NOTE; A 'high' or 'low' ratio gear is in reference to it's performance, not it's numerical number. To illustrate - a 'high' FD ratio will give 'higher road speed', but will have a numerically low figure. A 'low' FD ratio will give lower road speed, but have a numerically high figure. A by-product of this will be reduced acceleration capability on the 'high' ratio, increased acceleration capability on the 'low' ratio. The main gearbox gears work in the exact same way.
Elsewhere we've considered what alternative standard production ratios are available - but that still leaves you with the power-consuming and limited-ratio alternatives helical tooth type gears. Not desirable in a competition orientated car. The solution to this comes in the form of several types of straight-cut gear sets (teeth are cut at 90 degrees across the gear as opposed to the angled helical type). There are quite a number of kits on the market to fit everything from early 3-syncro up to the latest A+ 4-syncro gearboxes The accompanying table gives all relevant details of ratios/fitments/applications available.
Something I would like to point out right now is that contrary to popular opinion, a decently manufactured straight-cut gear set DOES NOT make a lot of noise. Largely because they’re drowned in oil which helps damp out the noise. The row (tasteful whine?) created by straight-cut gears is generated mainly by the drop-gears and final-drive.
When selecting a gear ratio set, the most important influences will be the engine’s power band and usage. A Mini used as a weekend warrior but used everyday on the road may have an engine built to a compromise - giving a relatively broad, usable power-band (say 2,200rpm wide) whilst sacrificing top-end power. Consequently a super-close-ratio gear set and low FD isn’t needed. On the other hand, an out-and-out race engine built for maximum power above anything else will need a very close-ratio box and low FD to ‘keep it on the boil’ - its ‘peak’ power-band being much narrower (probably only 700 to1,100rpm).
Apart from ratio selection, serious competitors consider the all-hallowed ‘dog-box’ a must. Seemingly the balk-ringed syncro selection is considered troublesome and unreliable. Well, the news is dog-type engagement isn’t the be-all and end-all that it’s claimed to be. It’s not even an accurate description, as all gears work by ‘dog’ engagement - it’s really ‘non-syncro’. Many folk go along with dog gearboxes because they’re lead to believe it’s what they must have. But not many folk understand the whys and wherefores. So here's the run down…
In the beginning…
We’re all up to speed (sorry!) on the history of crash gearboxes (outlined in the other 'Gearbox-' articles), their demise caused by development and application of the balk-ring/syncro mechanism, and the absolute transformation in gear-change quality this brought about. The dog-box design appears a backward step, so why use it?
Formula One cars used to use helical gears with syncro-mesh. Fact. No - really. Everything was fine until a certain Mr Hewland stuck his oar in. He could see a way to make a gear kit that was easier to use AND alter ‘in the field’. You see Mr Hewland was a keen motorcyclist/racer. Motorcycles had been using non-syncro engagement gearboxes since time began (for them anyway). Applying the same system, but beefed up for cars, would provide teams with a rapid system for altering gear ratios in the pits without the hassle of syncro-hub balls and springs careening around the garage like buck shot! Wear was increased as technology of that time wasn’t as advanced, but just having the aforementioned benefits coupled with ever increasing budgets made them a must - faster lap times being the ONLY objective.
Us Mini owners/racers are generally not quite as well ‘heeled’ as Formula One race teams, so what’s it mean for us? Well, perhaps the most significant point to get hold of is that motorbike gearboxes run at between half and two-thirds-engine speed. Our Mini gearboxes run at engine speed - and this is where the problems lie. I stated earlier that the dog-box was actually a ‘non-syncro’ set-up. Exclusion of the syncro system means that the dog-box has gone back in time to the crash-box type of selection. Unless perfect rpm selection is made by the driver/user, severe damage can be experienced causing very short gearbox life. We’re talking inertia and impact here. Imagine two gears with teeth sticking out of each side (the dog-teeth) spinning at several thousand rpm, each at slightly different speeds, then shoving them together in an attempt to slot those teeth into each other. This is exactly what’s needed to engage gears. The shear number of dog-teeth involved, and the fact that both gears aren’t rotating at the same speed, means that first time engagement is VERY difficult, seemingly impossible, to achieve. To ease this, dog-gears generally have fewer dog-teeth than the standard gears - but is still less than ideal.
What I’m alluding to here is the onus is very much on the driver/user. No syncro means exacting user ability. A dog-box in the hands of a poor user is far worse than a syncro box in anybody’s hands. Inaccurate use of a dog-box means slower/missed gear changes (slower lap times/engine damage through over-revving), and horrendous damage to the gears/selectors, causing engine-destroying metal particles/fragments in the oil. Consequently the potential/reasons for fitting a dog-box in the first place have been completely negated - and the much flaunted ‘reliability’. Unless the choice was purely for ratio availability, a syncro gearbox is far more user friendly. Frequent rebuilds of a badly used dog-box are monumental in cost. Frequent ‘service’ rebuilds of a syncro gearbox bearable. Having said all that - you just can’t beat a skillfully used dog- box!
Speed of change is down to the user too. The only reliable quick and sure changes after the syncro system are by semi-automatic or air-shift systems. Both make automatic adjustments to correct /match gear/hub speeds to ensure a perfect, first time selection. Formula One technology - and as yet beyond the realms of affordability for the average Mini racer.
STRAIGHT CUT GEAR RATIO AVAILABILITY AND APPLICATION CHART
C-AJJ3371 - 3-Syncro Mini gear set
C-AJJ3319 - 3-Syncro Sprite gear set
C-STN39 - Clubman gear set - available for A+ gearboxes only
C-AJJ4014 - 4-Syncro Remote-type gear set
C-STN76 - 4-Syncro Pre-A+ rod-change gear set
C-STN77 - 4-Syncro A+ rod-change gear set
All gears are available individually for each set, see site for part numbers.
See 'Gearbox - Up-rating diffs, FDs and ancillaries' for further information on gearbox components.