donut
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| posted on 15/8/07 at 01:43 PM |
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Working out diff ratio
My diff is out of the casing so i thought i would work out the ratio. How do you work the ratio out? (In numpty language please...none of this pinion
gear, lay gear stuff as i don't understand!!  )
Andy
When I die, I want to go peacefully like my Grandfather did, in his sleep -- not screaming, like the passengers in his car.
http://www.flickr.com/photos/andywest1/
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02GF74
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| posted on 15/8/07 at 01:48 PM |
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here is a diff; 
you can see the big round thing in the centre (the crown wheel), count the wobbly bits around the edge (teeth).
on the right you see a knob shaped thing (pinion) whose wobbly bits engage with wobbly bits in the big round thing - count the wobbly bits on that
(pinion teeth)
diff ratio = teeth on crown wheel divided by teeth on pinion.
[Edited on 15/8/07 by 02GF74]
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matt_claydon
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| posted on 15/8/07 at 01:49 PM |
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Couldn't have put it better myself 
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The Great Fandango
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| posted on 15/8/07 at 02:19 PM |
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I really hope your knob doesn't look like that!...
He Who Dies With The Most Toys Dies The Happiest
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donut
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| posted on 15/8/07 at 02:20 PM |
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Brilliant!! Fully understood, thanks very much.
Andy
When I die, I want to go peacefully like my Grandfather did, in his sleep -- not screaming, like the passengers in his car.
http://www.flickr.com/photos/andywest1/
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donut
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| posted on 15/8/07 at 02:28 PM |
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Ok, big wheel (crown) = 35 teeth
Pinion (knob) = 9 teeth
=3:88
I have a 1600 xflow and type 9 box so question is:
Is this diff good for acceloration or motorway cruising?
Andy
When I die, I want to go peacefully like my Grandfather did, in his sleep -- not screaming, like the passengers in his car.
http://www.flickr.com/photos/andywest1/
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David Jenkins
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| posted on 15/8/07 at 02:41 PM |
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Depends on your wheel size! I have also have a 1600 xflow, type 9 box, 3.89:1 diff, with 185/70 x 13" tyres, which means I'm just over
3000rpm at 70mph - very relaxed.
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donut
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| posted on 15/8/07 at 02:47 PM |
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sounds good to me.
I'll be using 13" wheels with 185:70:13's
[Edited on 15/8/07 by donut]
Andy
When I die, I want to go peacefully like my Grandfather did, in his sleep -- not screaming, like the passengers in his car.
http://www.flickr.com/photos/andywest1/
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matt_claydon
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| posted on 15/8/07 at 02:51 PM |
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quote:
Originally posted by donut
Ok, big wheel (crown) = 35 teeth
Pinion (knob) = 9 teeth
=3:88
To be correct, it's 1:3.88 ratio - 3:88 would be extremely low gearing
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donut
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| posted on 15/8/07 at 02:52 PM |
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Ahh ok, thanks.
Told you i was a numpty with these things!!
Andy
When I die, I want to go peacefully like my Grandfather did, in his sleep -- not screaming, like the passengers in his car.
http://www.flickr.com/photos/andywest1/
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mookaloid
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| posted on 15/8/07 at 03:50 PM |
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The quick escorts - rallying etc use 4.44:1 or even lower for good acceleration.
3.88 will be a good compromise.
Cheers
Mark
"That thing you're thinking - it wont be that."
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tul214
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| posted on 15/8/07 at 03:56 PM |
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i know it has been posted before but if you know your gearbox ratios this is handy;
http://www.westfield-world.com/gtc.html
1.6 Raw Super6 sold
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DarrenW
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| posted on 15/8/07 at 04:03 PM |
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Just taking this further - is it easy to work out if the diff is still in the casing and the ID tag is missing?
i seem to recall you can turn the prop then count revs of wheel but only if both wheels move same amount - i tried this when building car and seem to
recall getting both wheels to turn same amount wasnt easy in practice.
And is this right - if you have a 3.62 and change to higher number (eg 3.88) do you get better acceleration, and if you change to lower number better
top end?
Is the higher the number the higher the gearing or the other way round?
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DIY Si
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| posted on 15/8/07 at 04:07 PM |
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The higher the number, the quicker the acceleration. I've never been 100% about the higher/lower gearing term myself, as I can't remember
which way round it goes!
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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mookaloid
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| posted on 15/8/07 at 04:09 PM |
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quote:
Originally posted by DarrenW
Just taking this further - is it easy to work out if the diff is still in the casing and the ID tag is missing?
i seem to recall you can turn the prop then count revs of wheel but only if both wheels move same amount - i tried this when building car and seem to
recall getting both wheels to turn same amount wasnt easy in practice.
And is this right - if you have a 3.62 and change to higher number (eg 3.88) do you get better acceleration, and if you change to lower number better
top end?
Is the higher the number the higher the gearing or the other way round?
A 3.92:1 should accelerate faster than a 3.62:1 diff.
A 4.44:1 will accelerate much faster than the two above - however you'll need more gear changes to get up to a given speed.
Cheers
Mark
"That thing you're thinking - it wont be that."
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mookaloid
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| posted on 15/8/07 at 04:11 PM |
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quote:
Originally posted by DIY Si
The higher the number, the quicker the acceleration. I've never been 100% about the higher/lower gearing term myself, as I can't remember
which way round it goes!
IMHO the higher the number the lower the gearing. so 4.44:1 is low geared 3.38:1 is high geared.
Cheers
Mark
"That thing you're thinking - it wont be that."
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JimSpencer
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| posted on 15/8/07 at 05:26 PM |
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quote:
Originally posted by DarrenW
, and if you change to lower number better top end?
Hi Just an thought/observation..
Gearing on a 7 for 'top end' is a bit academic as you'll need over 200bhp to see much over 120 in any case?
So is you've got a crossflow that revs to say 6,500 you'll be wanting around a 1:4.4 otherwise most of 5th gear would be totally redundant
as cruising on a motorway isn't really what these are about?
I'm running a 4AGE - redlined at 8k, diff ratio is 1:4.7 and that's giving me a theoretical top speed of around 128.. though it runs out
of puff a bit before then anyway
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NS Dev
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| posted on 15/8/07 at 05:43 PM |
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quote:
Originally posted by mookaloid
quote:
Originally posted by DIY Si
The higher the number, the quicker the acceleration. I've never been 100% about the higher/lower gearing term myself, as I can't remember
which way round it goes!
IMHO the higher the number the lower the gearing. so 4.44:1 is low geared 3.38:1 is high geared.
Cheers
Mark
Yep same here.
On the old mk 2 forest car we used to run a 5.1 diff!!!
That was with a millington diamond 16v engine that went to 10,000, and I seem to recall that 9000 revs was around 115 mph in top gear!! 
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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procomp
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| posted on 15/8/07 at 06:57 PM |
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Hi. And boy oh boy do they sound nice at those revs.
cheers matt
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Dingz
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| posted on 15/8/07 at 09:24 PM |
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quote:
i seem to recall you can turn the prop then count revs of wheel but only if both wheels move same amount - i tried this when building car and seem to
recall getting both wheels to turn same amount wasnt easy in practice.
Jack up one wheel so the other is on the ground, turn the free wheel 1 turn and count accurately the number of turns the pinion does, divide that by 2
and Bobs your auntie, thats the ratio.
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snapper
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| posted on 15/8/07 at 10:01 PM |
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Then add in the gearbox ratios and you open another can of worms.
I make some assumptions here so please bear with me, i have been working on the ideal gear ratios for my LSIS for some time and with the available
gearboxes it's always a compramise. The type 9 has 2 different 1st gear ratios, 3.6 and 3.3 (V6 box) this means that to get a usefull first gear
you need a long diff 3.6 or above however 4th gear top revs has problems with achieving peak power.
Wheel rolling radius can also alter the speed in gears.
The basics of high diff ratio more acceleration low diff ratio more top end do apply but with caveats.
There are some good excel spreadsheets available that let you put in wheel diameter, tyre size and profile, diff ratios, gearbox ratios and peak
revs.
Off the shelf, best ratios in a gearbox i could find before going Quafe or BGH is the T5 cossie, with these ratios its then upto tyre wheel size and
power. 130 to 150 bhp then 3.9 diff is good over that then you can push through a 3.6 this is with CEC engines, V8's like 3.38 diffs.
Bike engines need the longer diffs because of the drop down ratios built in to the gearbox effectively multiplying torque and the fact they run bigger
wheels.
My solution so far is a V6 Type 9 and 2 diffs, 3.9 & 3.6
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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David Jenkins
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| posted on 16/8/07 at 08:06 AM |
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In general, my gearbox and diff ratios suit me perfectly (3.89:1 diff, Type 9 box, 185/70 x 13" tyres). Once I'm on the move everything
works very well for the sort of driving I do - fast & twisty country lanes, generally not exceeding the speed limit, but not slowing down much
either! 
The only fly in my ointment is the Type 9's 1st gear, which is far too low - when I'm in a hurry I have to change into 2nd at around 25 -
28 mph, and I get to that point all too quickly! My planned solution is to get another box and replace the 1st gear-set, plus an overhaul, ready for
a changeover at a later date.
Otherwise I'm very happy with my set-up - as Donut will be happy with his, once he's up and running again.
DJ
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