Quit Whining – the truth about straight cut gears.
My recent flag-waving about the amazing performance increase merely altering the gearing gives has fanned the flames of the ‘straight-cut gears are far too noisy’ discussions. Not that I said to use straight-cut gears, but the shortage of new components for the old S/1275GT close ratio conversion – most notably pre-A+ second and third gears – has lead to further discussions on the pro’s and con’s of straight-cut gears. Several close-ratio options are readily available, but not all are suited to most road-going engines. So let’s delve!
Whether the gears produce the pronounced and widely recognised signature ‘whine’ or not is mainly dependant on three areas – where they are located in the engine, their design, and then manufacture.
Location. There are three separate areas where gears work within out trusty Mini gearboxes – the transfer (drop) gears, main gear cluster, and final drive (FD/diff).
Strangely the transfer gears are so called because they transfer drive from crankshaft to gearbox, constituting three gears. Primary gear on the crank, input gear on the first motion gear supplying drive to the gearbox, and idler gear suspended between the two moving drive from one to the other. These are the noisiest of the trio by a LONG way, simply because they aren’t drowned in oil. Consequently, in-built ‘features’ of manufacture and design are amplified.
The main gear cluster makes up the collection of gears, first through to top. Nestling down in what effectively is the Mini’s ‘sump’ it’s constantly running in an oil bath. Consequently the noise produced by the straight-cut teeth is very much damped out.
Giving motion to the car ultimately is the final drive (FD or ‘diff’ as it’s generally addressed). Comprising two gears – the small pinion gear mounted on the mainshaft supplying drive to the much larger crown wheel bolted to the diff assembly – which run partly submersed in oil. The top half on the crown wheel runs in oil ‘splash’ thrown up by the lower half that is submersed. So it’s half way between the other two as far as ‘damped’ noise is concerned.
Design. The original idea behind straight-cut gears (cut at 90 degrees across the gear) was really ease of manufacture. Where limited numbers of gears are made for development – be it standard or motorsport – straight-cut gears are far cheaper and easier to produce. A multitude of gear ratio options being available by ‘correcting’ proper tooth profiles to suit - excruciatingly difficult and not economically viable with helical teeth where very much more expensive tooling’s needed. Other bonuses were reduced loads and drag inherent in the helical (cut at an angle across the gear) design – therefore leaving more power to drive the car. That straight-cuts are stronger than helical gears is a myth. Helical cut teeth run quieter as they’re almost constantly engaged, also eliminating shock loads. They’re stronger through better tooth root design and more load-bearing area – the helical angle increasing overall tooth length and surface area by a small but appreciable amount.
Mini FD gears are a special case. Originally these were straight-cut too, but the very weird size differential between pinion and crown wheel meant problems. To get the sort of ratios suitable for motorsport, it’s necessary to make the pinions very small, necessitating massive tooth profile ‘correcting’. This gave rise to at least premature wear, closely followed by regular failure. To improve matters, a very slight helical angle was applied – so now they’re classified as ‘semi-helical’.
Manufacture. The tooth profiles are literally ‘chopped’ out of round steel billets. Straight-cuts are far easier to produce as there’s no complex angle to set up. This can be witnessed in certain manufacturer’s gears where lines can be seen running across the teeth – almost ‘serrations’. How prominent these are is dependant on how fast the ‘chopping’ machine’s run. The faster it runs, the quicker (cheaper) the gears are made, but the more pronounced these serrations are. This is a bad thing, as they rub against each other in use – promoting both more noise and accelerated wear. Worst problem with this in our Minis is that the metal particles shaved off end up in the oil pump/engine! To avoid this the gear teeth should either be cut slowly or ground afterwards. The smoother the tooth, the quieter it’ll run and the longer it’ll last.
Conclusion. Straight-cut gears will ‘chatter’ at idle as there’s no initial load to instigate a stabilising side thrust. Rapid manufacture will increase noise generated via increased friction, and amplify the chatter by excessive lash between the gears. The more oil the gears run in, the greater the damping effect on noise production. So the main gear cluster actual develops very little noise at all. Barely discernable. Final drive gears produce more noise than their semi-submersed situation implies simply because of the area/quantity/design of teeth doing the work. The noisiest of all are the transfer gears – lots of activity and very little oil. The good news then is using a close ratio straight-cut gear cluster isn’t going to make you deaf!
Panel – ‘Recommendation for the road – If the helical S/1275GT set-up proves illusive, use Mini Spares Centre’s ‘Clubman’ kit. Practically the same ratios as the old 3-syncro straight-cut cluster (recognised as probably the best ratios for fast road use) but fits four syncro boxes. Wicked.