Ideal Front-to-Rear Weight Distribution?

[Dr G]

A while back on Bovey's Targa Truck thread there was a discussion regarding front to rear weight distribution. As part of this discussion I theorized that the oft discussed "50/50" front/rear distribution might not be the ideal, but maybe it was somewhere closer to 45/55% to aid in braking and acceleration. It is frankly something I've always wondered about. Somewhere near the beginning of that discussion on Bovey's thread I posed the question to Mark Ortiz, who writes a monthly column in "Racecar Engineering" magazine called "The Consultant". In this column Mark answers questions from readers regarding anything related to racecar chassis theory and setup.

Though I emailed Mr. Ortiz my question, I got no response, and figured it was not an interesting enough question for him to address. Then to my surprise in the latest issue of Racecar Engineering, there is my question, and his answer.

Note that I posed the question in very specific terms, with a lot of assumptions (ignore aero, polar moment effects etc). So his answer is somewhat idealized. Although he does not give a specific recommendation as to a single ideal front to rear weight distribution, I take two points from his answer: (1) Front-to-rear tire width stagger should match front-to-rear weight distribution for optimum steady state cornering and (2) Moving weight to the rear will aid both braking and acceleration. Although he does not directly state it, the conclusion of this would be that a rear heavy car with big rear tires to match the weight distribution is optimum for a rear wheel drive racecar on asphalt.

Obviously as you start to reintroduce a lot of items that were ignored in the question (aero, polar moment etc), then this changes the answer. Anyone who watched this year's Rolex 24 Hours of Daytona will have heard about the "new" Porsche factory 911 racecars, which had moved the engine forward from the traditional location behind/over the rear wheels. These new cars were faster, but it was stated that this was not due to decreased polar moment, but more due to the fact that moving the engine forward allowed for a bigger rear diffuser. The point being that in the rear world, with rules and physical limitations, the answers to idealized questions are often different from what one might expect.

Note that part (1) of Mark Ortiz's answer above supports my belief that for a Pro Touring truck, with likely a medium to high front weight distribution, you really want the biggest front tires you can fit if cornering speed is on your agenda. A lot of others believe this also (Ron Sutton comes to mind). It's why many (most?) of the faster PT trucks (and cars) have 'square' setups (equal width tires front and rear). The beauty of most trucks is that they actually do have room for wide front tires, unlike guys with 1st gen Camaro's and Mustangs and such, who sort of struggle with this due to space limitations.

Mark Ortiz is very open with his information. If you email him he'll put you on the list for his monthly newsletter, which he distributes for free and encourages folks to share. An archive of these newsletters can be found at this link, under "Chassis":

http://www.auto-ware.com/techref/lib_index.htm


Here is the Mark Ortiz response to my weight distribution question: