For a new race car, an important early decision in the design process is the choice of front to rear weight distribution.
Equally, if we are racing an existing car it’s important to know what our weight distribution is, so we can make decisions about the suspension set-up of the car.
How do we measure weight distribution?
If we have the corner weights, we can determine the weight at the front and rear axles as a proportion of the total weight. E.g. We may have 53% of the total weight at the front axle, leaving 47% of the total weight at the rear axle.
How does weight distribution affect the balance of the car? What is the effect of more front weight? Or more rear weight?
To get the answer, let’s look at the problem from the vehicle dynamics engineer’s perspective. They are trained in the application of a universal simple model of handling known as the “bicycle model” or “single track model”.
At Racing Car Technology, we’ve developed a schematic diagram of the simple vehicle dynamics model:
Looking at the car from above, when the driver applies a steering input, the forward momentum of the car forces slip angles at the front tyres, building lateral force FF, turning the car at the front. The resulting rotation of the chassis then forces slip angles and lateral force FR at the rear.
The yaw moment FFxa turns the car into the corner. The yaw moment FRxb is the balancing moment at the rear.
What we want is for our race car to turn as hard and fast as possible yet still with sufficient stability for the driver to maintain control.
In cornering, both the set-up of the car and driver technique can influence the balance of the car.
When the driver rotates the steering wheel in corner entry we are building the turning moment at the front axle.
If the net affect of the set up and driver technique adds to the front turning moment we are building more agility (oversteer).
If the net affect of the set-up and driver technique reduces the front turning moment we have more stability (understeer).
The influence of increasing agility is that the car will turn better. If the car gets to be too agile (too much oversteer), the car will be “nervous” and almost every motion of the car in cornering requires a driver correction.
The influence of increasing stability is that car will be, to at least some extent, more self-correcting. Stability adds to driver confidence. When the car is too stable (too much understeer) the car will not turn well.
Looking at our schematic diagram, if front and rear moments are balanced, i.e. FFxa = FRxb, we say the car is in “steady state” cornering. The car is travelling on a fixed radius with fixed throttle and steering.
If the centre of gravity is positioned at the midpoint between front and rear axles, then a = b. Given a = b and if we have the same tyres all round, then front and rear tyres will reach their maximum grip at the same time. At the limit therefore, the car can generate the maximum mid-corner lateral G available from the tyres. Front and rear tyres reach the limit of grip at the same time.
If the centre of gravity is forward of the midpoint between the front and rear axles (like many front engine production cars), then a is less than b. The front moment has a shorter lever arm and the rear moment has a proportionally longer lever arm.
This means for the front and rear moments to be balanced, in steady state cornering where FFxa= FRxb, because lever arm a is shorter, and lever arm b is longer, the front tyres are using up more of the available grip than the rears.
For a car with higher front weight percentage, the front tyres will reach the limit of grip first, leaving some unused grip availability at the rear of the car. The car will be a natural understeerer.
If the centre of gravity is rearward of the midpoint between front and rear axles e.g. mid-engine race cars, then b is less than a. The rear moment has a proportionally longer lever arm than the front.
For the front and rear moments to be balanced, the rear tyres are using up more of the available grip than the fronts.
For a car with higher rear weight percentage, the rear tyres will reach the limit of grip first, leaving some unused grip availability at the front. The car will be a natural oversteerer.
This turns on it's head the notion that the end with greatest vertical load on the tyres will gain lateral grip compared to the other .
It should be noted we never want to add weight to our race car. The lighter race car will always be faster. Better acceleration, braking and cornering. So, for this exercise just consider we are adding weight to get to a minimum weight requirement for our category of racing.
We could add the weight either forward or rearward of the existing centre of gravity.
Our first inclination is to think we are going to help stick the end with the additional weight.
For acceleration and braking this is true. The rear engine car is always going to launch off the line better than a front engine car. Similar thinking for braking. The rear engine car will have have the tyres more evenly loaded under brakes compared to the front engine car. You adjust the brake bias towards the front of the car to take advantage of the extra grip available at the front and also to reduce the possibility of locking up the more lightly loaded rear tyres.
Hands up if you've ever carried this thinking on to cornering capability as well. I think everybody will have their hand up, including me. (With apologies to any vehicle dynamicists reading this. Guys, you should have told us earlier.)
For cornering (lateral acceleration), as we know from our understanding of the vehicle dynamics model of handling, the affect of adding weight to the front or rear of the car is the exact opposite of "sticking the end with the extra weight":
Adding weight forward of the centre of gravity will increase understeer. Adding weight rearward of the centre of gravity will increase oversteer.
For me, this answers a lot of questions in the back of my mind for a helluva long time. Exactly why is the mid-engine layout the best for circuit racing? The answer is clear and does not come with any of the downsides we thought might have been there.
Another biggie to consider is what happens with weight transfer when the car pitches forward under brakes and rearward under acceleration? At this point, I think many racing drivers will be questioning what we are told about trail braking. I’ll make that the subject of my next blog.
You may be thinking, how can traditional ideas about weight transfer get it so wrong? It’s just that racers have not taken the time to look at the problem by investigating the simple vehicle dynamics model of handling (the bicycle model or single track model).
For 2020, at Racing Car Technology, we've decided to put the vehicle dynamics simple model front and centre in our thinking when looking at vehicle dynamics problems. The proof and demonstration of the value of the vehicle dynamics model goes back decades. If an idea seems right according to the model, there’s a big chance it is right ahead of any thinking to the contrary.
The mathematical model itself is what drives engineering design work and race car simulation. If we see it working for the engineers, it should work for us also, giving us confidence in the conclusions we are drawing.
But of course, without the ability to do the maths, we do have to interpret carefully. I’m thinking some of those with more in-depth knowledge of the vehicle dynamics model might have better explanations.
For a high performance road car, what is the ideal weight distribution?
For RWD front engine, 50-50 weight distribution. It’s the layout of choice for RWD performance cars from BMW, Mercedes etc for many years. By 50-50 we mean the centre of gravity (where the weight of the vehicle is concentrated) is at a point midway between front and rear axles.
50-50 cars display a degree of natural agility yet with a certain calmness that aids driver confidence. It’s pretty easy to get the final balance you want with correctly sized anti-roll bars front and rear.
With front engine RWD production cars for racing, at Racing Car Technology we find we can still get good balance with weight distribution between 53% front weight percentage and 47% front weight percentage. (We set the baseline balance of the car with the Racing Car Technology Weight Transfer WorkSheet™.) If we get to 55% and greater front weight percentage, we’re giving away some handling performance.
In a recent Car and Driver comparo between the Mustang GT500 and Camaro ZL1, it was stated the Mustang has 55% + front weight percentage and the Camaro closer to 50-50. According to the article, the Camaro is the better handling car. It seems the high front weight percentage of the Mustang is a result of different design compromises between the two cars. A high front weight percentage would be something for the Mustang handling package to overcome as it is for some other sports cars such as the Toyota 86.
What’s the point of having a larger rear weight percentage? For circuit racing, it aids agility and with more weight over the rear axle, gives better straight line acceleration.
The preferred layout for circuit racing and super high performance cars on the road is mid-engine.
Racing cars are likely to have a high power to weight ratio and could therefore be grip limited at the rear. More rear weight percentage is desirable to add vertical load to the rear tyres and therefore create more grip for straight line acceleration.
As we have seen more rear weight adds to agility. The mid-engine car will turn better. (The idea that the car could turn better with more weight over the front wheels doesn’t stack up, as discussed earlier).
It almost goes without saying - any cornering performance improvement will trump just about any associated downside. This is true for aero downforce, for instance. The gain in cornering performance from the downforce is worth way more than the loss in straight line performance from the aero drag.
When first introduced in the 50’s and 60’s, mid-engine race cars dominated F1 and the Indianapolis 500 straight off, with no appreciable downsides to consider. The mid-engine layout was a no brainer.
If we have the same tyres all round, the limit in rear weight percentage is around 55%. This would be the case for most Formula Fords, for instance. But with wider section rear tyres and high power to weight, rear weight is most likely best around 60%.
Especially with rear engine cars such as the Porsche 911, we have difficulty getting away from the idea that the rear weight is going to help lateral grip at the rear. Motoring journalists perpetuate the myth with comments like this: In a recent comparo between the earliest and latest 911 GT3s, the author says of the latest GT3, “Turn-in, typically a 911 weakness, is incredible thanks to the wider front track, rear wheel steering and aggressive geometry”.
When we consider the high rear weight percentage on a 911, we might conclude the exact opposite. A 911 turns in too aggressively. Perhaps the rear wheel steering and geometry tweaks of the late model car make more cornering power by calming down the aggressive behavior of the 911 layout at critical points in the corner.
Since the Mini, front wheel drive has been popular in racing. Even more so today with all the hot hatches on the market. With the advent of TCR touring cars, we have a mainstream front wheel drive only category for the first time.
Around 60% front weight would appear to be the norm. In fact, TCR have mandated 60-40 weight distribution in the rules.
If you were allowed a larger tyre size on the front, it would be a distinct advantage. But with the same tyre all round some slightly lesser front weight than 60-40 may be an advantage. In 2 litre touring cars some years ago, the teams would modify the weight distribution by rolling the engine back in the chassis. The cost was prohibitive, and the gain was small – around 2 to 3 % gain in rear weight percentage.
There’s more to the simple vehicle dynamics model than what we can cover here. Get our new E Book for more details and see how to use the ideas to tune your suspension - “7 Little-Known Hacks to Suspension Set Up Mastery.”
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