Bench Racing thought problem

This is a story premise that gets pitched at our editorial meetings from time to time. We're working on a way to do it that would actually be "doable" and produce meaningful results (or show us that the results are meaningless). But it's an intriguing question. Or maybe it isn't, but I'll ask it anyway:

So, assuming the same chassis configuration (like, we're talking about the same car being used, although parts can obviously be swapped out). The question is, for a given power to weight ratio, what will the "sweet spot" be. Let's say you have to maintain a 1/10 power to weight ratio in a BMW 3 Series, and there are no limits to either. Do you build a 1000lb car with 100 horsepower or a 4000lb car with 400hp?

Obviously there are a lot of variables (what track, what other performance metrics, aero, etc), but I'd be very curious as to what a graph of lap times would look like as you changed the weight and power.

If you were handed such a ruleset, where would you start? Would you automatically go for the lowest weight and take the power penalty, or would you crank up the power and find a way to hustle porky around the track?

jg

I like small cars so I would go small and light plus that is cheaper to a point.

I'd start with finding out what track we'd be running.

JG, you need to get a PS3 and join us on Wednesdays. We "answer" this question fairly often.

This last race i went with the "light as a feather with only 220hp" route over the "3000lb+ with 455hp" route.

Unfortunately, we were at Daytona road course, and the feather was slower, despite having a similar maybe even BETTER power/weight ratio.

Torque.

That is the answer. I have a very light race car that makes good power but it makes it up top and is more of a "momentum" car. I struggle to maintain speed in traffic because if I lift - I am losing a lot of time. I get beat to hell by cars weighing 500-700lbs more because they make a proportional amount of HP to keep the P/W but also have grunt. They spend money de-stroking or restricting bigger motors to cut power where I went smaller bore with more RPMs. I lose.

In clean air I can out pace them, out brake them, on raw lap time and top end - but I can't fight them head to head because they know they can pause going back to the throttle, make me back out of it then run away.

This story probably does not answer your question at all - but... going forward in a power to weight class, I'll sacrifice ultimate weight savings in favor a strong powertrain with a fat torque advantage and work the HP down and weight up to make the class.

If I were faced with such an open ruleset, in that I am limited only by power/weight, I would reach for the flattest torque curve crate engine available. Crate because easily replaceable and rebuildable and likely cheap parts. I'd take that engine, and build a car around it to meet the current weight.

That's not really what you're asking, though, you're asking 'which is faster', I think. If tire size is naturally limited by being in a sedan type body, then lighter is probably better to load the tires less. If that variable is eliminated by somehow normalizing the contact patch by the weight of the car, then it probably would be track dependant. Fun test, look forward to it.

I think the thing to gauge it on is the biggest tire you can fit in the example chassis. If it can only take a 195-width tire, power and weight are just liabilities.

A dehybridized Formula Hybrid car, 25 horsepower and 450 lbs, is far faster than any road car with 18 pounds per horsepower.

I think you have multiple variables.

Not just the PowerWeight, which you've consolidated to a single variable, but the next thing that occurs to me is that while at a relatively modest power you'd probably max out tire sizes, if you're playing way down in the featherweight/low-power area, you may need to run smaller tires just to work them hard enough to make them work. So you'd have to cycle through those multiple times finding the sweet spot for tire size for a 1000-lb, 100-hp car. Of course, you could simplify the editorial by starting at a spot where you were pretty sure you could use all the tire you could get on the car.

I assume you'd have to use something like an A-Mod car, or a locost or some other chassis that's rigid and capable of being more or less the same car from a very low weight and power and with the addition of more motor and ballast, still be equipped to deal with being a high-power car.

I would think that the sweet spot for any given car would be the least power and weight that was powerful enough to make use of the biggest tire that would fit. Below that you're leaving the opportunity for more power on the table, above that you're hauling around extra weight but are losing tire size relative to mass.

Maybe there's a sweet spot close to that sanity range, but I suspect the nature of power delivery and that particular chassis' habits would determine whether you could make use of a little more power at the expense of a bit more weight.

Of course, I keep thinking autocross, but more power at a fixed power/weight ratio becomes more beneficial as speeds climb.

As folks above have referenced, coming up with several power levels which have very similar torque curves could also be interesting... How closely must they parallel, or is it enough to have similar-after-scaling area under the curve?

I've only had a third of a coke this morning. I hope to heck I'm making some kind of sense...

The answer is probably not the lightest car, but the lightest car with the best engine- as others have said, flattest torque curve, best delivery, etc.

chaparral wrote: A dehybridized Formula Hybrid car, 25 horsepower and 450 lbs, is far faster than any road car with 18 pounds per horsepower.

It is but a 500lb one with 20hp and wide flat torque curve will beat it.

I would pick the power train configuration that gave me the best overall driveability. In other words, I would look for a good torque to weight balance.

Oh. Hey. How does aerodynamics play into this? Given the same shape, does the higher horsepower car have the advantage?

Totally depends on the track, the more high speed aero dominated the bigger advantage for the higher hp car. It takes so much hp to punch a hole through the wind regardless if weight. That being said to see how it works on most tracks, Look at the NASA performance touring uimited groups. Only rules are a modified hp/weight ratio.

One of the biggest modifiers is weight, you get penalized for being light, and get points back for being heavy. I bet they've put lots is thought and data into this

In reply to Alan Cesar:

Yes it does. I was just going to bring this up. The lighter car should have better cornering traction because the tires will be closer to equal loading and will be less stressed, this car will also brake far far faster and will be much more nimble. It's the obvious choice for a tight course. However, if the aerodynamics are the same, a very heavy very powerful car will destroy the lighter car in top speed and will have similar acceleration in a straight line (perhaps even a little better due to additional weight transfer, assuming RWD).

JohnyHachi6 wrote: this car will also brake far far faster

I take issue with this. Braking is a function of tire traction. A heavy car will create more traction than a light car even though it has more momentum to stop. I think it's not that simple. I even think in some cases a heavier car will be able to outbrake a lighter car.

In reply to tuna55:

Isn't it down again to tire loading? Once we've got all the tire we can fit on the car, and once the car is heavy enough to work those tires to their limit, get them to temperature and all, any weight beyond that has the same disadvantages that it does in cornering.

It comes back to the same principle: You get more traction by pressing down harder, but it's almost always less than 1:1, so while the tires will generate more grip, it will be less grip in proportion to the forces needed for the heavier car.

Alan Cesar wrote: Oh. Hey. How does aerodynamics play into this? Given the same shape, does the higher horsepower car have the advantage?

Oh, not at all. That is why you never see race cars with any spoilers, wings or anything

Do I get to pick the course? For a single run on a short autocross, I shall skip the ICE entirely and build an electric car with just enough battery to get around the course once.

Instant, consistent torque trumps all.

Giant Purple Snorklewacker wrote: Torque.

This. SOMETIMES.

I am not a good driver yet. My 3,600-pound lead sled has mediocre 300-treadwear tires with only a 255 width. However, on more "open" courses (such as at an autoX course on an airfield), I can keep up with better drivers in modified Miatas with better tires.

However, put me on a tighter course, and those Miatas will drive circles around me.

My point is that this question is far too complex to be reduced to only 1 or 2 variables. A heavier car might be more solid. A lighter car might be smaller, more nimble, and have more options in terms of a line to take through a corner due to having more room to maneuver.

Sky_Render wrote:

Giant Purple Snorklewacker wrote: Torque.

This. SOMETIMES.

That. ALWAYS.

There are limits to how heavy you can compensate for - but the weight of the larger engine, beefier components and chassis to manage it fit well under that umbrella. In a pure P/W class you wouldn't be 3600lbs unless you make crazy HP. You would also run slicks and make room for bigger ones if need be.

If it makes too much power at the weight we shave it to then we put some back in a better location and restrict the intake until it does not - but we tune for torque.

ALWAYS.

JG Pasterjak wrote: This is a story premise that gets pitched at our editorial meetings from time to time. We're working on a way to do it that would actually be "doable" and produce meaningful results (or show us that the results are meaningless).

Get a very tunable car. Get a good tuner, and get on the dyno to make 5 different power level setups, using ignition timing to reduce power across the board. Say 140, 160, 180, 200, 220 hp.
Set up weight ballasts as well.

Smaller power increases would necessitate less weight change, meaning less suspension impact.

Will produce meaningful results, but not quite the same as a ground up car build where brakes, wheels, tires, suspension etc. will all be different.

JG Pasterjak wrote: Obviously there are a lot of variables (what track, what other performance metrics, aero, etc), but I'd be very curious as to what a graph of lap times would look like as you changed the weight and power. If you were handed such a ruleset, where would you start? Would you automatically go for the lowest weight and take the power penalty, or would you crank up the power and find a way to hustle porky around the track?

If I you had the track data (basically XYZ 2d layout + elevation), you could setup a matlab script, or even an excel file with known weight, powerband, CdA, and approx max. cornering force to get a rough approximation of what's going to offer you the best setup. Just start with some obscene number like 4000lb/400hp and work your way down, reducing power across the board and weight by 10% each increment.

Hell, I'd like to see track data just to play with this myself.

To me it all comes down to tires. If the tire is speced build what works best to that tire. Some tires don't handle lite cars well others will be happy lightly loaded but all have max limits push that to far and your toast

If your going to try the balast weight experiment add the weight at the CG. John Block can help with that.

As a bench racing experiment I like the proposed problem.
As many have said there are a alot of variables and they all have to be weighed in combination with the entire package.
I pose the historic Chapparal cars as a likely source of data. They were matched in weight/size/power/tire to competing cars, but had other advantages... and were imeediately banned.

There are about a million variables. Obviously, we chose the light weight route since we are building a Challenge Lotus. The answer to your question lies in the nice simulation programs that are available to civilians now. We ran close to that million different combinations through the drag and autocross simulation before we went with 250hp and 1600lbs. You can test your theories easily with the software and then maybe do a car or two to validate each end of the spectrum. I can tell you that the drag results from the software seem to match up to real results. A few more variables in the autocross.