AR7E - Alcester Racing 7's Equipe
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Right, well this job doesn't really qualify as routine maintenance, but it is of relevance to a few, unfortunate people, so here goes...
The job in hand is the replacement of the propshaft (this takes drive from the gearbox to the differential). Normally, prop maintenance consists merely of greasing the UJs - a two minute job every few thousand miles if the car is already on stands. If this small job is not regularly performed, however, the UJs can 'dry out' and eventually wear themselves out. Typically, such progressive damage will show up as a transmission vibration that becomes evident above a certain speed (80mph seems common - out of a survey of two cars!). The UJs can be damaged in other ways (such as a shunt, or with ridiculous torque and power demands)- but neglect is the most usual cause. A more common issue is that the prop itself can become unbalanced - balancing of the entire prop and UJ unit is achieved by spot-welding (usually two) small steel plates to the propshaft itself. If one (or more) of these weights should be lost, you will once again experience transmission vibration. In either case, the prop will need to come out - either for replacement or simple re-balancing if the UJs are still OK. So what is the problem? (as we so often ask...)Well, it comes back to the fact that the 7 is constructed from a bunch of bits stolen from other cars. Most 7's now have De Dion suspension at the rear- this utilises a differential that is fixed to the chassis (with no real scope for movement). The rear wheels (and suspension) move relative to the diff. The diff is therefore (for all intents and purposes) fixed relative to the gearbox. In a simplistic world, you would simply unbolt the prop from the diff end and do the same at the gearbox end. The prop would now just drop out, and in five minutes a new prop could be popped back in place... ...but the gearbox comes from a car that was designed with live-axle rear suspension. This means that the rear wheels are fixed relative to the diff, and the entire diff, axle and wheel unit moves relative to the chassis. All well and good - but in reality this means that the diff travels in an arc about the suspension pivot points - and this means that the diff actually gets closer to the gearbox when the suspension is compressed... The propshaft effectively needs to have a variable length... Now, obviously it is a bit tricky to stretch or compress a heavy steel pipe as the car goes over a series of bumps - so another mechanism is required. In reality, the output shaft of the gearbox has a set of longitudinal splines running along it. The propshaft is a large diameter than the output shaft and has a corresponding set of splines on the inside of the tube at one end. The propshaft can therefore slide in and out over the gearbox output shaft - thus solving the variable length problem (the other end of the prop can be fixed to the diff as before). Hurrah!
Click Image for Full Size. Key: A - Handbrake cable/pulley B - Splined Gearbox output shaft C - Top of Transmission tunnel
Click Image for Full Size Key: A - old propshaft B - new propshaft C - cover protecting sliding shaft So where (we ask once again) is the problem?Well, the propshaft is caged into the transmission tunnel of the 7 by a number of chassis cross-members - unfortunately, it is not possible to just undo the diff end, drop that end of the prop down - and then slide the remainder out of the gearbox. Damn! Basically, either the engine and gearbox need to be removed (forget it!), or the diff has to come out... So diff it is. So what is the....Weeelll, usually, the easiest way to take something apart is to reverse the construction order (and visa versa) - only problem being that we'd be following the dreaded 'engine-out' method... So we'll be pulling the rear suspension and removing the diff instead. |
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