L.F …. Thanks for giving us this opportunity for a chat, Mr. McGurk.
McG …. No Problems …. You can call me Chas.
L.F …. Thanks …. With a record of DNFs like yours, Chas, there must be some tips you can pass on to the many readers of Loose Fillings, both here in Australia and overseas to help improve the reliability of our motorcycle-engined race cars .
MeG …. Certainly …. I’d be pleased to do that. You choose a topic.
L.F …. OK. let’s talk about drive chains. All bike-engined cars use chains, and they always seem to be falling off or breaking or whatever. What can we do to keep chains on sprockets and lasting longer?
MeG ….Well to get any sort of long life out of a chain there are three basic requirements. The first is correct alignment of the sprockets. You would be surprised how often the engine to clutch or gearbox to rear sprockets are out of alignment. This means the poor old chain has to constantly twist sideways to stay on the sprockets. If the chain gets at all slack it climbs off the teeth of the sprock- et and breaks or bends something. If it’s not the chain itself that breaks, then it will probably be the engine or gearbox mountings, or casings, or mainshafts. To check alignment just put a straight edge along the sprockets to make sure they all line up to within no more than a sixteenth of an inch error.
Usually rear sprockets are fixed later- ally, so it may mean first spacing the gearbox sideways to get the rear chain to line up. Then check the clutch and engine sprockets and space the engine accord- ingly. It’s worth noting that the engine sprocket on a speedway JAP can move a good half inch sideways on a splined extension of the mainshaft. This allows self-alignment of the chain, (and also easy gearing changes). An excellent de- sign .
- … That’s good stuff Chas …. Now what’s the next chain tip?
MeG … Lubrication. If you can keep a chain lubed it will do big mileages.
Run it dry and it will overheat, break rollers and wear itself out in minutes. The problem we have with our bike-engined cars is that for lots of reasons we can’t run our chains in an oil bath. That means we have to lube the chain internally before we use it, and externally as best we can when it’s on the car. Internal lubrication is best done by immersing the chain in oil for a couple of days before fitting. External lubrication can be done on the car with an aerosol spray and the applica- tion of heavy grease on the INSIDE run of the chain between EVERY race. The grease won’t get inside the chain rollers, but it will lubricate the outside of the rollers and the sprocket teeth, which are sub- ject to some very heavy impact loadings.
A chainguard is a very desirable item because it stops broken chains flying past your ear. It also helps keep the car clean. Naturally the above notes are more rele- vant to the primary or front chain because it is constantly changing direction around an engine sprocket rotating between three and six thousand revs a minute. It is therefore good to use as big an engine sprocket as possible and adjust the over- all gearing at the gearbox or rear sprock- et. A big engine sprocket not only makes life easier for the primary chain but also for the clutch, because the clutch will run at higher RPM reducing the torque on it. The rear or secondary chain has a much easier life because in top gear it runs around the sprockets at only about half the speed of the primary cha in. In all the lower gears the rotational speed of the rear chain relative to the front chain is even less. A squirt of oil on the rear chain now and again, and it will last forever. And this is important.
Don’t be tempted to fit a modern O-ring chain to address the lubrication problem. An O-ring primary chain will cost real horsepower because of the friction of the rubber O-rings between the plates. We speak from experience. Three seconds a lap slower at Mallala. Just bend an O- ring chain in your hands and you’ll see for yourself.
L.F …. And what is your last chain maintenance tip today for us Chas?
MeG .. . It’s all about chain tension. Firstly you very seldom get constant tension. Turn the engine or wheels and the tension varies . Find the tightest position and give the chain about half an inch (each direction) vertical slack. Studies show that as chain slack increases, so does power loss. Correct chain tension is very important. Never deliberately run the chain tight.
There’s enough loading on the crankshaft and gearbox bearings already. And something else. Because of its slower rotational speed, the rear chain always has more torque, or in simple terms, more pull than the primary chain. This means that the rear chain will always overcome the opposite pull of the primary chain and want to move the gearbox backwards.
This explains why, if we get both chains tensioned perfectly, and run the car, we may then find the rear chain loose and the front chain drum tight. If this happens regularly it suggests the gearbox is not positively located, even with the bolts dead tight. This may be due to the rear chain adjuster on the gearbox being located on the opposite side of the gearbox away from the chain side where it is less effective in holding the gearbox tight and square . If so, transfer and re-engineer the rear chain adjuster.
And one other thing …. Always try to use an endless rivetted chain … particularly on the primary. If you are forced to use a spring clip with a connecting link, smother it with silicone …. it might stay there a couple more laps before leaving home …. So that’s about it for chains right now.
L.F…. Thank you Chas …..lt sounds as though you’ve had to find these things out the hard way. Many thanks for sharing. McG … It’s been my pleasure …… But now excuse me. I’ve got a dental appointment, and I’m due soon for another DNF …. probably the clutch. Cheers.