37 replies to this topic

[mini13]

std helical gear sets are known to start breaking at around 150 hp, where as sc kits are being used up to around 250hp+ ( not dog type)

 

also dont confuse longevity with strength,

[Cooperman]

SCCR boxes really are only suitable for competition cars with very 'lumpy' cams (typically the 286, 296, 649, etc) where the torque is minimal below about 3000 rpm and the power band only strong between around 5000 and 7000 rpm. The close ratios are to enable the engine to stay 'on the cam' when gear-changes are made. There are only disadvantages when using a SCCR box on a road car, especially the very high 1st gear requiring a lot of clutch slipping when pulling away from rest with a consequent loss of acceleration and high clutch wear. I once drove a 1330 Cooper 'S' with a SCCR box, a 510 cam and a 3.44:1 FDR. It was very difficult to get a quick start from rest and the clutch wore out very quickly.

 

If you are nor driving at very high revs all the time, then a SCCR box is unsuitable.

 

A much better 'box for a mildly-tuned road engine, for example with a 266, 276 or equivalent cam is the old 998 Cooper and Cooper 'S' ratios. They are a bit closer and make for a very nice gearbox.

[Avtovaz]

 

straight cut boxes are stronger, fact.

 

 

If your thinking is open (and I suggest that's a good thing), then can I ask you have a good read of this thread, it was discussed at length and with some proven facts;-

 

http://www.theminifo...ears-beginners/

 

Of course, I'm all ears if you disagree, but I'll ask you to read that thread and then come back.

 

I do love an open, frank and most important, respectful discussion, but I'm not interested in arguing any points.

 

 

 

 

hi mate, thankyou, i shall try to explain but bear in mind ive been up since 4am, worked 11 hours should be in bed and am pooped!

 

 

 

so you have an engine , that makes power. To make the best of this for a given circumstance, we have gears.

 

So in a standard mini engine, you have 4 forward, 1 reverse. one of the forward gears usually is locking the shafts together 1:1 ratio but in some cases like on Lada Samara you have 5 serperate gears!

 

So helical cut in the mini, as the gears pass each other its like when you put a knife over a grind stone? That is all can can think of atm to compare it to, so your running it along the gear. This is rolling point contact.  when you drive, the shafts in the box take some axial loads, and this is taken by the bronze bushings in the box. Mini has ball bearing bearings.

 

So, what is a standard box good for? Its quiet firstly, and quiet cars sell well. You have a first gear to launch it up any hill, get the mass of the car moving, second to keep it going, and third usually is miles away as you need to get to the 1:1 of 4th.

 

 

 

A competition box, its after something different, you want to keep the engine in a range to transmit power to the diff to the wheels in the most efficient way you can. So back in the day, if you had a mk2 escort, you could pick the best of a bad bunch and use say a rocket box, 4 speed, not bad ratios, not massively strong, but  good enough for £50 second hand! Problem is, when you put power threw the box, its trying to push its self apart, Those bronze bushings now taking a good pounding, and your on and off the power all the time, which any play in those bushes then becomes 10 times worse as your not slide hammering the gears back and forth. Now also the gears dont line up as well, and dont mesh quite as well, the casings wear , and one day your box explodes.

 

So you use straight cut gears, this has moving line engagement. Now your not hammering those bushes as much, and your using the outer casing more to stop the gears from forcing them selves apart. I had a quick scan on that thread as as it says, helical GEARS are stronger than straight cut, that is true, look at the length of the cut and the shape of the gears, straight cut ones its peek trough peek trough, but, the contact patch is different, the way they mesh is different, its more efficient. Down side is its bloody noisey.

 

Now that your not loading the bearings up as much axialy , you can use roller bearings, that is the difference between a type 9 clubman and pro box, and because the bearings is superior, same od, bigger id, so you can make that part of the shaft much thicker. So its a lot stronger.

 

 

So, as i said, a SCCR ratio box is stronger than a standard box. The gears them selves will as a gear brake before a helical one as its stronger, but as an overall package, they have more strength, less transmission losses, they are designed for a specific purpose, fact.

 

 

 

right, ok,,  so why do i have a is200 6 speed helical gear box specially adapted to fit my bmw road rally car?

 

The gearing of a car means a lot, think about what you want the car to do, and how you are going to do it. So a sccr box is 5k, i dont have 5 k or do i want to listen to the noise the bmw boxes make, So i ve made the best of a bad job. I maybe on a hill on a control, and i need to launch 1200kg. My car does, 20-40-60-80-10-110mhp threw the gears, with a  final driver of 5.0:1 and 195/65/15 maxsports :)) My mate has a puma ex works rece car, 1400, cams in it, and on a hill it just could not pull itsself, it was painful watching it, and all those seconds he made up, just lost. The box in his car is geared to take him off the line on a sv rally, use the other 4 speeds then to keep it buzzing, then on hairpins down into this long multi purpose first and off again.

 

R&D 7 speed box used in the escort cossie, first gear was very short, get the mass of the car moving which is the hard part, and then use the torque to pull it .

 

 

If you have a motor that is only power from 5-7k maybe fine accelerating in a race, but when you come to a corner, and its inbetween 2 gears do you scream it in the lower one, and top out? Do you drop a gear and let it bog down? If you have a spread of torque, you can overlap the power, you down want it just to hit max revs, then drop to the start of when it comes on cam and short shift up and use the power to pull you threw.

 

 

SCCR not exactly recomended for the road, but good strong boxes, sharpens up the gearing, makes for a good car.

 

the ones that may brake sold by some mini shop that i wont say about as i have no problem with them or experience with them, is it the gears? Is it the bearings? Who builds the box?

 

anyway time to see if mat green has an update and tehn bed ;)

[Spider]

Cheers Avtovaz for that.

 

I think I gotya on most of that.

 

I'll say that your summation of helical gears (the teeth ?) passing over each other like a sharpening stone I don't agree with. There is a wipe on the teeth, yes, for sure, but from near the tip to near the root of the tooth - and this happens with any tooth form (straight or helical), but the wiping does not occur axially, though from loaded to unloaded they maybe some (while taking up end float) but does not occur on a continual basis while running.The gears would have to move axially with respect to each other for this to happen.

 

In respect of the Thrust Washers - those on the Laygear - I am yet to replace one yet from wear, and this includes them from standard to competition (on helicals). In respect of the drop gears, yes, I agree here, this is a real headache in the post 1979 type Boxes (I'm working on a conversion, but that's quite aside from this discussion). The problem here is that the thrust face / area in the alloy housings is too small for the type of material that it is, but not the amount of thrust that occurs. In respect of the bearings in the gearbox on the 1st Motion and Main Shafts, again, I've not found this an issue and if there was, it would jump out of gear on power and coast regularly (auto-disengagement as the factory liked to refer to it as). In summery and going back to losses and strengths of the SC and HC types, I really don't see this as being an issue and I say that not only from an experience viewpoint, but also an engineering one.

 

While I have said - gear for gear - Helicals are stronger than Straight Cuts this is true. By Gear for gear, I mean, made from the same materials and treated in the same way. However, when going out in to the marketplace to buy gears to fit, some SC Gears are made from different, superior materials and undergo appropriate treatments. Not all you buy will be made this way !!!!!!!!!!!!!!  Certainly these better material gears will most likely be stronger than stock helicals, but only for these reasons.

 

Likewise a cheaper Straight Cut set will be weaker than any helical gear. Gears are very time consuming to make, no matter what machines are used and require a high level of inspection before being deemed 'suitable'. This all costs loads. eg, I had a small production batch of FD Gears made a couple of years back and on a per set basis (2 gears) this was over a grand (around 550 quid). That's just for 2 gears and on production quantities to get the price down. Do the maths from there. The size of the gear and number of teeth has little to do with the price of them. (these were Helical Cut by the way for the highest strength)

 

I put the HP rating of stock Helicals at around125 HP, but I say this to be conservative and for some realistic reliability of the gear set. Matt Read (in these parts) regularly runs them to 175 HP. I know of a few Minis in the UK that run well in to the 200's through them.

 

However,,,,,,,

 

To run these higher levels of HP (Torque really) through them there is a few other factors that come in to play here to do this. No. 1 Key critical is getting the right gearcase. If the machining is out, then forget it with any gear - Straight Cut or otherwise, even if made from the most space age expensive alloy steels. The cases vary a LOT. When building up special and select gearboxes, it's not unusual for me to go through around a dozen cases to get one that I can work with. None I've picked up have been perfect, they all need work and it's finding that one that can be re-worked the right way. The last gearbox I did this way took over 3 weeks to build up, measuring the cases is very time consuming. Then you need to match up a transfer case. This also goes hand-in-hand with losses through the gearbox.

 

Next up is how it's driven. One thing any gear - especially SC Gears in space age material - won't take is impact loading. They'll do it a few times, but not too many. Impact loading is doing things like dropping the clutch and getting airborne.

[mini13]

good points there regarding quality of gear kits, generally for turbo motors most people use the minispares Evo gear kit, this is tried and tested at 200+ hp by lots of people, personally I run the minispares abbingdon kit as I am on a remote box, but this is essentially the evo kit with a different 1st.

 

also a good caveat about "gear or gear", i would be interested to try the new wider  minispares helical kit, but would like to se it in a wider ratio

 

http://www.minispare...px|Back to shop

[Avtovaz]

thanks for the reply moke spider, its a pleasure to type to you! I had a feeling by the way you asked for a reply you had actual hands on knollage!

 

About the knife thing, i was tired and could not think of how to put into words what i was thinking.

 

 

Core shift they call it done they on the castings when stuff is out of alignment with the casings? I ported a head for my lada and i got a lecture of the guy rae sharret who later built the motor all about how this can effect things.

 

 

Can you tell me, i have a compact bmw , medium diff my mate has the the same diff in his race car, rae built both the diffs, i dont see rae any more, he is maybe 70+ now, but he has machined the edges off the CW teath with a die grinder maybe, there must be a reason but i dont know why.

 

the information ive learned about boxes its been passed to me over maybe 20+ years of being into rallying, and it can be very interesting listening to what people do to resolve a certain problem.

 

 

You must also know why do minis have sc first and reverse!

 

thankyou... ;)

[Spider]

Avtovas - no worries.

 

I'm sure many of us can discuss many things, sometimes with differences of opinion and ideas in respectful manner.

 

Gotya on being tired - it's how I feel a lot these days !!

 

I think I can see what you were trying to convey in respect of the wiping action, To see what's going on here for yourself, if you have a couple of helical gears in a box and are fairly dry of oil, put a tiny dab of point on the centre of the tip of 1 tooth and spin the gears over a couple of times. See where the paint goes. You'll find it will make a stripe down the teeth but not across (unless it was too thick when put on).

 

Straight cuts were used for 1st and Reverse as Reverse does not use a syncho hub have but is 'crash' engaged by meshing and unmeshing the teeth themselves. To get this action to occur smother and with less 'baulking' (inability to select) the gear teeth need a lead in and will also fully select much easier as a straight cut. As Reverse uses First, this shows why First and Reverse are cut this way. Also and probably the bigger 'issue' here (when in Reverse) is also that on power on, the gears will tend to mesh 'deeper' but on over-run the gears will want to 'screw' apart (from side thrust) and so jump out of reverse. This is why in probably 95% of cars, they use a straight cut reverse.

 

For the short time that First gears and Reverse are used for, this seems acceptable in regards to noise.

 

I don't think it's core shift in regards to the gearbox cases that's the issue, but just plain poor tolerancing in machining. They are really woeful. A simple example of how bad they are is the drip that every Rod Shift Owners knows all too well. These drip oil not because they are low down in the box, but because the machining is so poor that the shafting here floggs about, well beyond the range of the seal that used. Only last week I gave some consideration to having my own cases cast, so I can machine them from scratch as well as correct some of the weaknesses in them. There's not much to them.

 

I can't comment specifically about what your friend did with his BMW CW, however until I had my Final Drive Gears made, I used to sit down for about an hour with the die grinder on the factory CW & P, not so much to re-shape any part but look for defects that could give be a stress riser and polish / blend them out.

[Avtovaz]

Right!!! now i see about the sc gears !

 

the machining, its 60 year old design or more, i look at the mini and i look at the bmw, its time machine stuff, but the mini is still loads of fun!

 

I never thought, he removed the weak points, my diff is a genuine bmw motorsport 5.0:1 he took little bits off of it, my mates came from motorsporttools.com, nice piece, lots cheaper but he has removed teh rough spots off of the gears!  Years ago, in escorts they had 3 piece shafts in the atlas axels which kept braking with pinto power, time has moved on but, the fix to make them stronger was to put them in the lathe and take off the roughness.. They didnt brake then ... And that is what rae has done to our diffs. i do wonder what 3 piece shafts are made of now, escorts with milly s are doing not far off 400 bhp now i suspect here for "2.5cc" ahem ahem... ;)

 

 

 

My bmw engine right, its not a massively complex thing, its a m44b19, 136bhp factory, i bought diesel M47 crank, etc, i could have got machining done to get it where it is now, but i said to rae, here is the parts, how much to build it. £600.. If i had built it, im sure it would have granaded by now , and have taken me ages to build.

 

Motor came back, clean, and painted LOL! pistons machined, crank installed properly, arp etc which i bought separate, sump came with windage tray, extra capacity, not had it open to look more, my cams timed in properly and front casings machined true too. list goes on.. i cant buy that knollage, its hands on.. it made by the way, 180 bhp and 160 lbs...

 

edit; if an engine builder is good, he wont build for a low price, he has a reputation, and will build it ocd as best as he can , to me my £600 was a lot, but its not is it,, think about my boss charges me out at work wor build tipper trucks at £36 an hour i last herd. so rae is cheep! They also wont build you a turd as of reputation, hence why i have windage tray, extra capacity, and a shopping list of oil cooler, this oil , dont use that, run it in like this, dont do that... once you have done that... GO AND HAVE SOME FUN!

 

 

thanks for talking. ;)

Edited by Avtovaz, 19 October 2017 - 08:23 PM.