Austin Seven Ruby
Matt attempts to reduce waste and extend engine life
Having stripped my Seven’s original Ruby body of all the parts I would need for the ‘special’ build, I didn’t have the heart to simply scrap the rest. I knew that it would be useful to someone.
That someone was Ian Tillman at Oxfordshire Sevens, so I hitched the trailer and delivered it to his Bicester workshop. Ian was like a kid in a sweet shop when I arrived, picking through the pieces of rusty metal falling off the shell in lumps. Ian will use every salvageable piece of the Ruby body either as panel cuts, as templates for his growing range of Ruby repair panels or as spare parts for his vast stock. This means it’ll go on to save countless other Sevens even in death. And that is good classic car-ma.
Good vibrations
After leaving Ian’s, it was just a ten minute drive to see Steve Smith at Vibration Free. I’ve known Steve for some time, and his words on an industry visit with the Oxford Universities Motorsport Foundation have stayed with me. ‘Vibration is wasted energy. There’s no point in spending time and money on go faster bits if you’re going to throw those gains away through imbalance.’
Steve’s workshop is an engineering delight. Extraordinarily accurate digital balancing machines allow him to precisely balance anything from my Austin Seven’s crankshaft right up to a fully built-up aero engine on his big rig.
We started by placing my new Phoenix crankshaft on Steve’s smaller machine and completing a calibration run, with a 5g mass of plasticine added to strategic points at each end of the crankshaft to calibrate the machine to the specific job at hand. The next run revealed that both ends of the crankshaft were out of balance and would need material carefully grinding away to remove unwanted centrifugal loads.
Imagine the feeling of an unbalanced wheel on your car, and the vibration that is transferred through the wheel bearing, track rod, steering rack, pinion, column and wheel. Not only is this wasted energy that could be better used propelling the car forwards, but it also produces unwanted loads on all of those components. It’s the same with an engine. A small imbalance on a crankshaft at low rpm will increase with the square rule, so if you double the speed, you will quadruple the force it produces, increasing side loads on the main bearings and reducing their service life.
Do the math(s)
As an example, a five gram imbalance, 5cm from the centreline of the crank, would exert a force of 0.4 Newtons, or 40gf, at 400rpm, but 40N, or 4kgf, at 4000rpm – a 100 fold increase in load compared to a 10 fold increase in revolutions. It’s easy to see, then, how a
perfectly balanced engine will not only produce more power and be able to rev higher, but will also last longer. And it’s that reliable power that’s the name of the game with this Seven’s engine.
Once the crankshaft is in perfect balance, Steve will mount the flywheel to ensure that it, too, is perfectly balanced. We discussed the pros and cons of lightening the flywheel and, given the propensity for two-bearing Austin Seven cranks to ‘whip’ (i.e. to bend under load), decided that the benefits of fast acceleration given by a super light flywheel would be outweighed (if you’ll excuse the pun) by the smoothing effects of a heavier flywheel for road use and longevity.
With all the rotating components now in hand, our attention turned to the reciprocating parts of the engine – pistons, gudgeon pins and rings. We’ll get to connecting rods shortly…
The pistons were all weighed and a discrepancy of 2.4g was found between the heaviest and lightest. Relative masses were marked on top of the pistons. Gudgeon pins, ring packs and circlips were all checked, too. ‘It’s not enough to simply match overall weights of con rods’
It’s vitally important that the cumulative masses of all the reciprocating components in each cylinder are matched to one another. Before removing any material from the piston or pins, though, the connecting rods needed factoring in. The little end of the connecting rod is a reciprocating component, whereas the big end is a rotating mass. Therefore, it’s not sufficient to simply match the overall weights of the con rods - the masses need separating.
We ran the numbers to discover that the little ends of the connecting rods were 10 grams different in weight to one another. ‘What I need to do now is to mix and match heavy pistons with light little ends before either removing the difference in mass from non-critical areas or adding heavy metal inserts to the centres of the gudgeon pins. Then I can think about balancing the big ends.’ This was going to be a challenge, I could tell. I left Steve to it…