Sprocketbox

The Frazer-Nash “Chain Gang” sports car of the ‘20’s and ‘30’s was a surprisingly effective device. For those of you who are not familiar with it, this was a light and Spartan British sports car very much in the Vintage mould, which featured a singular system of chain-drive transmission. Drive was taken from the clutch, via a primary drive shaft, to a bevel box and cross-shaft just aft of the driver’s seat. Several sets of exposed sprockets and chains, each engageable as required by a dog clutch, connected this shaft directly to the rear axle. The arrangement was light, simple, easy to repair and modify, and capable of handling considerable amounts of torque.

However, simplicity was taken to the point of dispensing with a differential, an attempt to mitigate the effect of that omission being made in the form of a relatively narrow rear track. To some extent the rear-axle-induced understeer that nevertheless resulted was compensated by the oversteer-inducing extreme of rear roll stiffness, which was just as well as what roll movement there was put torsional loads on the chains and thus considerably shortened their lives. Chains did break from time to time, but they were repaired, because the “Chain Gang” was that sort of car. All in all they did what they were supposed to do extremely well, and a small minority continue to swear by them to this day.

While it is neither necessary or desirable to render the transmission system of a rough-and-ready Vintage-style sports car completely maintenance-free, it is certainly possible to improve the system to eliminate the major shortcomings. To this end I propose: 1. incorporating a differential concentric with the rear sprocket set and 2. mounting the rear shaft to the chassis by bearings and using an independent or DeDion rear suspension. The system may readily be assembled today, using a crown-wheel-and-pinion set out of a conventional rear axle, a collection of motorcycle chains, and the differential out of the aforementioned axle. The most difficult part would be making up the dog-clutch units. Most practical would be back-to-back units that each serve two sprockets, which would allow a conventional gated shift pattern. It is also entirely possible for these to be narrow enough to allow only a flat-bar reaction arm and shift arm between adjacent chains, to keep the entire system more or less within the width of a conventional rearend third member.

The process of calculating the torque capacity and physical size of the system gave rise to a few further ideas and observations. The torque capacity of the system is proportional to the size of the sprockets and the strength of the chains. The greatest capacity is required of the first-speed set; chains for the other speeds may be progressively lighter, provided the driven sprockets are the same size. This has also the advantage that all the dog-clutch/sprocket-pair assemblies in the system would be the same. Taking, for example, a seven-speed arrangement with a 3.89:1 primary reduction, a 3.28:1 first gear ratio, and 3.66”/12” (91mm/300mm) sprockets gives a torque capacity of around 300lb.ft using common high-performance motorcycle chain, or over three times as much using certain types of racing chain. Reducing the sprocket sizes reduces the torque capacity correspondingly.

It makes sense to mount the dog-clutch assemblies on the driven shaft, as running them on the driving shaft would cause overspeeding of the lower-speed chains when cruising at speed. The only exception would be (in the above example) the 6th/7th dog-clutch unit, if 7th is an overdrive ratio. Mounting this on the driving shaft would allow the 7th speed driven sprocket to be smaller than the other driven sprockets. If the dog-clutch units are identical they can also be swopped about in the system as they wear: the 2nd/3rd unit will see the most wear, and may therefore see duty as the 4th/5th, 6th/7th, and 1st/reverse units before finally being reconditioned. That way only that which is worn out needs to be reconditioned.