Transfer box and diff ratio calculations for 33s

[d_r_1989]

Am i correct in thinking that some vitara ring and pinions 5.1's with a samurai transfer box 1.409 will be a similar ratio to samurai 3.7 diffs with a rocklobster 4.16? Or am i getting way too confused in this magic art. Just looking at ways to reduce the gearing without breaking the bank

[steve greenc3]

Rocklobster will only change low range. 5.12 diffs will work with 33`s.

[ScottieJ]

presume this is for the 33s as mentioned in another thread.

I do not recomend 5.12 for tyres below 35" tall, They are geared too low. but then I don't recomend them for 35" tyres eithewr as they are weak

33s I would go 4.3 or 4.6, 4.3 is available on jimny diffs so you can use two jimny rears, just swap your front side gears into one for the front unless you are running chromo 26 spline front shafts in which case just use two rears as they are

4.3 does benefit from a 410 or rocklobster tbox on road to drop the high ratio a bit more though.

[ScottieJ]

Rocklobster will only change low range. 5.12 diffs will work with 33`s.

Rocklobster also reduces high range slightly over a samurai box.

[ScottieJ]

Thought while I'm here i'd slap some combos into my gear calculator as it doesn't take long.

All speeds in 5th gear high (and then 5th gear low in brackets).

Stock samurai tyres and gearing on road = 68.5mph(42.5mph) @4000rpm, You want to keep on road gearing as close to this as possible IMO, suzuki engines are designed to rev, but obviously you don't want it screaming while cruising along at 55mph+

33" with 5.12s and samurai tbox (1.409 high) = 63.3mph(39.3mph) @4000rpm, its gonna be close to 4500rpm at 70mph in high.

33" with 4.6 and samurai tbox = 70.4mph(43.7mph) @4000rpm

33" with 4.6 and a rocklobster (1.580 high)= 62.8mph (23.9mph) @4000rpm
(with 410 tbox low is 39.5mph@4k)

33" with 4.3 and a rocklobster = 67.2mph(25.5mph) @4000rpm
(with 410 tbox low is 42.3mph@4k)

33" with 3.909 diffs (jimny rears or 413) and a rocklobster = 73.9mph(28.1mph) @ 4000 rpm
(with 410 tbox low is 46.5mph@4k)

33" with 3.73 diffs and a rocklobster (1.580 high) =77.5mph(29.4mph) @ 4000rpm, way too tall! Its actually not that bad at all if you have a 1.6 fitted, I have been running this for years but I am about to fit some 3.9 or 4.3 diffs.

However it's offroad where running a rocklobster really shines, the reduction in the samurai box isn't enough for big tyres and I would always recommend a rocklobster. Then adjust the diff gearing to correct the rpm for road use, nothing worse than an sj with it screaming at 50mph because the diffs are geared too low. You could opt for 3.909/4.3 with a 410tbox as it will be the same on road as a rocklobster but low won't be good enough for really technical stuff, ok for green laning though. (added these speeds in brackets below the rocklobster lines)

Hopefully tomorrow i'll be uploading an excel file that Tim pink set up that I've made some tweaks to so that you can select your ratios and tyre sizes with a drop down so that you can see all the differences every change makes.

If you are running a diesel engine then you will probably want to run taller gearing than stock for the bigger tyres as well, unless you fancy a 50mph top speed

[d_r_1989]

Thats some excellent information there scottie thanks
Yes im planning on running 33s as with the virtual lift i think 31s will be too small.
It has a 1.6 carb engine so hopefully that will help
Are the jimny 4.3 rear diffs rare? Strong as sj diffs?
I have seen your write up on the homemade rocklobster but im not confident i can do the pressing of gears plus i havent sourced a 410 tbox yet. Might just have to fork out and buy a rockwatt made one.
How much are these 26 spline chromoly shafts you speak of and are they worth it?

[d_r_1989]

Iv just found the excel calculator in an old post but its showing different speeds to what you got. I thought i changed the ratios so they were all correct for a samurai. Bloody dark art this stuff

[ScottieJ]

How different were the speeds? They shouldn't be any different as thats an old verion of the same sheet. Did you make sure 5th gear had the correct ratio as there are two different ratios available.

Also I always input ratios to 3 decimal places.

Jimny rear diffs are EXACTLY the same diffs as 413/samurai diffs, so equal strength.

I always got to rockwatt for rocklobsters. He is the best at making them in the UK.

Chromoly 26 spline front shafts (and CVs) are around £400 once iported with all the taxes and shipping etc. deffo worth it if you never want to blow up another cv joint

[d_r_1989]

Just a couple of examples, im probably doing something wrong

[d_r_1989]

I guess regardless of speeds, the 4.3 diffs and 1.58 high box is closest to standard with 33s. Thats in both our calculations.
Just for green laning the 5.12 and samurai tbox dont look too bad