Yes, another column on tyres....
People with front-wheel cars will know that if you wish to reduce understeer and get a better turn-in, it's often the rear sway bar that you increase in size as a first step. If the back is kept flatter, the front also stays flatter (unless your car has the torsional rigidity of a marshmallow), and with both front wheels having tyres more perpendicular to the road, good things can happen. (In the same situation a big front sway bar will tend to lift the inner wheel, giving less front grip.) That's a broad-brush of the accepted front-wheel drive wisdom, but less discussed is the effect that rear tyres will have on a front-wheel drive's handling.
In fact, to be honest, until today I'd never even thought about it in any detail.
Regulars will know that I recently bought a '89 Saab 900 Turbo, a car which has some extremely strong points - and also a not-quite-so-high quotient of bad points. The good points include quite amazing stability - at high speed it tracks like a modern all-wheel drive - and steering which is simply superbly weighted and progressive. In fact, while it may look like a Seventies design, the Saab's on-road stability feels closer to a RS Liberty or WRX than a VB Commodore. Truly! In fact, one of the tests I performed on the car before buying it was to throw it into some 60 km/h violent swerve and recovers. You turned the wheel, the car responded quickly, and as you recovered your previous path, it remained obedient. There was no lurching, no feeling that it was getting away from you - just very good stability.
But then I changed the rear tyres....
The car came with an odd mix of tyre brands and models - all were the factory 195/60 15's, but the fronts were Pirelli P6000s, one rear a Goodyear Eagle, and the other rear a Pirelli P4000. But as I said, it still handled very well. However, the Goodyear got a puncture (interestingly, through the inner sidewall) and rather than stay with asymmetric rear tyres, I decided to buy a pair. I wanted something middle-of-the-road and chose Michelin Vivacy - no, I had never heard of them either. But they were in the right price range, were in stock - and when all is said and done, the Saab's not really a high performance car - despite having no less than three turbo badges! I got the new tyres placed on the back, thinking that I'd find out which end - Pirelli P6000 or Michelin Vivacy - had most grip and then swap them or leave them depending on the balance it gave the car.
But I was thinking in terms of cornering hard.... not just changing lanes!
What?
When travelling home along the freeway after having the new tyres fitted the first thing I noticed was that the steering had lost some of its progression. Turn the wheel enough to change lanes and the car felt darty. It was almost as if the back end initially didn't want to follow the front - and then all of a sudden gave in and changed direction. Then I found that when making sudden, major steering inputs there was a distinct lurch from the rear.... in this area the car felt nothing like it had previously with the other rear tyres. The actual levels of rear end grip seemed fine - it was the tyres' behaviour in directional transitions that now felt horrible.
Hmmm.
Maybe tyre pressures? If the new Michelins had only about 25 psi in them (I normally run them just above the Saab heavy load recommendation of 35-36) then I'd expect the car to feel like this. A check showed that in fact they were at 30 psi - but lifting this to 38 didn't make the difference I had hoped for. In fact, further testing showed that the car could become completely unstable in some situations. Push the Saab through a consecutive series of S-shaped swerve-and-recovers and the rear end would get more and more out of synch with the front... a bit like a passive rear wheel steer system gone horribly wrong.
Simply, with the Michelin Vivacys on the back, the Saab was dangerous.
So why not put them on the front instead? If this disconcerting behaviour was being caused by the Michelins' sidewalls flexing, swapping them to the front should give just a slightly slower turn-in, easily able to be countered by the amount of initial steering lock used. And that proved to be the case. With the Vivacys on the front, the steering has now lost a little of its linearity and progressive feel, but the rear again feels obedient to instructions coming from the front.
So if you have a front-wheel drive that heavily works the rear (ie the back suspension has a high roll stiffness), think very carefully about your rear tyre selection. What works on the front may not work on the back - and vice versa!
When I recently fitted VDO Dayton navigation to my other car, I had the opportunity to make a change to its subwoofer system, one that I had been contemplating for some time. (The connection between the two is that the main unit of the nav system was mounted right next to the subwoofer.) The sub installation process was covered at "Installing a Free-Air Sub"; basically it uses a free-air sub mounted so that it fires through a vent installed behind the centre armrest of the rear seat, while the rear of the speaker (and its associated amp) is protected and obscured by a cover. At the time of installation I used a cover comprising carpet on top of a perforated particleboard - the holes in the board designed to relieve backpressure on the cone. (In a free-air system if air is trapped behind the cone the resonant frequency of the system will rise, decreasing bass response.) In addition to all the small holes, the cover had a gap around its edges - plenty of opportunity for air to escape as needed, I had thought.
However, after the cover had been made and installed, I noted a drop in bass response. At that stage I didn't feel like undoing the handiwork I had just completed, and so simply resolved to make a change to the design of the cover the next time it was removed. About a year later, with the installation of the nav system, that opportunity presented itself.
What was needed was a vent (or perhaps two vents) that would let the air move more freely between the boot space and the area directly behind the sub. I went along to a major hardware store and looked over their range of ceiling vents - ornate and fancy things in white plastic. I didn't figure that plastic would last long in the boot - what with all those missiles flying around - so I looked in another department. And in the area specialising in brickwork I found some heavy duty galvanised steel vents with a very open mesh design. A can of black spray paint later, some work with a jigsaw and screwdriver, and the vents were installed.
And they make a radical improvement to the level of bass output - obviously my original design had a major deficiency in trapping too much air behind the subwoofer driver....
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