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Old 10-05-2010, 06:11 AM   #1 (permalink)
Mof
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Default Re-Gearing for bigger tyres

Hey Guys
I have a 93 Samurai Softtop DD
I have fitted 30" tyres which put the gearing way up.
I changed the tranfer case for a SJ410 case which brought the gearing back down to where it used to be but ive lost a bit of power.
Im doing the same revs to road-speed as when it had the smaller tyres, but now its gutless.

My question is would I be better to change the diff ratios and refit the original Sammi t-case?

My thoughts for this is the output from the gearbox is spinning the speed it used to, but the driveshaft is spinning slowly.
Would i get more torque by lowering the ratio as close to the end of the drive train as I can instead of at the t-case?
Or am I just talkin out a hole in my head?

Cheers
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Last edited by Mof; 10-05-2010 at 06:14 AM.
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Old 10-05-2010, 08:29 AM   #2 (permalink)
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Quote:
Originally Posted by Mof View Post
Hey Guys
I have a 93 Samurai Softtop DD
I have fitted 30" tyres which put the gearing way up.
I changed the tranfer case for a SJ410 case which brought the gearing back down to where it used to be but ive lost a bit of power.
Im doing the same revs to road-speed as when it had the smaller tyres, but now its gutless.

My question is would I be better to change the diff ratios and refit the original Sammi t-case?

My thoughts for this is the output from the gearbox is spinning the speed it used to, but the driveshaft is spinning slowly.
Would i get more torque by lowering the ratio as close to the end of the drive train as I can instead of at the t-case?
Or am I just talkin out a hole in my head?

Cheers
Mof
For those tires, I would suggest using the Sammi case with a set of 4.16 gears. This will get you where you need to be. Larger tires would be better with the lower gears (33" - 4.89/5.10 for example).
Replacing the R&P changes everything equally, and you don't want that if you are still going to drive it on the road.
The t-case gear sets on the market have been engineered to drop the low side farther than the high side. Hopefully that makes sense this early in the morning...
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Old 10-05-2010, 09:00 AM   #3 (permalink)
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I always thought changing everything equally doing an R&P gear swap for On-road was fine. changing the low end of the T-case to be lower was better for off road applications.


I think the problem you're experiencing with the vehicel being gutless is because you've increased the rotational mass. You can change the gearing so that the engine turns the same RPM's for the Same speed down the road, but your engine has to work a lot harder to spin those larger heavier tires. You also have more rolling resistance at the contact patch where the rubber meets the road. the inertia of the drive shafts spinning faster or slower isn't going to make much difference. If anything, you'll have more friction from the the front differential and drive shaft u-joints as they will be working harder and revolving more per wheel rotation.

Gearing it lower than stock is the best thing to do, wheather it's at the differential or the t-case is up to where you primarily drive it.
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Old 10-06-2010, 12:07 AM   #4 (permalink)
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Thanks guys,
Im not sure why you say the front diff/drive shaft and not the rear/both..
Maybe im just not reading it right

at 100ks (60mph, i work in metric) im doing 3500rpm already so I dont want to go any lower, and im happy with the low box I dont need crawler gears.

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Old 10-06-2010, 07:44 AM   #5 (permalink)
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Quote:
Originally Posted by Mof View Post
at 100ks (60mph, i work in metric) im doing 3500rpm already so I dont want to go any lower, and im happy with the low box I dont need crawler gears.
Hi Mof,

If I understand it correctly, you might not be able to get speed, crawl and torque in one setup if the change is just with the R&P (or else a bigger engine). I had to sacrifice speed by going with 5.12 diffs and 3.05 tcase ratios and ended up with a topspeed of 80kph (@4000rpm).

Might be able to get away with it with the tcase mods.

Just my opinion.
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Old 10-06-2010, 09:53 AM   #6 (permalink)
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I am running '87 Sami with 16v, 6.4 t-case, and 4.30 r&p on 31" tires.
I am running about 4100rpm at 65mph and still have a crawl ratio of 100:1.
When decide to go to a 33" tire, my rpm should drop down under 4k @ 65mph.
AND this setup has my speedo/odometer working almost spot on, not that it is really that important but kinda nice to have.

I was previously running stock t-case, 5.12 r&p on 31" tires. This had me running 4200rpm at 65mph and a crawl ratio of 42:1.
Sucked off road and couldn't gear the t-case if I wanted to still run it down the freeway.

Here is a good online gearing calculator, give it a try... it has been spot on through out my different gearing changes :
Gear Ratio Calculator

Here is the Trail Tough article on gearing ; t-case vs ring & pinion :
Question - Transfer case gears or axle gears?
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Old 10-07-2010, 05:50 AM   #7 (permalink)
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Ok does anyone know what the factory t-case high/low ratios are for the

Samurai? (leaf not coil)
SJ413?
Sj410?

and for that matter the diffs

The only thing I can find online is one listing for Samurai 1986
But im sure 1986 would be a SJ413 not samurai
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Old 10-07-2010, 06:54 AM   #8 (permalink)
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Hi Mof,

If I understand it correctly, you might not be able to get speed, crawl and torque in one setup if the change is just with the R&P (or else a bigger engine). I had to sacrifice speed by going with 5.12 diffs and 3.05 tcase ratios and ended up with a topspeed of 80kph (@4000rpm).

Might be able to get away with it with the tcase mods.

Just my opinion.
I've got 5.12 on my diffs pulled out of a Vitara, but my TCase is 6.5:1, high is 1.6:1 approximately. I also get 4000 RPM at 80 Km/h, which is quite annoying. Any faster than that, and the coolant temperature starts shooting up. I've got 33" tires. I'm thinking of putting back a stock Tcase to bring the ratio closer to stock.

Our terrain is mainly sand dunes, and in dune bashing you need to be able to have a broad range of speed in one single gear, and that I have lost with those conversions. Because once you attempt to change gears while climbing a sand hill at high speed, your momentum is lost.
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Old 10-07-2010, 07:45 AM   #9 (permalink)
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Quote:
Originally Posted by alternator View Post
I've got 5.12 on my diffs pulled out of a Vitara, but my TCase is 6.5:1, high is 1.6:1 approximately. I also get 4000 RPM at 80 Km/h, which is quite annoying. Any faster than that, and the coolant temperature starts shooting up. I've got 33" tires. I'm thinking of putting back a stock Tcase to bring the ratio closer to stock.
Hey hey!!! Same here, 5.12 from a Vitara (front R&Ps) and 3.05 Tcase from an LJ and on 32s.

Maybe we should form a club? [email protected]? hahahaha...
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Old 10-07-2010, 08:58 AM   #10 (permalink)
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Quote:
Originally Posted by Mof View Post
Thanks guys,
Im not sure why you say the front diff/drive shaft and not the rear/both..
Maybe im just not reading it right

at 100ks (60mph, i work in metric) im doing 3500rpm already so I dont want to go any lower, and im happy with the low box I dont need crawler gears.
sorry about that, typo. I did mean the front and rear. don't worry, it's not supposed to be a cryptic message...I'm just confusing.
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