I have always told people that moving the position of an engine does not make that much of a difference in the car’s weight distribution and that moving a battery from the engine compartment to the truck can make more difference than moving the engine quite a bit. We showed how just how much moving the battery affects corner weight and weight distribution a few years back here.
Now we did a similar experiment on Dai Yoshihara’s unlimited drift car to show just how much of an effect moving the engine can make. The unlimited car has a semi-tube frame and an unequal-length A-arm front suspension that Chris Eimer built, kinda as a fun project, it would not be legal in Formula Drift and Dai drives this car for appearances and exhibitions.
Because of the tube frame construction, it was easy to build a simple aluminum rail to temporarily move the Chevy LS-based V8 engine around quickly for the experiment. The bare engine weighs about 460 lbs without its accessories, turbo, and intake manifold. When the car was in FD legal trim, it had a 51/49 weight distribution and weighed 3180 lbs. Now it weighs 2700 lbs and has a little bit more weight in the rear than the front. As a note, to be simpler, we did this experiment with no clutch or transmission in the car and that weight would have added 0.03% or so to the total front weight.
When Chris built the tube frame front end, He clipped the car, fabricated a new more rearward firewall, and set the engine back a full 6.5″ in the chassis to turn the car into a front-mid engine configuration. On the Subaru BRZ chassis, this is a pretty huge difference as the car had a pretty short flat 4 FA20 engine from the factory and a short engine compartment. The LS engine is now entirely in the back of the front axle centerline.
With the bare engine, 49.93% of the car’s total weight is on the front wheels. 50,07% is on the back wheels. With a fully dressed engine with a turbo system, clutch, and transmission, it is still a bit shy of 50/50. For the sake of this experiment, only the first number is important.
Next, we slid the engine forward on the temporary aluminum rails Chris had built for the experiment to see what effect that would have on the weight distribution.
We slid the engine 3.5″ forward, this is a pretty decent amount and you would think it would make a huge difference.
Huh… yeah, OK. Thinking, it’s a big chunk of mass but in the grand scheme of things moving it 6.5 inches is going to have much less effect than moving something that’s maybe a tenth the weight 6.5 feet (numbers out of thin air). But obviously does get to a point where the answer is “do everything you’re allowed to”.
Just musing; do you have general thoughts on when weight bias effects overwhelm mass centralization or vice versa? For example, and pretending you can get crossweight even either way, when would you prefer to put a hundred pounds of ballast in the passenger seat area vs the trunk?
Think of it more like using a breaker bar to break a really tight lug nut, as opposed to using a stubby wrench to break the same lug nut. With the battery being at the very front or very rear of the car, it could have the same effect as the much heavier engine toward the center of the car.
Just a thought.
“moving it 6.5 inches is going to have much less effect than moving something that’s maybe a tenth the weight 6.5 feet”
yeah i think that’s what he was saying
Right. My question was when fighting for more rear weight or fighting for better moment gets more critical to actual lap times… or at least a viewpoint?
On my semi-scca prepared 68 Triumph GT6, at 2000 lbs, moving the battery from front to rear made a 5 point difference in weight distribution. In autocross wins, I added the rear spare fully to the rear to further improve weight distribution. Later for Summit Point raceway track days, the rear spare was replaced with a 9/16″ rear swaybar I designed using Fred Puhn’s great suspension book, to detrmine roll stiffness balance.
The GT6 OEM battery location:
One important thing to consider when thinking about weight distribution is rotational inertia. While drift cars mostly rotate around the front axle, so moving an engine doesn’t cause a huge difference, in regular racing or street driving while making a hairpin car will have a massive amount of rotational inertia because the engine moved forward and the rotation axle is somewhere near the center of the car. M(R^2) – formula for the moment of inertia and also the name of the car designed around its mitigation.
Simplified and not totally correct example.
Let’s assume our car has a wheelbase of 2.76m, let’s assume that our’s suspension rotation center is in the middle. Let’s assume our engine’s length is 0.65m and mass is M. If we put the engine inside the wheelbase inertia’s arm (distance between the rotational center of the car and the center of mass of the engine) would be 0.73m, so the moment would be 0.54M.
If we put half of the engine outside: the arm would be 1.055m and the moment would be 1.11M – more than two times increase of resistance.
I tracked a Porsche 924Turbo at Summit Point Raceway, and with the rear transaxle, the car had a high mass moment of inertia. It was easy to handle on 60+mph corners, as with slight rear roll resistance bias, the back end would come out in slow motion. I was following an insrtuctor’s 944 and his did exactly the same thing.
When setting up my light steel flweel on my 93 RX7-TT, to compare polar moments of inertia wit the stock flyweel, I suspended each in a horizontal plane with 3 strings of a specific length, and timed the periods of oscilation to do the calculations. Also measured the presssure plate this was. Net was a ~ 30% reduction in inertia, for quicker 1st gear acceleration … easier to spin the wheels on take off.