the change in grip vs lateral acceleration?  That's so dependent on the car design, I'm not sure how you calculate that- suspension design, car CG, tires.  One can assume grip will stay the same, but it does not when you make a car wider, it does not when you move the CG down, it does not when you design the suspension to make the tires work better...

Although, perhaps the constraints Stampie has, the calculation can be done....