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Simexs (well big tyres) and gearing


Hybrid_From_Hell
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With the suspension just about done, the next thing is to sort out the gearing.#

Its helps when you have 300 BHP on tap, but, the gearing is not right.

Please can some kind souls confirm the ratios for a LT85 Santana 5 Speed gearbox ??

1st : 3.6497

2nd : 2.1804

3rd : 1.4363

4th : 1.00

5th .7951

Tbox : 1.222 (Genuine V8 90)

So

I have a program written yonks ago that calcs speeds,

I have picked say 4000 RPM in 5th

on 205s (std factory V8) - 98 MPH

on 7.50s(fac option) - 106 MPH

as is now (34s) - 118 MPH

Yep, its over geared. :blink::huh::(

I have calculated.............

LT85 & 1.222 Plus say going to 4.11s 102 MPH

3.9s 107 MPH

For those who have been through this.

1,. Whats have you got GBox TBOX Diffs and Tyres ?....

2. If I am to buy a Pair of HD R&P who is best ?..Ashcroft top of my list most likely ? :)

3. Any other options re the above ? :)

Nige

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Why do you think it is overgeared? Is 5th too high to use? I doubt it with 300 hp. If you come to a hilll, shift to 4th and you get 5th for the flats and lower noise and better mileage. As long as 1st high is low enough to start under any road circumstances and you can use 5th on the flats, then leave it alone. I never understand why people think that if they have to shift to 4th on a hill they are overgeared.

A 4.1 r&p will be weaker than stock and that is not what you need.

If it were me, I would get the Ashcroft crawler gears. You would have a lovely choice of gears and definitely be low enough off road for any situation.

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Nige - have you got a power curve for your Eales? I can easily work out when the power of the engine is overtaken by the aerodynamic drag power, and give you an 'ideal' figure to work towards.

FWIW, I've got a Tdi, R380, 1.2 T-box and 235/85 TracEdges. I find the gearing quite pleasant, I have to change down to 4th for motorway hills etc but can beetle along quite happily. Moving to the 285/75 Grizzlys, 5th becomes more of a cruising gear and 3rd is more useful around town. I'd say its overgeared on these tyres.

On the 235/85s, 4500rpm in 3rd is 70mph, 4th is 99mph and 5th is 128mph. I've not done the above drag/power exercise for it, because the 1.2 box was to reduce noise on the motorway from a 2000rpm oilburner clattering away... B)

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Nige, welocme to the problem that's had my attention for quite a while now. I'm not up on LT85s but can you get differant ratio t.box centres for them?

Assuming you can't here's what I see as your options: R&Ps, low ratio transfer boxes or an underdrive.

The first is the only option that allows you to keep your current gearbox/transferbox setup. The major dissadvantage is strength, the second is supply. AFAIK, KAM does not make gears for Ashcroft although I may be wrong. My inclination would be to go no lower than 4.11s just for strength.

The second and third will mean you need to fit an LT230. That'll mean you need an R380 with TVR spec internals, oil cooler etc. It should take the power, though ([wisper]or you could go auto[wisper]). With No 2 you can get one of Maxi Drive's gearsets. They'll offer you a reduction of 30%, 40% or 70% in low range with any factory high range gearset. Ashcrofts also do their gearset although high will remain at 1.22. I've looked at both and decided not to go for them.

The last option is the one I chose. That way you keep the naturally strongest R&P setup, you have some legs in low range (useful on a speed section) and, if you choose high range carefully, you cangain a useful extra ratio. In my case I chose a 1.66 t.box to make driving on raod better and to give me a useful reduction on low range when high ratio and underdrive are engaged (about 35%).

One last thing to thin about, Nige. Don't underestimate the rolling resistance of Simex. I don't think my truck is any faster with a 1.66 tbox than with a 1.41. It should be, though, as it was massivly overgeared with the 1.41. The rolling resistance seems to kill it at about 75 (on private roads, of course).

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Buy em from Kam - he supplies the 4.11s to Ashcrofts anyway so will be far cheaper. Kam also do other ratios that Ashcrofts do not.

Use the spreadsheet and work it out. All the standard gear ratios are on it anyway.

Jon

Hi Jon,

you are right in that KAM make more ratios than us but we don't buy our 4.11's from them, we make our own,

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Red 90 maybe its not the road work overgearing that is the issue as much as too fast in low 1st for decents etc.

although only a 200tdi i am running 33" tyres with LT77 (def) and 1.4 T box plus underdrive and for challenge events is great for just another 3" etc to the punch.

H range 4/5th as per normal road conditions, pulls 5th most of the time eg all motorway etc at 60mph.

L Range excellent for normal off road and towing/hitching up etc

Underdrive: wheels turn at winch speed roughly so theres no sudden grip then over running the winch rope then it getting shock loaded. rock crawling is also superb.

I did contemplate the replacement crawler box option although you can only get 1.2 H range which i doubt mine would pull, plus theres no overlap L to H. also you change the T box so cant take it with you as easily if seeling on the veh with an aspiration to keep your low gears etc.

Underdrive - bloody expensive, should retain value, easily transferable between veh and gives a third option as ultra low or H + underive option. also retains std LR pinions etc which i was keen to do.

1st geard, mph per 1000rpm

High = 5.2,

High +U/D= in between 5.2 and 2.2 cant remember ,

Low= 2.2 ,

Low + U/Drive = 1.3 (3rd gear = std low 1st approx)

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Aye LT230, and also worth noting that genuine V8 90 with LT85 should have a 1.1923 ratio transfer box and not 1.222 (typically disco/RR) as mentioned in Nige's first post.

AHB^gger....have to redo all the numbers.....I'll dig out some tech stuff on the LT230 and check ratio, you may well be right....

Nige

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Nige,

I have recently changed my T/box to 1.410 from 1.222 as the higher ratio was way too tall. I came the this outstanding conclusion after a short road test of 36K !!!! :lol:

The engine is making about 200ponies, with a V8 LT77s / LT230T. With the taller gearing on 235/85 rubber, fast A roads are tiresome with continually changing from 5th - 4 th etc. However, the M/ways were reasonably OK provided I was not towing, but the long steady climbs would warrant a change down. Another issue was in slow moving town traffic etc.

After hours of playing with excel and, I reached the conclusion that I would like to retain the existing standard diff ratios, therefore the change to 1.410 was a good solution. Looking at the figures I could probably go to 34 inch tyres without sacrificing the gearing.

Rather than look at the 4000rpm figure I tend to work on mph /1000rpm as this the norm and easily related to other vehicles.

Currently I get 24.54mph/1000rpm which is fine (98.15Mph @4000rpm)

The 1.222 was 28.31mph/1000rpm ( you wont find many, if any, vehicles with that gearing that high).

This change has completely altered the drivability of the vehicle ……… I wish I had done it 2 ½ years ago when I did the transplant :rolleyes: , but being lazy I waited until I needed to replace the T/box due to severe oil leak. Now I rarely come out of 5th and this gearing seems to really suit the torque curve of the V8.

As a point of interest the change has made absolutely no difference to the MPG …. but I expected this, as with the higher gearing the engine was working that much harder.

Ian

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Ummm, Will, he has an LT230. Remember LT85......

Nice long post though :D

Sorry, I've been up since 4.30 so I'm a bit slow tonight.

Thinking about your setup again, my advice for you would be to go for a 1.4 ratio setup and then get a MD 30% low range reduction if you think its too high. If youy didn't have the PTO pump I'd say an underdrive would be the best option, though.

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Nigel

I had this issue with the 90 4.2 with the 4sp auto with 35'' simex

With the original diff's I had the auto with BW transfure box would not change from 3rd to 4th gear till it hit 85/90 mph and would be jumping up and down all the time through the gears and alway felt under powered.

I fitted the 4.75 Kam diffs and shafts

now the truck will be flat out and about 90-95 tops and she drives like a lively standard Range Rover LSE on the road which I think is perfict set up as far as I'm conserned.

Off road (Low) she is grate with loads of control and if your too heavy on the power she will light up all four tyres but if you gentle she is very easy to control and very smooth.

Julian

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Nigel

I had this issue with the 90 4.2 with the 4sp auto with 35'' simex

With the original diff's I had the auto with BW transfure box would not change from 3rd to 4th gear till it hit 85/90 mph and would be jumping up and down all the time through the gears and alway felt under powered.

I fitted the 4.75 Kam diffs and shafts

now the truck will be flat out and about 90-95 tops and she drives like a lively standard Range Rover LSE on the road which I think is perfict set up as far as I'm conserned.

Off road (Low) she is grate with loads of control and if your too heavy on the power she will light up all four tyres but if you gentle she is very easy to control and very smooth.

Julian

Hopefully sorted with an order to the ever helpful Dave at Ashcroft, will let you know how it goers !

Nige

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