October 01, 1987

Straight Torque - Gearing and Tyres

OK trendsetters. So we've looked at Power and Torque, and we've spun it up on the dyno, and we've had a bit of a rave about what it all means. But hang on, all that P & T (let's just call it energy) is still up in the driveline somewhere. It doesn't do any good till it gets out onto the road, and that happens through the wheels.

You may have heard people rambling on about gear ratios. Very briefly, when the energy comes out of the motor it has to pass through several gear ratios: the gearbox, the transfer box, the diff centres and the tyres. (yes, that's right, the tyres!) Have a look at these figures for the 1300 Sierra. (Other models, just check the back of your workshop manual somewhere)
  • Lo range 2.268
  • Hi range 1.409
  • Diff Centres 3.909
  • 1st 3.650 
  • 2nd 1.946 
  • 3rd 1.422 
  • 4th 1.000 
  • 5th 0.793 
Now it's very simple. If you want to know the final gear ratio for an give gear combination, you simply multiply the three appropriate figures together.

For example, take a look at your main gear for 4WDing which is low range first gear (usually just called low-low). It would be 3.650 (first gearbox gear) X 2.268 (low transfer gear) X 3.909 (diff centre) = 32.827. Don't take my word for it… check it yourself.

The same goes for any other combination of gears … gearbox X transfer X diff = gear ratio. OK, so you might be thinking this is a really useless piece of information, and you might be right. Until you start comparing it with a few other 4WDs and asking a few question. We'll stick with low-low for a moment. Look at this…
  • Audi Quattro 14.000
  • Subaru Sportswagon 21.996
  • Suzuki Sierra 32.827
  • Mitsubishi Pajero SWB 35.440
  • Landcruiser Sahara 39.023
  • Toyota Hilux 43.583
  • Landrover 110 Diesel 47.837
  • Mercedes Benz 300GD 48.634
Notice anything? The really capable 4WDs all have one thing in common - a really low low-range gear ratio. They are all around the 40 to 50 range. By the way, gear ratios work back to front, the bigger the number, the lower the gear. Conversely, vehicles that are not generally regarded as "serious" 4WDs like the Audi Quattro and the Subaru, have quite high gearing (lower numbers).

That should immediately tell you something. Having a decent low range gearing is a very important characteristic if you intend to use your vehicle for "serious" offroad driving. So where does the Sierra fit in then?

At 32.8:1, it is the worst geared of the whole bunch if you don't count the Audi and the Subie, which are really just passenger car derivatives and not real 4WDs. Now, generally speaking, most Suzuki owners can get away with the gearing as it is because of their applications for the vehicle. Let's face it, the number of people you will find that use their Suzukis to go to Wirraba Ridge or Heartbreak Hill are in a definite minority. And because the vehicles are so light, and the motors are really quite efficient for their size, even the diehards can get away with the standard gearing in most situations.

But here is a problem to think about. The Sierra comes standard off the showroom floor wearing LR78/15 tyres. When Joe Public picks up his new Suzy and starts to think about modifications, he generally wants to start with different tyres (you know, rooly big ones!), and so he heads down to the local tyre dealer (who spends most of his time working with passenger sedans, so often doesn't think about this stuff), and gets a set of 10Rs fitted. They look great, but when the guy drives it out on the road, he suddenly wonders why it doesn't go fast anymore. It gets along ok on a flat road, but then dies in the arse at the first sign of a hill.

What this unsuspecting chap has actually done is to raise all his gear ratios. The original tyres were about 28 inches tall; his new ones are about 31 inches tall … that's about a 10% increase in all of his gear ratios! The poor little motor was never designed to pull that around… it simply doesn't develop the amount of torque needed to keep the wheels turning. Even if it did, when you went off road, you would have to do everything at far too great a speed just so the motor wouldn't stall. It would be like driving everywhere in high range.

The final result of all this is that it is very unwise to go to a much bigger tyre (in diameter) unless you are prepared to do some serious modifications to the rest of the drivetrain to bring the final gear ratio back to where you started from (or lower!) If you're not prepared to do these mods, and you still want your car to perform well, then don't use any tyre bigger than a 215/75-15, or about 28.5 inches in diameter.

On the other hand, if you really don't care how much of a dog it is to drive, and you don't mind using second gear to climb hills, and the farthest offroad you get is your front lawn, but you don't care because at least it looks mean, then go for it. Try Unimog wheels.

Chris Betcher
October 1987

September 01, 1987

Straight Torque - Running the Numbers

Righto. Heads still reeling from last months little discussion? Good! This month we will take a look at these figures in practise and see how all this waffle actually gets to the wheels.

As promised, CB-2801 revved itself silly on ARB's dynamometer last week and spewed forth a collection of power and torque numbers. Before we look at them, pick up a copy of Suzuki's 1300 Sierra brochure, flip over to the back page and look up the figures for maximum power and maximum torque. You'll find that maximum power is 47 kw (kilowatts, remember them?) at 6000 rpm, and maximum torque is 100 Nm (Newton-metres) developed at 3500 rpm.

Now when I tell you that the Mobile Fueltank developed only 40 kw at 6000 rpm, you might think "What a wreck! It's 7 kw below standard specs!" But it also developed maximum torque of 1500 Nm at 3000 rpm. That's 15 times the standard spec!

Whoa! What's going on here? Down on power, up on torque? Well, the first point to note here is that the output figures quoted in most manufacturer's brochures are measured at the flywheel, not the rear wheels. Remember, that power is "the engine's ability to do work". And turning all those gears, shafts, wheels, tyres, etc, is hard work! So by the time it gets to the back wheels it has lost a bit of its oomph. Around 30% of its oomph, in fact.

And the torque figures? Well, you see, a gear box goes by another name… engineers refer to a gearbox as a "torque multiplier". And simply stated, the torque that goes in one end gets multiplied so there is more of it at the other end. By the time you put in 100 Nm, when it comes out the other end, the gear reduction ratio is responsible for my ensuing 1.5 kNm, or 1500 Nm.

So, you see, Betcher's Beast is not really that breathless after all; in fact it is quite respectable… 47 kw minus 30% drive train loss is about 33 kw. Mine had nearly 40 kw. You will also notice that the Cockroach developed maximum torque at 3000 rpm, 500 revs lower than standard spec. All this can be attributed to, I suppose, the extractors, sports exhaust, Finer Filter, Nulon oil additives, good HT leads and new plugs. All these things add up and they do seem to make a difference.

Now, while we are on the subject of gear ratios, you have all heard the common old sierra whinge "the gearing is too high". What does that mean exactly? We all want to do our hard 4WDing at the optimum speed - very slowly! But when you go down to a nice slow crawl, the motor slows down as well. When the motor is just ticking over, say 800 to 1000 rpm, it is developing bugger all torque, or turning force, and of course it takes very little to make it stall. The motor simply cannot keep turning the load being placed upon it, and so it gives up and stalls.

What you really want is a system whereby the motor is able to get up higher into the torque band, say at least 2000 rpm, preferably higher, but to still allow the car to crawl along at a very slow pace. This is what's meant by having lower gearing. 1.3 Sierras are geared low-low at 33:1. The early 2-stroke Suzukis were geared low-low at around 56:1 … the needed to be geared like that because 2-stroke motors only really develop torque if they are spinning quite fast. But that gearing is about 40% lower than a Sierra. Disgusting isn't it?

You can work out the end ratios by looking at the Suzuki brochure again, finding the gear ratios for Transfer Box, Gearbox and Diff Centre (sometimes called the Final Drive) and multiplying the three together. For the 2-stroke, it's 3.012 x 3.835 x 4.875 = 56.324:1.

Why? Because in the modern day rush to get from place to place quicker, Suzuki (and indeed nearly every other 4WD manufacturer) has made a compromise between a very low first gear and a reasonably tall fifth cruising gear. It is simply easier to build it that way because the transfer case can be made a reasonable size… having a big difference between the high and low range means a big difference in the actual cog sizes… hard to fit in a compact case.

In the 1300s, you can drop a 1 litre transfer box into it, but it will lower your high range a little too. You can pop in some lower ratio diff centres, but again, down come all your ratios in every gear. Maxi Drive in Queensland were talking of designing an interconnecting gearbox to fit between the back of the gearbox and the front of the transfer case, which would be a third box with 1:1 and then something lower again. So, when low-low wasn't enough, engage the interbox and you have low-low-low, if you get what I mean. Sounds like a great idea, but when will it surface? Don't hold your breath waiting - the total percentage of zooks that would even consider the use of such a device is pretty minimal, so it's no surprise it isn't on the 'things to do' list of most manufacturers. Wouldn't it be good if the club could design and manufacture one? Something to think about anyway!

Chris Betcher
September 1987

PS - Since this article was written back in 87, there are a couple of solutions to the gearing problem. The best one seems to the Rockhopper, a set of replacement transfer case gears which reduces the final drive by about 83%!

May 22, 1987

Poor Rod's Tent

Through the air the frisbee went.
And in the catch, the catcher leant
Upon Rod’s tent…
And down it went!
I’m sure the tent was never meant
To take such stress, and so it bent
Poor Rod’s tent…
Down it went.
Into the fire the tent pole went.
To heat it up and fix the bent
Poor bent tent.
That never meant!
Then Andrew’s car in circles sent,
Went silly, for the tent it went.
Poor Rod’s tent …
Again it went.

By now the tent was looking bent.
You’d think that someone would invent
Poles for tent
Too strong to dent!

So, "sorry" Rod, about the tent
To bent the tent we never meant.
Poor Rod’s tent.
‘Tent for Rent’.

Chris Betcher
Volume 12. No. 5
May 1987