How much is AWD worth?

goingnowherefast said:

jstein77 said:

What's AWD worth?  In a Focus, about 2 seconds on a 40 second course:

Did you just use a RS vs. ST with different drivers and 3 runs as reference as an apples to apples comparison? lol. 

 

This was an example.  This same interval has been pretty consistent over close to 3 years of racing the RS, even at events where I was not the winner.  Also look at 2nd place in both classes - same 2 second interval.

OFFS, I must have stumbled into a SRS BZNS thread. My bad.

I read the OP, considered the percentage of nationally competitive mod car drivers I might expect to see on GRM, factored in the number of those guys who have first hand experience with an AWD mod car, divided that by the requirement to retain "street utility," and concluded that JinKS did not post here expecting a spreadsheet of apples to apples, perfect data.

Maybe just me, but having built my own EM, I tend to look at "street utility" and "national level mod car" as mutually exclusive ideas. In that regard, discrediting the real world examples given in this thread turns an entertaining discussion into an unfun debate.

I would presume the reason you don't see AWD Emods at Nationals is because the 300lbs penalty at that level of competition makes it unappealing. That is a different scenario than I would assume we are discussing here if we are attempting to build something with street utility. As such, we would be building an AWD autoX car the fits into EM and in that case the 300lbs is less significant at the regional level. It still matters if you are chasing overall PAX trophies because mod PAX sucks for under prepped cars, but less so if your regional competition is also running at a less than Nationals level of prep(common).

I don't have Nationals aspirations. I like to autocross this car, but I have no desire to prep it to the point that I can only enjoy it for a few minutes a month. It has been (completely and totally outgunned) in XPrep for some time now, and I am perfectly at home with the idea of a car that shouldn't be where it is. And my wife may divorce me if she can't take this car to the store from time to time.

The easy button is to spend a couple grand rebuilding the current setup, but if I'm going to the trouble of making it right, I might as well go nuts. "Anything worth doing is worth doing to excess", or something. 

So here we are. Sounds to me like AWD is the answer, given the parameters. Still not sure why Kiesel's Sprite and LooseCannon's MGB aren't AWD if it really makes as much sense as my calculations say, but to hell with it.

My arguments in favor of AWD really apply more to street-based vehicles rather than built-from-scratch pure race cars.  In the mod classes, I don't think I've seen any AWD cars competing nationally, most likely because of the weight penalty previously mentioned.

81cpcamaro said:

One thing I am thinking, if, in Emod AWD was an advantage, wouldn't there be more AWD cars in Emod, especially at Nationals?

In Amod, the low minimum weights probably are difficult to reach with an AWD vehicle, which may be why we don't see them there. Edit: John beat me to the question.

It's also really, really, REALLY hard to make a mid engined AWD vehicle.

(I think I have a way, with Easily Junkyardable Parts, but it's a little quirky, and probably would not be able to handle much more than 250 horsepower.  And if you want a manual transmission, that's also possible but much more expensive)

I just had a thought: The calculations on page 1 have to be wrong somewhere.  They don't show the AWD car being able to run at a meaningfully higher power setting at any point without wheelspin and they show a few points where the AWD car has a lower power limit before wheelspin.  Based on that, it can't possibly accelerate 50% faster than the RWD car. 

rslifkin said:

I just had a thought: The calculations on page 1 have to be wrong somewhere.  They don't show the AWD car being able to run at a meaningfully higher power setting at any point without wheelspin and they show a few points where the AWD car has a lower power limit before wheelspin.  Based on that, it can't possibly accelerate 50% faster than the RWD car. 

Good point.  I never looked closely, but the theoretical throttle (requested HP I'm guessing) is higher on the RWD car despite weighing less and having less forward traction.  Something is up with the spreadsheet.

Did that chart say 3470 ft/lbs of torque?  Building a tractor puller or Top Fuel car?

In reply to maj75 :

You're right, that's a typo. Not ft-lbs of torque but lbs of thrust potentially delivered through contact patches.

__________

The % throttle is based on comparison of traction and thrust at given rpm. 

In my head, the AWD should pull... Harder at a give throttle position because forward thrust acts at both ends of the car rather than just one. Twice as much force with about 40% traction available means faster acceleration with somewhat lower throttle opening before wheelspin.

RWD, 1750lbs, 1.5 coef of friction, 65% rear weight bias yields max acceleration of just shy of 32ft/s^2. AWD, 2050lbs, 1.5 cord, 65% bias yields max acceleration of 48ft/s^2.

I feel like I'm forgetting something painfully simple. Again, I'd be happy to have somebody spot-check my math, I will email the file if you want to play with it.

In reply to JohnInKansas :

Like for instance, my AWD figures are based on "engine makes power, driveline sends power to both ends of car, EACH pair of drive wheels makes xx thrust (same as the one set for RWD)". AWD can make twice the forward thrust from the same torque because it pushes from both ends.

I'm not at all confident that's correct.

If not, then the difference between R and AWD is as simple as traction. And the decision between the two is a matter of acceleration vs additional weight and its affect on cornering.

The more I think about it, the more the second option makes sense. 

Damn I hate when I say something dumb in public.

JohnInKansas said:

Damn I hate when I say something dumb in public.

What's worse is when you say nothing so you never correct yourself before you DO something dumb in public!

There's a lot of value in "rubber duck debugging."

Played with this some more and the punchline is this:

Nobody does this because 300 pounds is a significant enough weight penalty that you have to be really traction limited to make the extra traction do more for you than the added weight robs you of (even without considering cornering issues associated with additional weight).

I built an iterative calculation to refine the weight transfer rather than setting transfer as a constant for each gear, and AWD is quicker through first gear by almost .2 seconds. RWD walks that gap to nothing during the second gear pull, overtaking at 60 feet from launch and continuing to walk away to a 1 second gap at the 1/4 mile. As long as I can do most of my competitive driving in second gear (23-46mph) or higher, RWD is mo betta.

AWD carries the initial gap longer on slick surfaces or with high treadware tires, but is tied in the 1/4 mile.

RWD will entail quite a bit more throttle control to prevent ridiculous wheelspin.

As much as I like the concept of AWD for the sake of street driving, mid-engine rear drive appears to be the superior option given the SCCA-imposed parameters. Thanks all for being my rubber ducks.

In reply to JohnInKansas :

That agrees with what logic tells me in the sense that the more you can optimize the chassis in terms of weight distribution, etc. the harder it will be to get enough benefit from AWD to out-weigh its downsides.  In a car where you can't get the front end weight below 50%, however, you'd probably have a lot more to gain.  

To throw another shovelful of totally anecdotal information on this pile, there was a Tesla 3 Performance at Saturday's autocross.

That thing left even the slowest corners like a ball bearing out of a wrist rocket.

A soon as he got it rotated enough to see daylight, all he had to do was mat it and pick his next braking point.

. AWD can make twice the forward thrust from the same torque because it pushes from both ends.

I'm not at all confident that's correct.

That is not correct. That goes right back to conservation of energy theory, at least the way i look at it.

The way i would boil it down to its simplest form is that in terms of acceleration, an AWD car will be slower than a RWD car right up to the point that the RWD loses traction and the AWD doesn't.  In a powerful, light car that's frequently accelerating from slow speeds on corner exits, that will come into play constantly and the AWD car will be significantly quicker. 

Vigo said:

. AWD can make twice the forward thrust from the same torque because it pushes from both ends.

I'm not at all confident that's correct.

That is not correct. That goes right back to conservation of energy theory, at least the way i look at it.

The way i would boil it down to its simplest form is that in terms of acceleration, an AWD car will be slower than a RWD car right up to the point that the RWD loses traction and the AWD doesn't.  In a powerful, light car that's frequently accelerating from slow speeds on corner exits, that will come into play constantly and the AWD car will be significantly quicker. 

 

You could also look at history.  Any time all wheel drive and two wheel drive cars have had to compete on equal footing, all wheel drive dominated, even after being handicapped.

I'm not just thinking of stage rally, but also IMSA and Showroom Stock.  (Unless a 200hp, 3000lb car on a road course qualifies as lightweight and overpowered...)

Never underestimate the importance of being able to accelerate earlier out of a corner.  Hell, you could say that all driving techniques and theories boil down to "accelerate the earliest from the least disadvantageous speed"

Okay, dredging this back up for one more rehash, this time with a slightly different focus.

AWD accelerates better in and just beyond the traction-limited gears, but in my application, I'd have to hang the engine behind the rear wheels. While the added differential in the front and additional ballast should pretty much keep the Cg in the same location, the polar moment of inertia will change with the engine out behind the rear wheels. Will affect willingness to rotate quickly in a corner.

I'm not sure how to quantify the effect of p.m.o.i. Sure, I can get back in the throttle earlier, but how much harder will I have to work to get the car pointed the right way?