Weight Distribution & Transfer

[Caito]

Hi guys. Maybe you can help me clear up some doubts I have with regarding car weight distribution mainly.

Problem is I read several times that teams put weight on the front to improve front grip. That contradicts my beliefs. First of all if the tires are the same, suddenly you have (let's put a number) 20kg more at the front. 20kg give more grip, (CoF*Normal_force) but due to tire sensitivity you won't have "20kg of grip" more. So your front now has more weight and more inertia... how can that improve the front end? In my opinion the car would understeer even more now.

[Lycoming]

It changes inertia... but inertia isn't really directional, in that it doesn't make sense to have more front inertia. btw I assume inertia means yaw inertia, which is the sum of all masses multiplied by the square of the distance to the axis of rotation.

When you have a difference in front and rear grip, you potentially have a difference in lateral force generated at the front and rear, resulting in some yaw moment. This moment is what, in some senses, characterizes understeer/oversteer. 0 moment is a neutral car.

So instead of thinking in terms of a front force and a front mass, think in terms of a yaw moment and a yaw inertia, which encompasses the entire vehicle. If you move ballast to shift weight distribution forward, that does not necessarily increase your yaw inertia. Say my yaw axis is halfway between the front and rear axle on the vehicle centerline, and I shift weight distribution forward by moving ballast located at the rear axle to the center. That would decrease yaw inertia and increase front normal force, which potentially increases the yaw moment in the oversteer direction. If you move weight forward by moving ballast from the CG to say the front wing, you (potentially)increase oversteer moment, but increase yaw inertia. Is the net effect oversteer or understeer? depends on the situation.

That said, I don't think ballast is typically used to adjust balance in that sense. I remember reading an article by Jim Hall in which he says that weight distribution alone has no effect on the balance of a car.

[Greg Locock]

" I read several times that teams put weight on the front to improve front grip"

OK, where did you read this? What else did they change at the same time?

[Jersey Tom]

Problem is I read several times that teams put weight on the front to improve front grip.

I'm with Greg on this - where did you hear that? More importantly, front weight or front mass? You can add front "weight" in the sense of adding front downforce with a wing adjustment, and indeed that will give you more front... I won't use the word. Or similarly you can add front weight temporarily by means of holding a bit of brake pressure and transferring load forward. Adding front mass however, as you point you, is quite different and generally the opposite... and the teams in F1 are locked into a pretty narrow range of front mass are they not?

I try to never say never, and there may be some rare and extenuating circumstances I could think of by which moving mass forward had some freeing effect on balance - but the likelihood of any car being in that operating range is pretty slim IMO.

I remember reading an article by Jim Hall in which he says that weight distribution alone has no effect on the balance of a car.

I'd have to see the article to confirm that he said that. I'd say mass distribution is one of the most fundamental / powerful balance tuning tools out there.

[machin]

wrote:I remember reading an article by Jim Hall in which he says that weight distribution alone has no effect on the balance of a car.

I'd have to see the article to confirm that he said that. I'd say mass distribution is one of the most fundamental / powerful balance tuning tools out there.

I guess Jum Hall meant something along the lines of:-

If a racecar's tyre sizes, roll stiffness distribution and downforce are all in harmony with the mass distribution then it doesn't matter what that mass distribution is...

However, by inference, if you're locked into a certain car design then mass distribution, as JT points out, has a huge effect on balance....

[Belatti]

I guess caito is talking about the Bridgestone era where teams were putting ballast at the front because of tyre stiffness and stuff. I think that with the mandated weight distribution there is not much to do now, other than reducing polar moment of inertia in Z.

[Greg Locock]

If you just, simply, move mass forward in the car, you will increase the rake on the car, which will affect the aero signficantly, and might well increase the downforce from the undertray at the front, in particular. However, that is an unfeasible change typically.

[thisisatest]

for f1 in particular, I could see someone moving ballast forward if they were not generating enough heat in the front tires. before they narrowed the front tires was a good example.

[Jersey Tom]

for f1 in particular, I could see someone moving ballast forward if they were not generating enough heat in the front tires. before they narrowed the front tires was a good example.

Depends on if you consider tire temperature a primary or secondary influence.

In most applications, and by conventional fundamentals of vehicle dynamics, moving mass forward adds understeer. So if someone wants to play the temperature card it better be a big hitter.

And yes, people sometimes love to go on about tire temps but it isn't uncommon for larger factors to be at work.

[DaveW]

I was sure that this topic has been covered before, but a quick search failed to find it.

In most applications, and by conventional fundamentals of vehicle dynamics, moving mass forward adds understeer.

True statement, JT, but any loss in balance caused by moving the c.g. can be recovered by changing bar settings, I think, for a given suspension geometry, and within sensible limits. Other parameters may also help (geometry, differential settings, etc.). So now you have the prospect of comparing the performance of vehicles with changes in c.g. position, all with lateral balance.

I rather like Ben Bowlby's remark, quoted by machin. He brought tyres and downforce into the equation, & I think both are important. If handling is to be consistent over a speed range, then the c.g. must follow the aerodynamic centre of pressure quite closely, & I like to think that a well set-up vehicle should "work" its tyres in a sensible way.

My business is helping race teams with mechanical set-up, mostly for track (i.e. road course) performance. To help me (and my teams) I have developed a "Performance Index" (PI), a cost function that generates a single number intended to describe the mechanical performance of a vehicle using its response as measured on a multi-post rig. The precise contents of the PI are propriety, but it includes fairly obvious things like sprung mass damping ratios & load variations.

Generally, a rig test can improve performance on the track, often improving tyre "life", and also (with some insight) can sometimes help teams improve the vehicle in other ways. Interestingly, it has definite views about matching the vehicle to its tyres. Most F1 teams had to make large changes in c.g. position in 2007, when they all switched to the same Bridgstone tyres. That change was predicted by rig tests (actually before the first test day, I recall).

[Jersey Tom]

In most applications, and by conventional fundistribution of vehicle dynamics, moving mass forward adds understeer.

True statement, JT, but any loss in balance caused by moving the c.g. can be recovered by changing bar settings, I think, for a given suspension geometry, and within sensible limits.

To some degree, sure. Though there are also handling changes imposed by a front distribution adjustment which cannot be recovered with mechanical balance. Depends on tire properties but not all vertical load changes are equal.

[DaveW]

Though there are also handling changes imposed by a front distribution adjustment which cannot be recovered with mechanical balance.

I'm sure you are correct (we have had this discussion before). Fortunately, perhaps, the majority of tyre designers don't set out to be obstructive.

In any case, I didn't suggest a "front distribution adjustment" would necessarily improve performance, simply that such a change could be evaluated without the complication of changing balance.

[gixxer_drew]

I rather like Ben Bowlby's remark, quoted by machin. He brought tyres and downforce into the equation, & I think both are important. If handling is to be consistent over a speed range, then the c.g. must follow the aerodynamic centre of pressure quite closely, & I like to think that a well set-up vehicle should "work" its tyres in a sensible way.

Recently I am questioning this even though I used to follow that religiously. In the last couple years I've been breaking this rule and seeing improvements in lap time at certain types of tracks.

[Tommy Cookers]

I rather like Ben Bowlby's remark, quoted by machin. He brought tyres and downforce into the equation, & I think both are important. If handling is to be consistent over a speed range, then the c.g. must follow the aerodynamic centre of pressure quite closely, & I like to think that a well set-up vehicle should "work" its tyres in a sensible way.

Recently I am questioning this even though I used to follow that religiously. In the last couple years I've been breaking this rule and seeing improvements in lap time at certain types of tracks.

is there a pattern to your findings ?

do we want handling to be consistent over a speed range ?
surely so-called turn in (angular/yaw acceleration) is important in slow corners and unimportant in fast corners ?
(and downforce rather the other way)
so the handling should vary with speed ?

[gixxer_drew]

I rather like Ben Bowlby's remark, quoted by machin. He brought tyres and downforce into the equation, & I think both are important. If handling is to be consistent over a speed range, then the c.g. must follow the aerodynamic centre of pressure quite closely, & I like to think that a well set-up vehicle should "work" its tyres in a sensible way.

Recently I am questioning this even though I used to follow that religiously. In the last couple years I've been breaking this rule and seeing improvements in lap time at certain types of tracks.

is there a pattern to your findings ?

do we want handling to be consistent over a speed range ?
surely so-called turn in (angular/yaw acceleration) is important in slow corners and unimportant in fast corners ?
(and downforce rather the other way)
so the handling should vary with speed ?

Well basically the concept is not to upset the balance... but to use different means of compensating for a shifting balance at speed to make a "bad" aero balance work. Because it offers some advantage somewhere else on track thats worth more lap time. At first, I thought it was an undrivable scenario the sim was saying is quicker and only existed in some equation.. up until I figured out how to make it work in reality and the driver didn't even notice it was there but went very quick. I'm talking about a very specific sort of track though, but I realized after that it wasnt as uncommon of a scenario as I originally thought either. probably not use for F1.... more corners are high downforce.