(Originally posted 12/28/2016, Moved to WordPress 1/16/2017)
I’m not trying to stir up controversy by saying this, but I do not believe 50/50 is the optimum weight distribution for a rear wheel drive car (whose primary goal is setting fast laptimes on a road course). I wanted to preface this post with that disclaimer because I have read through a few forum discussions on this topic that have actually gotten pretty heated. If you read my first post a couple weeks ago, you may have noticed that my projected weight distribution is 40% F / 60% R. It may move a few percent toward 50/50 as the design progresses but I am definitely shooting for a rearward weight bias. Here’s why (it may get a bit technical):
The amount of weight transfer during acceleration (or deceleration) is related to the car’s wheelbase, center of gravity height, and the rate of acceleration. The TT1 has a wheelbase of 98.5” and the CG height should end up around 13”. This results in weight transfer of 13.2% per G of acceleration. So, when accelerating at 1 G (which is roughly the average acceleration for a 3 second 0-60) the vertical load on the rear tires is 13.2% higher than when it is sitting still (73.2% for the TT1). On a rear wheel drive car, this is good for traction when accelerating out of corners (the more vertical load on the tire, the more tractive force can be applied to the road without slipping).
When braking at 1 G the weight transfer is 13.2% higher on the front tires making the weight distribution 53.2% F / 46.8% R. Pretty evenly distributed across all four wheels, which is good since all four wheels have brakes. This means the front brakes don’t have to be massively oversized (and heavy) compared to the rears like they are on most street cars. In fact, the TT1 will use the same rotors and calipers on the front and rear:
Now, 1 G braking is not all that impressive for a track car and I expect the TT1 to be closer to the 1.5 – 2 G range, especially on R-Compound tires. This would put the dynamic weight distribution around 60% F / 40% R. So the front brakes will still be doing 50% more work than the rear. I’m hoping some good brake ducts will help keep the extra heat under control at the front.
Compare that to a C7 Corvette with a 50/50 static weight distribution, 106.7” wheelbase, and 17.5” CG height. Braking at 1.5 G the weight distribution would shift to 74.6% F / 25.4% R. The fronts are doing 3 times the work of the rear.
Now compare it to a sporty FWD car like the Civic Si with a 62% F / 38% R static distribution, 105.1” wheelbase, and an estimated 20” CG height. Braking at the same 1.5 G results in a dynamic weight distribution of 90.5% F / 9.5% R. The front brakes are doing 9 times the work of the rear!
What about cornering? Assuming the same tires front and rear you would expect a car with a rear weight bias to want to oversteer, but of course there is no rule saying that the front and rear tires have to be the same. The general rule of thumb is to try to match your “tire width distribution” to your static weight distribution. I have been designing the TT1 with 225mm front tires and 275 rear tires which only gives me a 45% F / 55% R Tire Width Distribution. I may end up going down to a 205mm front which would give me a 42.7% F / 57.3% R Distribution but those are the limits of sizes available for Hoosier A7’s so I can’t get any closer to my 40/60 Weight Distribution. For now, I am staying with the 225mm fronts just because it will be easy to fit a 205 under the fender if I need to, rather than designing for the 205 and then trying to cram a 225 in there if I end up needing more front grip.
Once you have your tire sizes roughly optimized for your rear weight bias, tuning your cornering behavior is the same as it would be for a 50/50 car (or a front-biased car) by adjusting front and rear roll stiffnesses via springs and/or swaybars (I’m not going to get into that in this post).
So… while I can certainly see how a 50/50 distribution goal just seems to make sense, logically, I also believe that the “zeal” with which some enthusiasts defend it is a result of the marketing of certain cars (one make in particular). That’s all I’ll say (really not trying to start an argument).
I hope this has been interesting to some of you. I don’t want these posts to become too technical and boring. Here’s another picture:
– Mickey Oswald