Straight Cut Gearboxes - a few questions
Discussion
Hi, I've mulling over the idea of a 4 speed straight cut box & I'm looking for a few answers if you might oblige chaps!
1. are they easy to use?
2. does one still involve the clutch?
3. are they reliable with 90-100 at the wheels - how can I expect it to last?
4. would you use one?
5. have you and what do you think?
Thanks people
1. are they easy to use?
2. does one still involve the clutch?
3. are they reliable with 90-100 at the wheels - how can I expect it to last?
4. would you use one?
5. have you and what do you think?
Thanks people
minimax said:
Hi, I've mulling over the idea of a 4 speed straight cut box & I'm looking for a few answers if you might oblige chaps!
1. are they easy to use?
2. does one still involve the clutch?
A standard straight cut box retains synchromesh operation, hence they are as easy to use as a standard helical box and yes, you need to use the clutch. If you want to try full power clutchless changes you will need a dog box, but for road use this really is massive overkill.
said:
3. are they reliable with 90-100 at the wheels - how can I expect it to last?
They are far more reliable than helical boxes due to the lack of side thrust on the main shaft bearing. Torque is the real killer of the A series box, so if you are running a N/A setup the box should be extrememly reliable, as long as you uprate the diff.
said:
4. would you use one?
5. have you and what do you think?
My brother and my sisters partner have had straight cut gears in their mini's for years with no problems. They also have the straight cut drops as they have turbo'd setup's and the drops are a known weak point. However, the drop gears vastly increase the noise, to the point where you probably wouldn't want to go on very long journeys. After driving from Plymouth to Silverstone and back for the 35th Aniversary bash, my ears were literly ringing for a couple of days. The noise of the drop gears even drowned out the 45DCOE under crusie conditions.
If you are having gearbox reliablity problems and you can tolerate an increase in gear noise, then a straight cut box is a great idea.
>> Edited by MR2Mike on Wednesday 6th August 11:52
minimax said:
Hi, I've mulling over the idea of a 4 speed straight cut box & I'm looking for a few answers if you might oblige chaps!
1. are they easy to use?
2. does one still involve the clutch?
3. are they reliable with 90-100 at the wheels - how can I expect it to last?
4. would you use one?
5. have you and what do you think?
Thanks people
1) Yes easy to use, same as a normal box.
2) Yes you still use the clutch; I believe a dog box can be changed without using a clutch.
3) Wouldn’t think you have any problems, worth getting a twin pin diff with that much power, LSD would be even better
4) If I had the money.
5) They are a little noisy but worth it, my friends car sounds like a touring car coming down the road
Straight cut gearboxes are great. Make sure you use straight cut drop gears and a cross (twin) pin diff as well.
Everyone will heard you coming down the road - a mate of mine thought it sounded like an UFO.
With a turbo engine, it sounds even better - whine, psst, whine, psst - magic stuff.
Everyone will heard you coming down the road - a mate of mine thought it sounded like an UFO.
With a turbo engine, it sounds even better - whine, psst, whine, psst - magic stuff.
My 2 pence.
If you want out and out performance: SC Box, SC Drop gears, X Pin Diff. Expensive, noisey, but sounds cool and good reliability.
Compromise. SC Box, Semi helical drop gears, and uprated Diff pin. Much cheaper, quiter, but more reliable than the Std box.
The reason the box is noisey is that the Drop gears are not surrounded by oil, so it is them that make all the "chatter" and whining, but it does sound soooo cool.
I'd go for the first option, except I hardly drive the minis any more and am hopefully getting a TVR at the end of themonth.
If you want out and out performance: SC Box, SC Drop gears, X Pin Diff. Expensive, noisey, but sounds cool and good reliability.
Compromise. SC Box, Semi helical drop gears, and uprated Diff pin. Much cheaper, quiter, but more reliable than the Std box.
The reason the box is noisey is that the Drop gears are not surrounded by oil, so it is them that make all the "chatter" and whining, but it does sound soooo cool.
I'd go for the first option, except I hardly drive the minis any more and am hopefully getting a TVR at the end of themonth.
Here's my bit, for what it's worth.
apart from the noise levels, as described above, the only problem is that the S/C boxes available now all seem to be the close ratio type, which was originally the 'race' spec ratios, rather than standard ratio S/C. This makes first gear a bit on the high side for rapid starts and if you use a hotter cam you will need to change the clutch more often. I wish I could get a standard rato S/C box for my rally car, then I could get better off-the-line performance. As it is I have to use the clutch as a torque converter to overcome the high 1st gear, and this is with a 3.9 diff which only gives 14.6 mph/1000 rpm in top on 10" wheels.
I don't use S/C drop gears and my 100+ bhp car has never caused a problem, even flat out on rough stages.
Also, I often do clutchless changes with my boxes, it's very easy. It'll probably break on this weekend's event now!!
apart from the noise levels, as described above, the only problem is that the S/C boxes available now all seem to be the close ratio type, which was originally the 'race' spec ratios, rather than standard ratio S/C. This makes first gear a bit on the high side for rapid starts and if you use a hotter cam you will need to change the clutch more often. I wish I could get a standard rato S/C box for my rally car, then I could get better off-the-line performance. As it is I have to use the clutch as a torque converter to overcome the high 1st gear, and this is with a 3.9 diff which only gives 14.6 mph/1000 rpm in top on 10" wheels.
I don't use S/C drop gears and my 100+ bhp car has never caused a problem, even flat out on rough stages.
Also, I often do clutchless changes with my boxes, it's very easy. It'll probably break on this weekend's event now!!
What is a dog gearbox? Is it like a semi-automatic or a tiptronic? Are there any advantages using the dog versus the straight-cut gearbox? What are the disavantages of using a dog?
I hear more about the straight-cut gearbox than the dog. What are the price difference? I have found straight-cut gearbox to be around 2k USD. Are there any for about $500 USD (used)? What is the price of a dog box and any Website that offer more info/pics on it? Thanks.
Blimey Cooperman - and I thought my 3.44:1 diff was frantic on the motorway
I really want a set (full whack) highly amusing I suppose you could get a different ration set specially made up, but god know what the cost would be
Oh, and as for not using a clutch - my aunt used to change her manual discovery without using the clutch - for some reason she got through a lot of gearboxes... (not helpful, but highly amusing)
I really want a set (full whack) highly amusing
I suppose you could get a different ration set specially made up, but god know what the cost would be Oh, and as for not using a clutch - my aunt used to change her manual discovery without using the clutch - for some reason she got through a lot of gearboxes... (not helpful, but highly amusing
)[quote=Fatboy]Blimey Cooperman - and I thought my 3.44:1 diff was frantic on the motorway
My son and I had a full rally spec 970 'S'a few years ago with a 286 cam and a 4.1 diff. This gives about 13 mph/1000 rpm in top, but it revved to nearly 8500. It had 83 bhp at 6500. I thought it was a horrible car, all revs and no real go, but then I was comparing it to a full 1293 rally car with lots of lovely torque. It was a good class winner in the up to 1000cc class, but we sold it quite quickly. It always handled beautifully, though, as it didn't have the power to really understeer like the big cars can.
My son and I had a full rally spec 970 'S'a few years ago with a 286 cam and a 4.1 diff. This gives about 13 mph/1000 rpm in top, but it revved to nearly 8500. It had 83 bhp at 6500. I thought it was a horrible car, all revs and no real go, but then I was comparing it to a full 1293 rally car with lots of lovely torque. It was a good class winner in the up to 1000cc class, but we sold it quite quickly. It always handled beautifully, though, as it didn't have the power to really understeer like the big cars can.
d3vine said:
What is a dog gearbox? Is it like a semi-automatic or a tiptronic? Are there any advantages using the dog versus the straight-cut gearbox? What are the disavantages of using a dog?
I hear more about the straight-cut gearbox than the dog. What are the price difference? I have found straight-cut gearbox to be around 2k USD. Are there any for about $500 USD (used)? What is the price of a dog box and any Website that offer more info/pics on it? Thanks.
A dog box refers to the method in which the gears (which are in continuous mesh) actually engage/disengage drive from the shaft on which they turn. A normal road gearbox uses synchro hubs which match the speed of the gear to the shaft before allowing it to engage. This gives a smooth, crunchless gear change. However, the synchro will utlimately limit how quickly you can change gear, and the selector forks can bend/break if you are too violent with gear changes.
A "dog box" uses a dog clutch on the gears. This is simply protrusions on the sides of the gears that mesh with simmilar protrusions on the hubs. This means that the dog clutch can only be engaged, or disengaged, it cannot slip to allow the gear to match speed with the shaft like a synchromesh system. The advantages are that you have nothing limiting the speed of your gear changes, and the dog clutches tend to be stronger than synchro hubs. Gearchange quality for slow shifts is very poor, essentialy you have to change gear very quickly to get a reasonably smooth change. The "dogs" get smashed into each other with a lot of force during fast changes so they tend to chip/wear quite quickly.
Note that there is no relation between a straight cut gear and the dog clutches, it's entirely possible to have a helical gear set with dog clutches (if someone made such a thing).
For road use you definately want to stick with a synchro box.
Cooperman said:
Fatboy said:
Blimey Cooperman - and I thought my 3.44:1 diff was frantic on the motorway
My son and I had a full rally spec 970 'S'a few years ago with a 286 cam and a 4.1 diff. This gives about 13 mph/1000 rpm in top, but it revved to nearly 8500. It had 83 bhp at 6500. I thought it was a horrible car, all revs and no real go, but then I was comparing it to a full 1293 rally car with lots of lovely torque. It was a good class winner in the up to 1000cc class, but we sold it quite quickly. It always handled beautifully, though, as it didn't have the power to really understeer like the big cars can.
Those over-square (as the 970 and 1071 S cars were IIRC?) blocks couldn't half rev
8500 rpm !!!!!! Wonder what it would have been like with a blower on it to up the torque a bit? (other than blown to bits 
) I love revvy engines - main reason I want to plonk a bike engine in my mini one day... a mini revving to 12000 would be something to behold
Edited to add Good luck this weekend!
>> Edited by Fatboy on Friday 8th August 18:03
dannylt said:
Note that there is no relation between a straight cut gear and the dog clutches, it's entirely possible to have a helical gear set with dog clutches (if someone made such a thing).
How about all motorbike gearboxes?
>> Edited by dannylt on Friday 29th August 09:44
Well, I was actually refering to a geer set for the mini box...
BTW, not all motorbike boxes use helical gears, certainly the small engined strokers I rebuilt in my youth used straight cut gears.
Curious, there seems to be a bit of scruples about dog boxes being a pain in the ass. Bikes use them and I have seen some seriously old gearboxes with the teeth in quite good order. I cannot see any reason why the 2 setups differ, unless bikes run their gears at a lower speed. Would I be right. A fast mesh of dogs would wreck the teeth and a slow one would probably mean there was enough torque to wreck the bearings on a car. I am considering using a dogbox in my forthcoming lotus twincam mini.
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