Compared with box-based steering systems, rack and pinion offers a number of advantages. The best known of these is that because the system has far fewer joints, there’s much less scope for free play and the steering is a lot more accurate. This is reflected in the amount of movement allowed at the steering wheel for MoT purposes; a box-based steering can have up to 75mm at the wheel. With a rack, however, the limit is normally 13mm, though the MoT manual also says; “where there are several joints between the steering wheel and the rack, movement up to 48mm on a 380mm diameter wheel may be accepted”.
However a steering rack has another big advantage over a box-based system; it’s more compact and thus a lot easier to fit into – or more accurately under – a crowded engine bay. This is particularly relevant on front-wheel drive vehicles where the extra space is taken up by the drive shafts and suspension, meaning that rack and pinion steering is usually the only possible option.
As was the case with so many ‘good ideas’, Citroën were the first European manufacturer to use rack and pinion steering on a production car, with the Traction Avant. Here in the UK, Morris/Nuffield used it from the late ‘Forties on – most notably on the Morris Minor, MO Oxford and then the MG Magnette and its Wolseley siblings. Following the formation of BMC, when more conservative Austin practice came to dominate engineering matters, steering racks were used less. However when the Mini arrived in 1959 there was no option but to use a rack – and doing so was in part responsible for the car’s excellent handling characteristics. By the end of the ‘Sixties, rack-based systems were pretty much universal, though a few manufacturers – including Mercedes-Benz – persisted with box-based systems for a few more years.
HOW IT WORKS
The key components within a steering rack are, simply, a rack and a pinion gear. The pinion gear turns with the steering wheel, and this causes the rack to move from side to side, creating the lateral movement that causes the steering action. Rubber gaiters at each end provide a flexible seal to keep lubricant inside the rack assembly and, more importantly, keep grit and dirt out.
As already mentioned, a rack doesn’t need the relatively complex rod/idler gear that has to accompany a box; all that’s needed is a single joint (or track rod end) on the end of each rod. There is, though, one additional joint as it’s rarely practical for the ‘turning’ shaft going into the rack to be in direct alignment with the steering wheel. This means that the steering column has to incorporate one or more universal joint-type arrangements to alleviate the problem. Sometimes this will be a ‘proper’ universal joint that’s effectively a smaller/simpler version of the type found in a rear-wheel drive-type propshaft. Alternatively, a circular rubber disc-type arrangement will be used with four holes in it – two taking a shaft into the joint and two taking one out. The flexibility within the rubber takes the movement through the necessary angle.
This arrangement is cheaper to build. Some also say it gives a smoother action; however the rubber joint doesn’t last forever and eventually wear, tear, heat and oil contamination may take their toll. Obviously a component that provides the only link between the steering wheel and steering gear is fairly crucial from a safety point of view and it should certainly be checked at MoT time.
The most common problem is failure of the rubber gaiters at each end of the rack. These stretch and contract with movement and eventually split. This is a MoT failure as if any dirt or grit gets in it’s likely to cause damage to the rack’s internals. Renewing the gaiter is usually straightforward – if a correct/original gaiter is no longer available universal-type gaiters are available with a range of different diameters at each end that can be cut it to the required size.
Changing a rack gaiter usually involves taking the track rod end off; they generally unscrew from the track rod. However at least one ‘end’ will normally incorporate the steering tracking adjustment; the further on the rod you fit the joint, the greater the toe-in. Therefore, to retain the tracking setting that existed before the gaiter was changed, you have to refit the track rod end in the same position. The fool-proof way of doing this is to count the number of ‘turns’ needed to remove the end and wind the new one on the same number. Some people reckon they can tell where it should be from where the clean/previously hidden threads end and the dirty exposed ones start, but I prefer to count.
Of course, if you’re renewing the rack, you’ll need to have the tracking reset anyway. You can buy DIY-quality kit to do this yourself if you wish; alternatively leave it to a garage/tyre depot, though you may need to supply the data. As regards lubrication, the method and type of lubricant varies from rack to rack; some require grease, some EP90 oil. Refer to your workshop manual. In all cases though, take care not to overdo it; in particular the rack should not be packed with oil in the same way as a steering joint, as the lubricant has to be free to move about to where it’s required by the action of steering.
The method of lubrication also varies from rack to rack – a few racks have a grease nipple, others a blanking screw that’s removed and replaced with a nipple, others a plug that’s unscrewed to facilitate putting oil in. And some racks are claimed not to need any routine lubrication at all. Again, refer to a good workshop manual.