Thread: How well does your Isuzu tow??

Ok. I stand corrected. I only know what I know and don't mean any harm. There are a lot of people around with limited knowledge of the bits that go up and down and round and round so it was more of a general rule to prevent g'box abuse. Are the people chasing the MS/XA isuzu boxes simply too abusive on the LT85 to make it a viable option for an overdrive? It would be a hell of a lot easier to fit an LT85 instead of LT95 for a bit beter highway RPM. It is great to hear from you Isuzurover about your box and the work it has done. I was under the impression of the folks who had struggled when trying to keep them together and when they went on to testify for the failure had been towing in 5th gear which is generally a no no for most g'goxes in most vehicles. If they were un aware of how to look after the box than no amount of oil was goint to save them. The direct 1:1 of 4th gear is strongest for towing because the box doesn't have to do the work.

It all seems to be a case by case type of thing with the owner partly to blame for the failure. Driving style seems to be the key.

Just my 2 cents,

Brian

Brian,

The issue is not so much towing as much as how much load is on the transmission.

You simply shouldn't make blanket statements like "don't tow in 5th gear,

With no load other than an small box trailer, or light trailer boat, like I have, cruising on relatively flat roads puts less load on the transmission than when loaded with camping gear, water etc. climbing hills. Edit in red: in the first example (towing) I would use 5th, in the 2nd (not towing) I would use direct or even a lower gear. Here again no blanket statement like "don't use 5th when driving up hill", it depends upon the load on the transmission.

The LT85 has it's weaknesses, few will deny that. And the Isuzu version of the LT85 are not that easy to come by, and these and other considerations should be weighed up in the choice between an LT95, LT85 or Isuzu gearbox.

I'll add mine bit too - I'd say that for as long as I can remember it's been generally accepted that the 5 speed LT85 is not as tough as the good old LT95. My gearbox had 1&1/2 rebuilds before I bought it, and I have done it and the transfer since.
The only really tired bit in the gearbox that I found was the input shaft bearing. What let it down was the rear mainshaft bearing had been spinning in the housing and was eating it's way into the retaining plate, so the mainshaft was able to slop forward and backwards. Everything else seemed good. Mine had a modified oil feed to the mainshaft splines so they were ok.
I still do however take on the bigger hills when towing or not in 4th gear as to me it just makes sense to have the box going through direct drive when it's under more load. Thanks for the comments guys.
Adam.

I don't understand this "more load in 5th" talk. The engine produces X amount of torque. It doesn't matter what gear your in it still only produces X torque, changing gears doesn't change this! So if your saying that the X torque in 5th will blow up your gearbox, you had better not load up your truck and drive up a steep hill in 2nd or 3rd either because you will still reach max torque.... no?

I don't have a isuzu or a lt85/95 etc. I have had a ranger and used a patrol for towing in 5th with no ill affect. I also tow in 6th in my 2.2 puma. I think towing in 4th would just be wasting fuel, unless of course your going up a hill and it just wont pull up it.

Originally Posted by debruiser

I don't understand this "more load in 5th" talk. The engine produces X amount of torque. It doesn't matter what gear your in it still only produces X torque, changing gears doesn't change this! So if your saying that the X torque in 5th will blow up your gearbox, you had better not load up your truck and drive up a steep hill in 2nd or 3rd either because you will still reach max torque.... no?

...

Engine torque is multiplied by the gear ratios. So in 5th you have less torque at the wheels than in any other gear.

Originally Posted by debruiser

I don't understand this "more load in 5th" talk. So if your saying that the X torque in 5th will blow up your gearbox, you had better not load up your truck and drive up a steep hill in 2nd or 3rd either because you will still reach max torque.... no?

I don't have a isuzu or a lt85/95 etc. I have had a ranger and used a patrol for towing in 5th with no ill affect. I also tow in 6th in my 2.2 puma. I think towing in 4th would just be wasting fuel, unless of course your going up a hill and it just wont pull up it.

I think you have misinterpreted what was said.

I don't recall anyone saying that. I certainly didn't.

Also you don't seem to understand what is going on with engine torque and gear reductions, load, etc. when you make incorrect statements like:

The engine produces X amount of torque. It doesn't matter what gear your in it still only produces X torque, changing gears doesn't change this!

There is a maximum/peak torque value that the engine is capable of producing, which changes across the usable rpm band.

The engine doesn't produce this amount of torque normally, it responds to load.

There will be a particular amount of torque required at the wheels, to overcome the resistances to motion, and inertia during acceleration/deceleration.

Wheel torque = resistance x radius of wheel

Now follow what happens to this torque at the wheels, along the drive train to the engine.

The crown wheel and pinion reduction in the differentials reduce the torque in the drive/tail shafts to:
driveshaft torque = wheel torque x ratio / mechanical efficiency of rear axle and diff (if crown wheel to pinion = 3.54:1 then ratio = 1/3.54 = 0.2825).

Next along the drive train is the transfer case reduction, so the torque at the gearbox mainshaft is:

mainshaft torque = driveshaft torque x transfer ratio / mechanical efficiency of transfer case

Then we come to the pinion shaft in the gearbox:

pinion shaft torque = mainshaft torque x gear ratio / mechanical efficiency of gearbox

Now the pinion shaft torque is the load on the engine and it will produce that amount of torque, if it is capable. If it isn't capable of producing that much torque it will slow down until it does (this is called torque back up, i.e. engines are designed so the torque curve shape provides more torque as rpm drop). If the engine is still not capable of that torque then the driver selects a lower gear (or the engine stalls).

It should be very clear that selecting a lower gear will reduce the torque that the engine produces. Put another way, for any given load, selecting a lower gear reduces the torque at the pinion shaft.

In lower gears you rarely pull maximum engine torque for long as the vehicle will accellerate. You can see this in your EGTs when offroad - they stay pretty low. By comparison you can be near maximum toque for hours on end in a high gear on the highway.

In a 1:1 gear in a normal automotive gearbox there is little radial load on bearings as there is no load transmitted through layshafts etc. By comparison, in an overdrive gear you're definitely able to put full torque through the 5th gear and shafts etc and they often arent built to withstand it.

Thats my take on it anyway

Steve

Originally Posted by Bush65

There is a maximum/peak torque value that the engine is capable of producing, which changes across the usable rpm band.

The engine doesn't produce this amount of torque normally, it responds to load.

There will be a particular amount of torque required at the wheels, to overcome the resistances to motion, and inertia during acceleration/deceleration.

Now the pinion shaft torque is the load on the engine and it will produce that amount of torque, if it is capable. If it isn't capable of producing that much torque it will slow down until it does (this is called torque back up, i.e. engines are designed so the torque curve shape provides more torque as rpm drop). If the engine is still not capable of that torque then the driver selects a lower gear (or the engine stalls).

It should be very clear that selecting a lower gear will reduce the torque that the engine produces. Put another way, for any given load, selecting a lower gear reduces the torque at the pinion shaft.

I understand what you have said. So what you are saying is that if you are towing something heavy, then drive slower? Isnt' that common sense? OR get a stronger gearbox?