Grant Denyer’s Tarmac Rally Crash – Is a Mid-Engined Car a Good Weapon for Rally Stages on Public Roads?

Uncategorized Mar 31, 2017

What?  I hear you cry.  The fastest cars in the world are mid-engine.  All the greats of motor racing have driven mid-engine cars since the 1950s.

Surely this layout is the best for any quick car where performance is paramount.  The mass located around the centre of the car, and the associated rotational inertia, should help the car to turn better.  What more does the driver need to go fast?

Faster, yes.  But stability might be the bigger requirement in high speed driving where there is no room for error.  With stability comes more control options, and the ability for the car to be self-correcting to at least some degree.

Here’s the problem.  In car design, there is always a trade-off between response and stability (ease of control).

Rotational inertia can work in one of two ways, depending on the car.  Either promote an increase in lateral velocity at the rear axle, build tyre slip angles faster at the rear, improve response, or decrease lateral velocity at the rear axle, slow response, easier to control.

There is a number used to express this level of agility known as the Dynamic Index (DI):
DI < 1 The agile car, where rotational inertia promotes response.
DI = 1 Rotational inertia has no influence on the car.
DI > 1 Rotational inertia promotes stability, easier to control.

It is said that most sporting cars may have a DI around .9 (indications are cars like Mazda MX5, BMW, Mercedes et al and all their rear wheel drive sporting competitors come in about that number.)

There is a design imperative for all modern cars, where driving dynamics are part of their mission statement, to have as much of the weight as possible packed inside the axles, reduce over-hang, so as to get the Dynamic Index as low as possible. 

We don’t know what the exact numbers are, but DI much over 1 must be considered undesirable.

What about mid-engined cars like the Lotus Elise, Toyota MR2, Porsche Boxster and many others?

First thing to note, they havn’t gone the mid-engine route for low DI.  Most mid-engine layouts are designed to get more of the weight over the rear axle, so as to improve acceleration.  Open wheeler race cars, for instance, are likely to have 60% of the weight over the rear, 40% front.  It also opens up the possibility of wider grippier tyres on the rear.

So where’s the problem?

It can be seen with some reflection, that a low mass, mid-engine car might err towards a DI that is too low. You may have a short wheelbase in an effort to reduce weight to a minimum.  Light cars such as the Elise and MR2 AW11 may be in this area.  Porsche Boxster may be easier to get right – it's a heavier car.  Open wheelers can get the number they want by putting driver and engine and accessories in the right location for the given wheelbase.

How will low DI play out in actual performance?

Below the limit, the car may have brilliant feel.  But consider what happens if you are already at the limit of adhesion at the rear tyres.  Any disturbance of the car, and the low DI number could cause an increase in the rear tyre slip angle, promoting the tendency to spin.

My old formula ford was too loose when on the limit.  With a low DI car in fast corner exits under full throttle, the car running wide and rear slip angles maxed out, you don’t have a lot of control options.  Even if you do get the car to turn a touch tighter, the rotational inertia could cause the rear to step out.  If you correct close to the edge of the circuit, it’s still going to leave the road.

Perhaps this was Grant’s situation.  The rear slip angles may have been sneaking up in that long corner (the driver can’t feel a slow build-up).  When he realizes he is running wide, he does manage to get some speed off the car, but not enough to regain control.

On gravel, the drivers can use large steering wheel movements to move the car around so they can feel the body slip angle.  (By feeling the rotation of the car, the driver can get advance warning of impending oversteer.)  On bitumen, you can’t do that.  To that extent, you have less feel, less control on bitumen.

I remember when Paddy Hopkirk and Rauno Aaltonen came out to drive at Bathurst in 1966.  In the first practice session, they were backing the cars into Forest Elbow, like they would do in a tarmac rally in Europe.  Doesn’t work too good on a race circuit.  Took them a couple of laps to work that out.

Ah, but it does work for drifters.  I saw a video the other day with a couple of the cars drifting down a narrow mountain pass.  For safety sake, they were doing the Scandinavian flick on corner entry – getting the tyres smoked up early, and rotating the car out then in, so they had a better chance of presenting the car accurately to a tight narrow corner.

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