How much is AWD worth?

Help me bench build to get through Monday.

As part of the initial planning phase of a Subaru-powered mid-engine, rear-drive E Mod car, I put together a spreadsheet that tabulates acceleration based on weight and weight balance, tire size, gear ratios, torque at given rpm, etc. I have been playing with the variables to see what makes a difference where.

Sending power to all four tires ought to fairly significantly impact acceleration, as you have the full weight of the vehicle available to help make traction, rather than just the percentage of the weight on the (rear) drive wheels.

The counterpoint is turn-in. Further forward weight bias means more weight on front wheels means more available cornering traction.

So. Looking for thoughts on mid-engine, rear drive with 60%ish static weight on rear vs rear-engine, all-drive with 70%ish static on the rear. 

Autocross build, retaining street utility. Rear drive weight target is 1750lb, AWD has a 300lb handicap for a total minimum weight of 2050. Not so much worried about "can it be done/how could it be done" and more about "is it really worth it".

AWD because acceleration.

cornering and braking can be balanced via component / tire selection and tuning.

If you're going to have that much of your weight on the rear wheels (plus some weight transfer under acceleration), I'd say the benefits of AWD are probably not all that large.  

Does your engine have enough power to overcome the rear wheel traction, with sticky fat tires?

That's my question as well - are you expecting to be traction limited?  Of course, we're looking at acceleration out of a corner and not just in a straight line.

300 lbs for the AWD penalty seems extreme for a driveshaft, diff and a couple of halfshafts - although you will have some packaging challenges getting the power to run forward off the back of the gearbox.

Based on a tire coefficient of 1.5 (which is an SWAG), both AWD and RWD will be traction limited through the first several gears. My spreadsheet could be wonky, but plugging in the numbers for a known baseline yields reasonably accurate results.

The 300 pound penalty is sanctioning body-imposed, not the actual weight of the components. 

Upon further review, the components required for AWD would be far enough forward that, with the ballast needed to make up the 300 pound handicap, rear weight bias could be as low as 53% or as high as 70%, depending on ballast location.

My teenager tried to talk me into the AWD version of the CX-9.  I told him "You never stop paying for AWD.  You pay up front when you buy, you pay with more weight and worse handling, you pay with drivetrain losses and worse fuel economy."  You might need it once or twice a winter around here.

Of course, that's from the perspective of a passenger car.  Race car?  I'd be in the RWD with fat, sticky tires camp.

Back to the CX-9, which came with sketchy Yokohama Geolanders on it (bricks), every time they spin pulling out, my son quips "See?  I told you you should have gotten AWD!"   Grrr.  I wonder how long it will take to use them up?

Keith Tanner said:

That's my question as well - are you expecting to be traction limited?  

Follow up question:

how traction limited will you be?  Whats your accel chart look like with and without AWD?

AWD

RWD

These are admittedly somewhat simplified, as they assume a linear throttle map and don't factor in slippage sigificant enough to trigger traction control.

The % throttle to prevent slippage doesn't change all that much between the two, but the rate of acceleration is about 50% higher with AWD.

If somebody smart wants to proofread my spreadsheet, I can email it. I'd welcome the input.

JohnInKansas said:

The % throttle to prevent slippage doesn't change all that much between the two, but the rate of acceleration is about 50% higher with AWD.

!!!!!!! Why are we still talking about this?? Good lord.

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

jstein77 said:

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

That comparison is pointless.

Not really pointless.  I have found, pretty consistently, that the RS is about 2 seconds faster than than an ST on most courses we run.  It's basically the same chassis, and though the RS has more horsepower, it also has more weight, so the power-to-weight ratio is actually pretty close.  The only real difference between them is that the RS can put the power down earlier (WAY earlier) in the corner exit and enter the straight carrying much higher speed.  And that, in my opinion, is the true advantage of AWD, and the reason why I posted this comparison.

1988RedT2 said:

My teenager tried to talk me into the AWD version of the CX-9.  I told him "You never stop paying for AWD.  You pay up front when you buy, you pay with more weight and worse handling, you pay with drivetrain losses and worse fuel economy."  You might need it once or twice a winter around here.

Oh, handling is waaaaaay better with all wheel drive.  I still get gigglefits when I do something that would be utterly stupid in a two wheel drive car (full throttle acceleration from a stop while turning) and the car just goes.  Physics should not work that way.

I'm also done with having to worry about tire spin in the rain.  My last front wheel drive car was turbocharged and it would spin the tires at 60mph if you weren't careful.  New car has twice the power and MUCH better grip in the rain...

Since it's a racecar being built for a type of racing that constantly has you slowing down to speeds where even average cars have tirespin issues, the AWD will pay for itself.  If it were purely a street build i would say it wouldn't be worth it as MR cars have near-awd levels of traction  when accelerating on a flat,smooth road. 

Results from Yesterday:

Three AWD cars ahead of my FE/RWD EM.
I could have been closer with an air temp higher than 45° and if the course wasn't setup with a long straight-line dig to the first slalom. Conditions will vary, but anything other than ideal = even bigger advantage for AWD.

Only on grm:

"For autox you must have awd but rwd is superior for rallyx."

Hahaha. Not that I disagree (in a class where they compete directly awd will be faster). Any awd amod cars?

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. 

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

You're right. I think you should personally get your hands on both cars, put the same tires on them, detune the RS to 247hp, and run them yourself on a longer course and report back so we can have better data for the $0 we're paying for it!  

I'm not sure how high the bar needs to be for trying to analogize real street car experiences that actually happened to a theoretical race car that doesn't actually exist. 

In reply to Vigo :

I cannot thumbs-up this more than once, but I really want to! 

Vigo said:

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

You're right. I think you should personally get your hands on both cars, put the same tires on them, detune the RS to 247hp, and run them yourself on a longer course and report back so we can have better data for the $0 we're paying for it!  

I'm not sure how high the bar needs to be for trying to analogize real street car experiences that actually happened to a theoretical race car that doesn't actually exist. 

Or just not do absolutely useless comparisons in the first place. 

There is a 100 HP delta between the two, there's two different drivers, from a stats point of view 3 runs is not even close to enough data to pull any conclusions with a confidence interval above ~50%, they could have different tires, different mods. IMO it's better to have no comparison at all than one that says absolutely nothing and could lead the OP the wrong direction and cost him/her a significant amount of time and money. 

So my only real concern is, while the real-world data I've plugged in returns reasonably accurate results, if AWD in a light vehicle really makes as much difference as my spreadsheet says it does, why doesn't anybody else in E Mod run AWD?

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.

Maybe no one has really developed the AWD emod car yet? do we know of a serious effort that failed?

I'm wondering what you could do with a WRX or DSM drivetrain in a locost-style frame build. Reduce the wheelbase and track width of course. Front weight might be an issue with WRX/audi style, but the transverse front AWD might be able to get the weight back behind the fronts enough. 

WRX is the proposed donor, and that's more or less the plan. Would either 1. Convert AWD transmission to FWD then install mid-engine in a tube chassis (Mid eng, RWD) ... OR ... 2. Reverse ring and pinion and put the drivetrain wrong way forward in tube chassis (Rear eng, AWD).

Front eng AWD is off the table because I'm stuck on a rear engine tub and rules say a cars using a rear engine tub have to have the engine behind centerline of the wheelbase.