15 replies to this topic

[Hewlett_T]

Note: Without bumping post, credit to guessworks engine decoder. It was correct about my original diff ratio, I'll include a photo. Clearly I have a mini gearbox, worm drive and Pinion. I'll likely replace the lot to keep the gear ratios original and the FD then use the metros box on the mini.

Thanks for the help Guessworks and Whistler.



Hey,

Additional Info:
Car is a 1985 Metro 1.0L so nothing special but a mini engine and box as you'd expect.

The speedo is wrong on the Metro and I can't work out why. (Oh and it is the original wheels 160/65/r315)

Speedo Readings (Actual Speed).
43(30)
70(50)
100(70)

Speedo is 1000TPM (I have swapped this as I had a spare and this wasn't the problem).

I've then worked out the diff is likely 3.44 I did this by turning the wheels until I had 10 precise turns of the crank. Then counting the wheel turns and the fraction divide by 2. As shown below.

Whole Turns: 5
Wheel Circumference: 64.75
Fractional Wheel Turn: 35.

Then did Wheel Turns + (Wheel Circumference / Fractional Wheel Turn) / 2 = 3.425 so basically 3.44.

This means the diff in the car is correct to my research, and I can't find any Pinion/Worm combination that would cause such as bad over-read. Stock is 17 Pinion and 5 worm I believe.

Therefore I am stumped. If you have any idea please let me know and I can look into them and hopefully fix this speedo.

Thanks.

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Edited by Hewlett_T, 08 December 2016 - 09:00 PM.

[whistler]

Try the engine number thingy on John Guess's site www.guess-works.com and put your engine number in. It's basically for Minis but try your engine number to confirm your FD and then use the Speedo TPM to help you with the speedo TPM. I used it for my mini with a different FD, wheels and tyres etc. It told me what TPM I need for accuracy so I just bought the appropriate speed to suit. Much easier than ripping out the speedo gears from the side of the gearbox.

[Hewlett_T]

Try the engine number thingy on John Guess's site www.guess-works.com and put your engine number in. It's basically for Minis but try your engine number to confirm your FD and then use the Speedo TPM to help you with the speedo TPM. I used it for my mini with a different FD, wheels and tyres etc. It told me what TPM I need for accuracy so I just bought the appropriate speed to suit. Much easier than ripping out the speedo gears from the side of the gearbox.

Hey,

Thanks for your response I used this and the details are below. I then used his speedo calculator and if the diff should be 3.65 (more likely 3.647 being a metro diff). If the Worm and Pinion weren't changed and the diff was it would result in a very minor over-read as a result of having 18 teeth instead of 17. However not as much as 20 MPH at times :/.

Thanks for your help though, I'm thinking I may have rip the box apart though as a aftermarket speedo would look out of place, the only downside being that the car is in constant use and it may result in not achieving anything which is why I'm trying to prod you all for knowledge.

Thanks again though :).

Source Vehicle : Metro
Engine type & CC : A+ 998 cc Transverse mounted
Fueling :
Engine features :
Lucas 16AC alternator with -ve Earth
1st Alternative compression ratio
1985 Requirements
Mechanical petrol pump
Gearbox features :
Std. Ratio Rod Change gearbox with Pot Joints
3.65:1 Final Drive

[whistler]

If you have used the speedo calculator then you know that if you vary the TPM setting until you get the accurate speedo reading then that'll tell you what TPM speedo to look out for. You'll find that Metros, like Minis, have different FD fitted in different models. Often a van version will have a lower FD than a car and it follows that the speedo could have a different TPM. Do some research on Metro speedos and the different types available.

At the moment I have a standard size Mini centre speedo with a 1376 TPM and I've recently bought a Austin 1300 100mm 1376 speedo because I want to move the instruments back over to the right side. Both are accurate as the TPM is the same. Going to bed now. My brain hurts. Lol

[Hewlett_T]

Thanks for your response, I did go through the worms and pinions on the site and because it's by RPM and not Miles shown. I couldn't use the information really due to not having a rev counter.

If anyone else should try this however I have came up with a solution. First workout the Diff the way I did if you don't know.

Second do this:

Diff Ratio: 3.44 (or your ratio).
Worm and Pinion: ?/? List below.
Speed; Speeds you know but actual speed not over-read or under-read this is the resulting number.

Then do this calculation on the list until you get a matching result:

Diff Ratio * Worm / Pinion * Speed = X

Examples: 3.44 * 7 / 17 * 30 = 42.49
3.44 * 7 / 17 * 50 = 70.82
3.44 * 7 / 17 * 70 = 99.15

Thus this confirms there's a problem and it's in my case with my worm drive, being far too large. Thus I have ordered a 5 tooth worm. I'm certain this is my problem.

List (Worms / Pinions)
7/15
7/16
7/17
7/18

6/16
6/17
6/18

5/16
5/17
5/18

Edited by Hewlett_T, 08 December 2016 - 04:21 PM.

[AndrewT]

I'm really struggling with the calculations here.

If the road wheels made 5 full turns plus 35 inches of a part turn (with a full turn being 64 inches) then isn't that a total of 5 plus fraction turn / full turn = 5 + 35/64 = 5.547 turns per 10 turns of the crank?

Then that makes 1.8 turns of the crank per 1 turn of the road wheels which makes a ratio of 1.8:1? But that's a really odd ratio.

And why do you divide by 2? Is that because the final drive ratio is based on firing cycles rather than engine revolutions?

I'm obviously missing something basic here.

[Hewlett_T]

Read me first: Upon writing this I feel I have gotten this wrong, I'll leave what I've written so far anyway however I have noticed I have not taken engine revolutions into consideration. Which would void my calculations and all the maths I did yesterday . Anyhow I believe the correct way would be:

 

Turns of Crank: 10

Turns of Wheel: 5

Circumference of Tyre: 64.75

Tyre Fractional Turn: 35

 

64.75 (Tyre Circumference) / 35 (Fractional Turn) = 1.85

5 (Turns of Wheel) + 1.85 (Fractional Turn) = 6.85

10 (Turns of Crank) / 6.85 (Turns of Wheel) * 2 = 2.919708029197080291970802919708

 

Essentially meaning it's a 2.91 Diff, I'm certain this new method is correct, and my old one was wrong as I was in such haste to get it done I never totally thought the maths through. This information can still be used to work out worm and pinion sizes though such as:

 

2.91 * 7 / 15 * 30 = 40.74

2.91 * 7 / 15 * 50 = 67.9

2.91 * 7 / 15 * 70 = 95.06

 

Obviously if you find an actual drive ratio your going to get closer to my over-read so using 50 only to minimise calculations:

 

2.95 * 7 / 15 * 50 = 68.83

3.105 * 7 / 15 * 50 = 72.45

 

Which is still within margin of error for my speedo, although I have only tried that pinion size and I'm sure I can find a closer match if I do my entire sheet again . As yes I did calculate every worm and pinion just to safe.

 

Obviously I now have multiple diff ratios to work with to find the closest one to my over-read, meaning double the calculations. You could minimise this for example by doing 10 - 15 or even 30 turns of the crank, to get a more definitive answer but I absolutely refuse to jack the car up again as the damn thing is too low for a jack (suspension issues).

 

Thanks to AndrewT however as without you posting that I wouldn't have 2nd thought it.

 

Below in quotes (what I was writing when I noticed my mistakes)

 

 

 

Hey,

 

Firstly sorry I never replied earlier I read it but never replied as my phone died.

​Anyhow, the reason the calculations aren't working for you is because you've done them backwards, and the calculation is basically saying how many times can I get this number into this number. So 35/64.75 would be 0.54 or the inverse calculation of what we wanted as we want to do the whole wheel divided by the fraction which is 1.85. This is a easy mistake to make as multiplication can be done in any order but division can't.

 

Therefore you need to do Wheel Circumference (64.75) / Wheel Fraction (35) + 5 = 6.85 then divide by 2 = 3.425 (I will explain).

 

The reason you divide by 2 is because when the car is moving under its own momentum both of the wheels are turning, when doing this test you would want to have just one wheel in the air and therefore one turn of that wheel, is only HALF a turn of both wheels. Therefore 6.85 of one wheel is 10 turns of my crank, or 3.425 turns of both of my wheels is 10 turns of my crank.

 

I should also mention that the car MUST be in 4th doing this test, as this means the main shaft of the gearbox and the crank on the engine are then turning at the same rate due to this being the 1:1 ratio in standard 4 syncro Box. If you have less or more gears you'd need to find out which is the gear with the 1:1 ratio.

 

Also note that 3.425 is not accurate to a mini drive ratio, this is likely due to slight inaccuracies in the test so your result is never going to be perfect by with all the diff ratios of standard and aftermarket diffs known you can easily look through then (especially on guesswork's site) and work out which is closest to your result. I went with 3.44 as its only 0.015 off.

 

I also recommend taking the plugs out to do this as then your not having to compress the engine (likely easier by rocking the wheel) which may skew your result due to having to move the wheel with force to compress it. I also recommend a bright chalk on the crank as this means not having to find TDC or BDC and you can just use the bright chalk mark. Make sure on the last rotation of the crank you align it PERFECTLY with the chalk mark you've made, and not run past it as this can over or undershoot your result.

 

In order to get the circumference I use a bendy tape measure or a ribbon which you can then measure to get the result.

 

In order to get the wheel fraction I recommend going past your first chalk mark and then one your dead on the crank at 10 rotations make a 2nd chalk mark and you measure from the 2nd mark to the first!. You also measure the way the wheel turned and not the part of the wheel that hasn't passed yet. (I'll put an image).

 

I'll also mention that 10 might not be your number if your looking at close diff ratios like 3.1 or 3.2 you might have a bit of trouble working this out unless you do more turns of the wheel.

Edited by Hewlett_T, 10 December 2016 - 08:59 PM.

[AndrewT]

Thanks for the reply.

I understand the dividing by 2 bit now however I still think you have the fractional bit wrong.

You need to do 35 / 64.75 = 0.54. That makes 5.54 total turns which if you then divide by 2 gives 2.77.
Then divide 10 engine turns by that to get your ratio gives 3.61:1 which is close enough to the metro size of 3.64 that you first thought it was to be due to a small measurement difference on your fractional turn. (32 rather than 35 would give exactly 3.64)

The other way of looking at it is that the way you have done it your fractional turn works out as more than a single turn which can't be right.

Hope this helps you find the speedo drive bits that you need.

Edit: A quick back calc on the worm and pinions listed on the guessworks shows that 7/18 is almost bang on for your over-reads. 5/18 will give you what you need.

Edited by AndrewT, 09 December 2016 - 10:27 PM.

[panky]

I'm sure my speedo must be wrong, it's got to be as I've just got a speeding ticket - in a Morris Minor

[AndrewT]

I'm sure my speedo must be wrong, it's got to be as I've just got a speeding ticket - in a Morris Minor

If it's a Morris Minor then we can safely assume that the speed camera is the thing that is wrong.

[panky]

Do you think that will stand up in court, I'll take the naughty boy course I think

The minor has been 'warmed' a bit, I was doing 35

[Hewlett_T]

Thanks for the reply.

I understand the dividing by 2 bit now however I still think you have the fractional bit wrong.

You need to do 35 / 64.75 = 0.54. That makes 5.54 total turns which if you then divide by 2 gives 2.77.
Then divide 10 engine turns by that to get your ratio gives 3.61:1 which is close enough to the metro size of 3.64 that you first thought it was to be due to a small measurement difference on your fractional turn. (32 rather than 35 would give exactly 3.64)

The other way of looking at it is that the way you have done it your fractional turn works out as more than a single turn which can't be right.

Hope this helps you find the speedo drive bits that you need.

Edit: A quick back calc on the worm and pinions listed on the guessworks shows that 7/18 is almost bang on for your over-reads. 5/18 will give you what you need.

 

I think your right, I've become confused anyway . Either way I need to crack the box open to change the worm so no worries . I'll mark your answer as right anyway as it looks right . Thank you xD.

[Hewlett_T]

Do you think that will stand up in court, I'll take the naughty boy course I think

The minor has been 'warmed' a bit, I was doing 35

 

I'm sure my speedo must be wrong, it's got to be as I've just got a speeding ticket - in a Morris Minor

 

 

Naughty Boy Course , send the car with a note xD. Speedo's I've learned are a huge pain in the arse . When they work they work, when they don't even by a little they're an arse.

[panky]

I hold my hands up. I fitted the correct speedo to match the 3.7 Midget diff I fitted (along withe the 12G940 head, Minisport inlet and Midget exhaust manifold)  and it's pretty close (according to the local 'slow down you boy racer type' speed indicator) so I've only got myself to blame  It does sound nice when it hits 3000 rpm, and beyond, in second when you come off  a round a bout........and there's a flippin mobile camera there waiting to photo-bomb you........so like I said I'll take the course

Sorry for the hi-jack, please forgive me.

[gazza82]

Tell them you didn't realise there was a tail wind!