An open response to John Milmont's article titled: Stop and weight! A 50:50 weight distribution is not ideal. (http://automotivethinker.com/chassis/stop-and-weight-a-5050-weight-distribution-is-not-optimal/)
Just to clarify a few misunderstandings on the part of many commenters...
All the points made by the author about the benefits of a rear-biased weight distribution on a RWD car are CORRECT. To summarise:
- Better acceleration as weight is over the driving wheels (rear). This can also help earlier corner exit. However, this is purely from the consideration of traction and doesn't take into account centrifugal forces acting on the car through the turn.
- Better braking because brake force is distributed better between front AND rear, instead of being more front-biased.
Now there's a whole lot more involved in engineering a car's handling attributes than simply looking at linear traction (frictional forces acting parallel to the longitudinal axis of the car). This is one reason why even mid/rear engine sportscars don't have their engines positioned all the way back.
There IS a context in which 50:50 weight distribution is IDEAL. If we are to consider ONLY the lateral (centrifugal) forces on a car during cornering, the ideal weight distribution would be 50:50. This is the weight distribution which would provide neutral stability in a car going through a constant radius turn, at a constant speed. As you increase the lateral forces (increase cornering speed), a front-heavy car would lose traction at the front, causing understeer. A rear-heavy car would oversteer. a balanced weight distribution would do neither as it would lose traction at the front and rear equally. These 3 characteristics translate into the steering stability of a car when it loses traction mid-corner.
see this picture:
http://image.slidesharecdn.com/basicaerodynamics-110324082120-phpapp02/95/basic-aerodynamics-35-728.jpg?cb=1300955896
- understeer = positive stability. when you reduce speed, the car will return to the original turning radius
- oversteer = negative stability. once you start to oversteer you continue to oversteer. This is neglecting the dynamics of counter-steering.
F1 cars are en excellent case study of how engineers try to find the best weight distribution taking into account ALL the variables. The loss of traction at the rear during cornering, is what will allow the centrifugal forces to rotate the car and destabilise it mid-corner. But the acceleration and braking benefits of a rear-heavy car ARE desirable. Amongst MANY design features that they encorporate to achieve the best possible lap times, a few basic principles applied are:
1. rear-biased weight distribution to improve braking + cornering.
2. aerodynamics to put more downforce on the rear driving wheels, and hence improve linear traction.
3. long wheelbase so that the lifting effect of the rear end during braking is minimised. A longer moment arm creates a larger counter-moment against this lifting effect to keep traction on the rear wheels.
4. the longer wheelbase also reduces the angular velocity of the oversteering car about its normal axis by increasing the radius of the rear end from the normal axis. This means that that recovery of control by counter-steering or regaining rear traction is easier.
5. Wider tyres on the rear to improve traction.
So in otherwords, although rear-bias weight distribution is not ideal for steering stability, they use other design features to remedy this problem such as suspension tuning (compression, rebound, etc), and steering geometry (wheelbase, track width, toe, camber, etc.).
However, for a road car, many of these features on F1 cars cannot be used as extensively.
Road cars need to meet passenger safety requirements, and have geometric limitations to accommodate passengers, luggage etc. Extra wide tyres are also far too expensive to be used on street cars, and most importantly the drivers are not professionals. The average joe cannot be trusted to effectively regain control of an oversteering car, so much so that many manufacturers use electronic stability controls and differentials which make rear-wheel drive cars behave like front-wheel drive cars. So in these applications, a 50:50 weight distribution IS ideal - such as the Mazda MX-5 Miata.
My credentials:
- Currently 2nd Year Bachelor of Mechanical Engineering
- Graduate with Bachelor of Aviation Degree
- Always been good at mechanics (physics)
- Car lover
No comments:
Post a Comment