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:

https://static1.pt-content.com/images/pt/2017/02/The20Weighting20Game205BMark20Ortiz5D-1.jpg

[bovey]

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 Mark 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.


Ack! Where on earth did you send the e-mail to? I don't recall seeing or receiving it! I answer all of my e-mails, even the crazy ones. And this is not crazy...

[Dr G]

Ack! Where on earth did you send the e-mail to? I don't recall seeing or receiving it! I answer all of my e-mails, even the crazy ones. And this is not crazy...No worries, I was referring to Mark Ortiz. I've edited my post to make that more clear. If you read it again it should make sense.

[bovey]

No worries, I was referring to Mark Ortiz. I've edited my post to make that more clear. If you read it again it should make sense.


OOOOHHHHHH. Hilarious! FYI, my first name is Mark. What a random coincidence.

[GoodysGotaCuda]

I would target 50/50 and no more than 40/60 if I could have it my way on a RWD vehicle. Unfortunately that's hard to do on these old cars.

[rustomatic]

One thing I've been curious about is potential for a pendulum effect (at limit cornering, possibly with bad driving or just grip inconsistencies) as a result of rear-mounted gas tanks and batteries (and ridiculous tail pipe efforts). C4 Corvettes always made me think of this (tanks were high and all the way back), but later ones (C5) went to split tanks, much like Beemers and Benzes (not mounted in the rear). It seems that generally, the more you can put between the axles, down low, the happier things will be, perhaps regardless of how the actual numerical weight balance winds up.

As far as rear-biased weight goes, what's better: keeping it as low as possible (like the typical NASCAR fuel cage setup), or as far forward as possible (just above the axle)?

[Dr G]

One thing I've been curious about is potential for a pendulum effect (at limit cornering, possibly with bad driving or just grip inconsistencies) as a result of rear-mounted gas tanks and batteries (and ridiculous tail pipe efforts). C4 Corvettes always made me think of this (tanks were high and all the way back), but later ones (C5) went to split tanks, much like Beemers and Benzes (not mounted in the rear). It seems that generally, the more you can put between the axles, down low, the happier things will be, perhaps regardless of how the actual numerical weight balance winds up.

As far as rear-biased weight goes, what's better: keeping it as low as possible (like the typical NASCAR fuel cage setup), or as far forward as possible (just above the axle)?

This is great question. And this is specifically why I posed my question to Mark Ortiz so as to ignore any effects from polar moment of intertia. This is what you are referring to when you mention various masses hanging out past the wheels and causing a pendulum effect. The more you can concentrate the mass of a vehicle towards the center of mass (looking down from above), the less resistance that vehicle will present to wanting to change direction (it will be more 'agile').

I do not think there is a single right answer to your question. But obviously the more you try to move weight rearwards to improve the balance on the rear tires, the more you are increasing the vehicle's polar moment. Especially if you start moving heavy items behind the rear axle line. So the two goals are at odds. Seems like this is always the case :)

[Peter Mc Mahon]

I like this topic, very interesting. A couple things that I think about are, if a 50/50 is ideal then under braking or acceleration and I guess cornering, the vehicle would not be 50/50 or even close to it. I would maybe guess that 50/50 measured at the most extreme point of cornering would be most balanced? Also, I have no examples, but if a vehicle is 50/50 wouldn't the spring rates front to rear be somewhat close? [I understand the front is getting heavy and the back is getting light while braking] Thoughts?

[Dr G]

I like this topic, very interesting. A couple things that I think about are, if a 50/50 is ideal then under braking or acceleration and I guess cornering, the vehicle would not be 50/50 or even close to it. I would maybe guess that 50/50 measured at the most extreme point of cornering would be most balanced? Also, I have no examples, but if a vehicle is 50/50 wouldn't the spring rates front to rear be somewhat close? [I understand the front is getting heavy and the back is getting light while braking] Thoughts?

At least regarding the response I posted above by Mark Ortiz, there is nothing particularly ideal about a 50/50 front/rear weight distribution. Ortiz's point is only that front and rear tire widths should be chosen to match the front-to-rear weight distribution, for steady state cornering performance.

To understand this subject it is helpful to be familiar with the the concept of "tire load sensitivity". This dictates that a pair of tires will create maximum grip when they are evenly loaded. This is because the friction coefficient of a tire is not linearly dependent on vertical load. Thus as load transfers from one tire to the other in the pair, the one that gains load gains less traction than the tire that loses load (which proportionally loses more traction). So the net total traction is reduced from the evenly loaded condition. This is the #1 reason that performance/race cars strive to have a low CG height. Because a low CG height reduces load transfer under cornering, braking and acceleration, and thus the tires on the vehicle remain more evenly loaded (be that side to side or front to rear).

Under hard braking there is an increase in load on the front tires, and a reduction in load on the rear tires. From the point of view of tire load sensitivity this is bad, and reduces overall braking traction. However, if the vehicle starts out being rear heavy, then under braking as weight shifts to the front tires, the front/rear weight distribution approaches an evenly loaded condition. The result is greater overall braking traction.

For acceleration of a RWD vehicle, tire load sensitivity is not an issue. The only thing that matters is that the more vertical load you can get on the rear tires, the more traction they will make. So for this case you want a vehicle that is rear heavy, and if fact having a high CG is helpful as well (think of a 'Gasser' drag racer for instance). As long as the thing does not wheelie end over end (as Ortiz mentions).

But these are idealized scenarios. In real life a performance vehicle must turn as well as brake and accelerate, so you have to consider the entire package.

[rustomatic]

Another funny thing or two to consider is the supposed minimal effective difference between moving an entire engine and transmission back five inches versus just putting the (heavy) battery in the back. On a numerical balance basis, the difference can be humiliating to the person who went to the effort of the engine move. Whether or not the numerical difference (weight balance) actually will result in the same (applied) driving difference, I would expect that the drivetrain move would be considerably more valuable/perceptible in handling, braking, and steering. The other funny thing is how BMW can achieve 50/50 with the engine (I6) hanging way out over the front axle line (with stock battery in trunk). Yuck, but it works amazingly well. Would the person who designs the race car version keep this? Likely no. Numbers lie.

[Dr G]

I was scratching around on my hard drive, looking for old tech articles on Ackermann, when I stumbled on this Mark Ortiz newsletter from 2006. It addresses a question I've seen asked frequently. What is the trade-off between moving weight back to improve front to rear weight distribution, vs. keeping weight centered in the vehicle for better polar moment (cornering responsiveness).

Mark addresses that particular question in this newsletter, towards the end. I've highlighted the text in Red and Green.
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[Dr G]

In case anyone is interested, there is a thread now on another forum relating to whether removing weight from the rear of a vehicle makes it faster, even if it screws up front to rear weight distribution:

http://corner-carvers.com/forums/showthread.php?t=49681

As usual with these subjects, there is not much agreement, but it is still interesting to read.

[rustomatic]

I just joined that site recently for reading purposes. There is some good tech in places, but there is so much tangential BS everywhere that, well, the site seems to have lost its meaning. The "weight balance" discussion seems to go nowhere, with (very basic) discussion of only front vs. back--Miatas and Euro-stuff have put batteries in the trunk for decades, and it's no big secret, except to those who can only think of a shiny 9-inch with shiny coilover fangs as the most modern thing. What about the middle? With the quantity of tirades on the internet welding quality rating system, you'd think someone there would have a bit more creativity to pee on the fire. Not so much.

[Dr G]

With the quantity of tirades on the internet welding quality rating system, you'd think someone there would have a bit more creativity to pee on the fire. Not so much.Man, you are so right about the welding quality discussions!

[RobNoLimit]

Good read. Ortiz is a sharp guy. 50/50 static would be hard to get in a truck. I've tried. I would agree to some extent that lighter is faster, even at the penalty of weight balance. Up to a point. I have done a lot of testing with tire size, and I agree that the best approach (cornering) would be to match the size of the contact patch per pound. If I did this on the HellBoy, I would have a 335 up front and a 305 in the rear. That may or may not corner better, but I would loose some straight line launch. So, not going to do it. On current builds, I am also focusing on weight placement with the tire centerlines. As if I drew a line from the center of each contact patch around the truck. Sort of a rectangle. Any weight outside that area, especially in the rear, can act as a pendulum, and pull the end of the truck around. And, that's no good. One last thing, 'Low' weight, as in down low to the ground, is not so bad. Like a boxed rail or a big X member, this 'Low' weight will effectively pull the CG down a little, and that is worth the price per pound.

[Ben@SpeedTech]

I'm the "creative designer and can't wrap my head around math" guy, so I have zero engineering insight to add here, take my thoughts as such. My car is around 50.7 % front and 49.3 % rear totally by accident. I planned to add on a fiberglass front bumper and hood and was worried that I may go lighter in the front and throw off that "50/50 is best" scenario. So I once asked in casual conversation with Ron Sutton in layman's terms what's the best weight distribution, 50/50? His short answer was basically yes. I asked if there was an advantage at all to be heavier in the rear, and tongue in cheek he said he likes it when his competitors build a car rear heavy, it makes it easier for him to win. That was the end of the conversation after a couple laughs. I'll let him explain what exactly he meant and the scenario behind that answer.

He also emphasized that a square tire size is a better idea. Again in layman's terms he basically said with muscle cars we come from a drag racing and pro street past so the wider rear tires is a look we're accustomed to and want to pull into Pro Touring too. He continued to explain that with a wider rear tire you end up detuning the rear suspension to balance the car and it didn't really make a bunch of sense to do that. Having square tire sizes makes more sense to me in a longevity view too, if your wheels have the same offset like mine, (or if you don't mind swapping tires on wheels) you can rotate them for longer life. He also mentioned that some winning cars he built in the past actually ran a wider tire up front. That of course depends on those specific cars so I'll leave it there for him to explain the situation.

[Dr G]

I have done a lot of testing with tire size, and I agree that the best approach (cornering) would be to match the size of the contact patch per pound. If I did this on the HellBoy, I would have a 335 up front and a 305 in the rear. That may or may not corner better, but I would loose some straight line launch. So, not going to do it.


He also emphasized that a square tire size is a better idea. Again in layman's terms he basically said with muscle cars we come from a drag racing and pro street past so the wider rear tires is a look we're accustomed to and want to pull into Pro Touring too.

I continue to be amused/perplexed by the seeming insistence of so many folks building "Pro Touring" cars to put wider tires on the rear, when the vehicle is front heavy. It seems so obvious that this would not lead to good cornering.

I agree that it's probably an ingrained thing from a drag racing heritage.

The funny thing is though, that for going straight in a fast line, wider rear tires are not necessarily the optimum solution. Sure, you can have narrow front tires, but what you really want in the rear is tires with a large outside diameter, not so much with a wide profile.

This follows from the theory of how tires develop grip, via slip angles (cornering) or slip ratios (acceleration and braking). It took me a lot of reading and studying over the years to get a handle on how tires really work. With mechanical friction and chemical grip (inter-molecular forces) each playing a part. There is a great book on the subject:

https://www.amazon.com/Racing-High-Performance-Tire-Using-Balance/dp/0768012414/ref=sr_1_1?s=books&ie=UTF8&qid=1490403103&sr=1-1&keywords=racing+and+high+performance+tire

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