13 replies to this topic

[walshy95]

Hi guys, looking at converting my gearbox to straight cut. I've found on mini spares the st Abingdon ratio gear set. How much different are the ratios? I'm rebuilding my 1380 with the intension of supercharging it, the vmaxscart website informs me to try and stay away from close ratio gear sets. So ideally I want as close to standard or standard ratio but in straight cut form!! Thanks in advance guys!!

[Ivor Badger]

Hi guys, looking at converting my gearbox to straight cut. I've found on mini spares the st Abingdon ratio gear set. How much different are the ratios? I'm rebuilding my 1380 with the intension of supercharging it, the vmaxscart website informs me to try and stay away from close ratio gear sets. So ideally I want as close to standard or standard ratio but in straight cut form!! Thanks in advance guys!!

 One has to ask, why stay away from the close ratio gears?

[Northernpower]

I can't understand why you want to stay away from the close ratio gear set if you're wanting it straight cut?

[Cooperman]

A super-charged engine will have high torque and should have a broader power band than a normally aspirated engine with similar power.
Thus an ultra-close-ratio gearbox is not ideal. Wider ratios will work better.

[Allrounder]

Be interested in what you find. After I broke my standard box ( broke the 2nd gear teeth off the lay shaft) I am looking for a stronger replacement but all the ratios are wrong for needs so now I'm just lifing the gear sets. Guess-works is making a road going s/c gear set with closer ratios to standard but is taking awhile to do.

[Allrounder]

Be interested in what you find. After I broke my standard box ( broke the 2nd gear teeth off the lay shaft) I am looking for a stronger replacement but all the ratios are wrong for needs so now I'm just lifing the gear sets. Guess-works is making a road going s/c gear set with closer ratios to standard but is taking awhile to do.

[mini13]

your a bit stuck on ratios for a forced induction engine, yeah you dont want them too close, but the ST or clubman set ( they are amost the same ratios) are the widest straight cuts available that I know of with their 2.5 1st gear ratio. you want to steer clear of the closer sets with a 2.3 fist gear.

 

at one point quaife did a gear set with a 2.7 first that would have been good, but it was on their dog box, and IIRC was only one batch sold through swiftune.

 

personally I have run the abingdon ratios in a turbo lump with a 3.1 diff and it was ok, first was a little long so not great for being in traffic.

[Northernpower]

Having read through some of the builds and comments I now understand why you need the wide ratio's. Never too old to learn.

[Spider]

A super-charged engine will have high torque and should have a broader power band than a normally aspirated engine with similar power.
Thus an ultra-close-ratio gearbox is not ideal. Wider ratios will work better.

 

Exactly to the word what I was going to say.

[mini13]

Until the teeth get pulled off the laygear...

[Cooperman]

There will always be strength issues with Mini transmissions. It is all a compromise. Remember, size-wise the gearbox was originally designed for 34 bhp (on a good day) with the torque of a limp wrist on a cucumber. Now we are trying to put huge power and, more importantly torque through similar sized components, albeit using better materials.

I have fitted every strengthened part possible to my 115 bhp 'S', but still the transmission is the weak link. I have stripped gears, broken drop gears and snapped drive-shafts. I have even broken a cross-pin diff unit (that took some doing!).

If very high output units are fitted to Minis and seriously aggressive driving then happens, breakages must be expected. 

Helical gears are no less strong than straight cut, in fact they may well be stronger and SC drops are simply horrible.

For a forced induction unit the standard Cooper/Cooper 'S' gear ratios are ideal and can be used with a relatively high FDR, maybe 3.1:1 for good cruising at sensible revs.

On a normally aspirated engine with, say, a 286 cam, high-comp gas-flowed head, big carb and a narrow power band a very close ratio box is needed to keep it 'on the cam' and a low FDR is needed as well which means high revs at cruising speed, but superb acceleration.

It all needs to match really for best overall performance. look at the cam graphs for the engine spec chosen, then see what the effective rev range is at best torque and gear it to that. It's called 'engineering'   .

Edited by Cooperman, 07 February 2017 - 07:32 PM.

[MGMiniman]

I did 20 seasons racing Mini Miglia in both 1000 and 1275 capacities never broke a gearbox!

Had engines break cranks, throw rods through the gearbox but never the gearbox's fault.

Close ratio Straight cut gear sets - straight cut drop gears - originally from Special Tuning later Minispares gear sets. 

On slicks - wheel spinning starts - over the kerbs - off track excursions etc. Ultimate power and torque.

Gearbox carefully assembled - the right clearances - good bearings - single pin diff.

Used proper steel baulk rings gleaned from stripping old original gearboxes - Probably the originator of the diff mod now copied by Minispares. Fitted phosphor bronze rocker bushes in the planet gears to counter diff pin wear.

Other than the right bits and careful assembly worked well for me.

[mini13]

all things being equal the standard gearset starts giving out around 140-150 ft lb, good straight cut sets ( and there are not so good ones out there) are ok to 220lb ft, mainly due to being made from better materials,

 

 

interestinly minispares have released an "uprated" helical kit, but your still stuck with the abbingdon ratios. and its not cheap.

 

http://minispares.co...px|Back to shop

 

a final note, the worst thing for killing gear parts ( and any parts) is rallying, or racing where your going from loose to tarmac etc, very hard on parts, and usually the cars are heavier, cages extinguishers etc...

[Cooperman]

Yes, it is serious rallying and rallycross which puts huge shock loads into the transmissions, especially if really 'trying'.
Drive shafts and diffs are especially vulnerable to breaking and I managed to break a cross-pin diff. I have also broken drop gears (but that was after 28 rallies) and a CV joint.