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TOPIC: Checking tyre sizes and diff ratios

Checking tyre sizes and diff ratios 07 Jun 2020 14:39 #211200

  • Dave_64
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Hi All. the "Pedantic Anorak" with more questions to annoy you!
Been done to death here as well as anywhere, working out Tyre diameters and Rear axle ratios.
So, how to check against published figure for the tyres we'll deal with first.
Depending on whom you listen to, or whose chart you look up, a 7.50 X 16 tyre SHOULD come out at between 31-31.6" depending on variances like who manufactured it.
Not content with that, (being a P.A.) I drew a chalk line on a new tyre (fitted on a rim and inflated), another chalk line on the garage floor, wheeled the tyre forward as straight as I could, when said tyre had done ONE complete revolution, laid down another chalk mark. Measured between the two came up with a figure of 95.75 inches, so far so good. Remembering a little basic geometry from skool, I then divided that by Pi (22/7) and came up with 30.5" (rounded off), so already lost a bit over manufacturers sizes, but not too worried yet.
Next measured from the floor of the garage up to a straight edge laid across the flat of the tyre, comes out at 31.0". Again, losing a little from published numbers.
So took the easy way out and split the difference between the two measurement I have as, 30.75" diameter, so thats what I will use.
Anyone kick any holes in my reasoning so far?

Part two, establishing the correct diff ratio when you have NO tags, only published figures and don't want to pull the diff centre out to either look up the numbers on the Crownwheel, or at worst, have to count both CW and pinion teeth. Being a Banjo type as opposed to a Salisbury type. If it was the latter I'd simply drop the backing plate off.

So, the good book (Google) says to jack up one side of the vehicle, mark the tyre off the ground with chalk, mark the floor of the garage where the two marks would meet, wind the wheel/tyre over TWICE counting how many times the pinion flange turns from another chalk mark on the flange extended to the housing. Again so far so good. Will only give you an approximate count. They suggest to do this exercise BUT rotating the wheel TWENTY times to give you a more accurate ratio count.
The way they say is if you rotate the wheel TWICE as recommended, count the rotations of the pinion flange (we will use only as an example that the pinion has turned FIVE full turns plus ended up in the four o'clock position that the ratio is close to 5 +1/3rd (or 5.33:1). Again by their reckoning if you were to rotate the wheel (or have somebody do it for you), and you can count the rotations of the pinion without losing count, you find that it (pinion) has turned FIFTY TWO FULL turns plus almost completed ANOTHER full turn, ending up between the ten and eleven o'clock position, so you say that you are close enough to either 5.28 OR 5.29:1 ratio.
Cross reference it against the drivers hanbook you find that the published ratios for that vehicle are say 4.88:1-5.28:1-6.33:1 so you now have an accurate ratio count without ripping the diff centre out. If you can't lay your hands on the PUBLISHED ratio figures, you may be in strife, but the 5.28/5.29:1 is a well known common ratio.
Again, anyone punch holes in my logic??
So, now we know the wheel diameter, the axle ratio, the "sweet spot" of the engine RPM (we'll say 2500RPM) put them into the Tremec app with a 1.00:1 transmission ratio and get 43MPH or close enough to 7okph.
How am I going so far?

You "Roads" scholars can possibly fine tune this for me. Purpose of the exercise was trying to find out what the top roadspeed of the ORIGINAL vehicle was with 6.00X13" wheels (measured the same way comes out 27.25"diameter).
The only published diff ratios I can find are in the drivers handbook, which are either 6.57:1 OR 4.875:1. With a 1:1 top gear ratio, I came up with 31MPH OR 42MPH using the above formula. (I favour the 6.57:1 ratio as the few people I spoke to who knew these particular vehicles, commented they were as slow as a wet week, but being so low geared, they could carry quite a load for their size).
Just to add to the confusion, there is NO guarantee that the drivers handbook is the correct one for that particular vehicle. It is a Pommy handbook and I have an Oz assembled vehicle. To the best of my knowledge, the vehicle ONLY came out in England with either 16 or 20 inch wheels. So, two things may have happened, either the Oz version had different size wheels/rims sourced from a local manufacturer, obviously with an extremely low loading height and what it was intended for, required no real high speeds.
Or the two published axle ratios were more to do with the 3 engines optioned, a 1.6 BMC diesel, a 4.203 diesel (Perkins) or the trusty 2.6 4 banger petrol engine.
I haven't got around to accurately count the wheel/pinion turns yet, chassis buried under "stock" inside garage, just wanted to make sure I was going about this the right way.
Cheers, Dave

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Checking tyre sizes and diff ratios 07 Jun 2020 15:53 #211203

  • Blackduck59
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Depending on the load on the vehicle you could take another 1"off the diameter. The accurate measurement is rolling radius, that is measure from the ground to the centre of the axle.
Cannot comment on the axle ratio working as I have had no experience doing it that way.
Cheers Steve
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Checking tyre sizes and diff ratios 07 Jun 2020 16:48 #211207

  • Zuffen
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I'm with Blackduck.

An unladen wheel and tyre combo will give a different rolling circumference than a loaded wheel and tyre.
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Checking tyre sizes and diff ratios 07 Jun 2020 18:39 #211211

  • Dave_64
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Thanks, adjusted my figures here at home, wasn't a huge difference in it.
Now, any takers on the method of determining the diff ratio as per the opening post??
Thanks, Dave

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Checking tyre sizes and diff ratios 07 Jun 2020 18:50 #211212

  • PaulFH
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Dave, your diff ratio calculations will be fine if revolutions are counted carefully.
Rolling radius is theoretically correct, but with modern steel belted tyres the full
circumference of the tread belt will be sent around each revolution.
As you say, small difference. Happy figuring. Paul.
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Checking tyre sizes and diff ratios 07 Jun 2020 21:10 #211215

  • Mrsmackpaul
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Dave the best way I know to check diff ratio is to tape a bit of string to the tail shaft and that way you dont need to watch it as it turns around
Just count the turns

Now if you wanna get precise I reckon using some basic school boy math you could accurately measure the diameter of the tail shaft
Then accurately mark the string at the start and finish and using that basic math work the circumference out and divide it into the string length

Its gunna be pretty spot on

Paul
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Checking tyre sizes and diff ratios 07 Jun 2020 21:23 #211216

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Now thats one trick I hadn't heard, Mrs! I" ll give that one a try when I get all the stuff off the chassis, damn side easier counting the amount of turns when your working by yourself!

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Checking tyre sizes and diff ratios 08 Jun 2020 08:07 #211225

  • wee-allis
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I don't get the bit about a loaded tyre having a different "rolling circumference" than an unloaded one. I know the radius changes, but the circumference?

If I get, say. a hose clamp, measure the circumference of it, then flatten one side, it is still the same distance around. What am I missing? Not doubting you blokes, just needing to be educated.
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Checking tyre sizes and diff ratios 08 Jun 2020 08:50 #211227

  • Zuffen
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You can't change the rolling radius without changing the rolling circumference.

If you could the tread would have to slowly move around the sidewall.

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Checking tyre sizes and diff ratios 08 Jun 2020 09:12 #211228

  • Swishy
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Me 2 cents worth sez:
identical size tyres can differ
due to Make, Tread, Pressure, Load



more speed can be obtained by increase n the loaded radius if all other variables are the same in comparisson

WotSezU?

cya
OF ALL THE THINGS EYE MISS ................. EYE MISS MY MIND THE MOST

There's more WORTH in KENWORTH
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