Require some technical advice/ help please...
Discussion
Require some technical advice/ help please...
I recently had an Automatic RX8 fail the SVA partly because of the gearing ratio.
Gear Ratios (MPH/2000 RPM) for each gear according to the model report should be;
1st - 11mph
2nd - 21mph
3rd - 31mph
4th – 50 mph
Now I forgot to ask at the time what the actual speeds where and whether they were read off the Speedo or checked with some special device (in case Speedo is off), which I’ll do on Monday… but I was told they’re out side the +/- 8% allowance.
The model report shows tyre sizes to be 225 / 55 R16 but my car has 18 inch wheels with 225/ 46 R18 tyres. This is ok for the model report as the rolling radius is with in the +/- 8% allowance for the tyres.
But could this be what is affecting the gearing ratios?
I recently had an Automatic RX8 fail the SVA partly because of the gearing ratio.
Gear Ratios (MPH/2000 RPM) for each gear according to the model report should be;
1st - 11mph
2nd - 21mph
3rd - 31mph
4th – 50 mph
Now I forgot to ask at the time what the actual speeds where and whether they were read off the Speedo or checked with some special device (in case Speedo is off), which I’ll do on Monday… but I was told they’re out side the +/- 8% allowance.
The model report shows tyre sizes to be 225 / 55 R16 but my car has 18 inch wheels with 225/ 46 R18 tyres. This is ok for the model report as the rolling radius is with in the +/- 8% allowance for the tyres.
But could this be what is affecting the gearing ratios?
Aftab said:
I was told they’re out side the +/- 8% allowance.
I think you need to get more information about what the problem is before you can sort it out. Is the speedo over-reading or under-reading, and how much by? What is the +/- 8% that you refer to? I thought the speedo wasn't allowed to under-read so specifying a +/- error tolerance doesn't make sense to me.The problem could well be that you've fitted oversize tyres on the driven wheels and effectively raised the gearing which would make the speedo under-read. But without a better understanding of the problem it'll be hard to be sure.
GreenV8S, as you might know, at the SVA (single vehicle approval scheme for imported cars) the car is checked against a model report which gives core technical information regarding the car when it was first manufactured.
There is usually a slight allowance because not every car will be identical. In this case at 2000 RPM, in first gear for example the speed of the car should be 11mph. The way I understand it (and I might be wrong), this will very rarely be the case so there is an allowance of 8% either way. So it can be 11.88mph(?) or 10.12 mph(?)
They put the car on rollers and check this electronically I believe. What I don’t know is if the reading was too high or two low. But what I’m trying to find out is if the size of tyres makes this difference or do I have a bigger problem? I know the test is very stringent and that they would not allow for 9% variance.
According to my calculations off here... http://www.gearboxman.co.uk/download/gearspeeds.v2...
My current 225/ 45 R18 tyres have a rolling radius of 6.80 feet. However with the original wheel and tyres set up (as per model report) of 225/ 55 R16 the rolling radius is 6.74 feet.
The tyres on their own are fine as they meet the 8% leeway on them. (So can be max rolling radius of 7.28 or min 6.20 feet.)
But CAN this make the difference (obviously depending on the how far out the gearing ratios were - if it was massive variance than it cant be the tyres?)
There is usually a slight allowance because not every car will be identical. In this case at 2000 RPM, in first gear for example the speed of the car should be 11mph. The way I understand it (and I might be wrong), this will very rarely be the case so there is an allowance of 8% either way. So it can be 11.88mph(?) or 10.12 mph(?)
They put the car on rollers and check this electronically I believe. What I don’t know is if the reading was too high or two low. But what I’m trying to find out is if the size of tyres makes this difference or do I have a bigger problem? I know the test is very stringent and that they would not allow for 9% variance.
According to my calculations off here... http://www.gearboxman.co.uk/download/gearspeeds.v2...
My current 225/ 45 R18 tyres have a rolling radius of 6.80 feet. However with the original wheel and tyres set up (as per model report) of 225/ 55 R16 the rolling radius is 6.74 feet.
The tyres on their own are fine as they meet the 8% leeway on them. (So can be max rolling radius of 7.28 or min 6.20 feet.)
But CAN this make the difference (obviously depending on the how far out the gearing ratios were - if it was massive variance than it cant be the tyres?)
Edited by Aftab on Sunday 22 July 13:33
Aftab said:
In this case at 2000 RPM, in first gear for example the speed of the car should be 11mph. The way I understand it (and I might be wrong),
On a manual this should be a consistent and easy to measure value, but on an auto surely the actual gear ratio can vary significantly due to the torque converter?It seems strange that they bother confirming that the gear ratios match the specification, but if you say that's what they're checking then I'm sure you're right. The overall gearing is determined by the product of the gearbox ratio, diff ratio and tyre rolling circumference. If you've fitted bigger tyres this will raise the gearing but the change you've made would only make a tiny difference, nothing like the 8% margin you say you're allowed. Perhaps you have a different diff ratio? These things are easily checked.
GreenV8S said:
It seems strange that they bother confirming that the gear ratios match the specification, but if you say that's what they're checking then I'm sure you're right. The overall gearing is determined by the product of the gearbox ratio, diff ratio and tyre rolling circumference. If you've fitted bigger tyres this will raise the gearing but the change you've made would only make a tiny difference, nothing like the 8% margin you say you're allowed. Perhaps you have a different diff ratio? These things are easily checked.
I spoke to SVA UK who also own the model report for the RX8. It’s likely to be the tyres they say. But as you suggest the difference in rolling radius is tiny and nothing like the 8% they allow for(?) I’ve also spoken to the SVA test centre where one of the guys told me the calculation is done manually. So they get a machine reading of the MPH at 2000rpm and then calculate the difference.
I’ve asked for the readings so will post them here to make sure the tester hasn’t got them wrong.
Even more confused now, according to the SVA guys the measurement is taken from their rollers at 2000RPM this is the result...
Model Report/ Reading/ Difference
11/ 14/ 21%
21/ 24/ 13%
31/ 37/ 16%
50/ 55/ 9%
The difference in tyres is only 4mm which apparently which doesn’t account for the difference above.
????
Model Report/ Reading/ Difference
11/ 14/ 21%
21/ 24/ 13%
31/ 37/ 16%
50/ 55/ 9%
The difference in tyres is only 4mm which apparently which doesn’t account for the difference above.
????
Aftab said:
Even more confused now, according to the SVA guys the measurement is taken from their rollers at 2000RPM this is the result...
Model Report/ Reading/ Difference
11/ 14/ 21%
21/ 24/ 13%
31/ 37/ 16%
50/ 55/ 9%
The difference in tyres is only 4mm which apparently which doesn’t account for the difference above.
????
Those differences don't correspond to the percentages you're giving. I guess they worked out the percentages and then rounded everything down? Model Report/ Reading/ Difference
11/ 14/ 21%
21/ 24/ 13%
31/ 37/ 16%
50/ 55/ 9%
The difference in tyres is only 4mm which apparently which doesn’t account for the difference above.
????
Since the discrepancy isn't constant across all the gears, it's clear that the gear ratios in your car are different to the ones in the standard spec. You should be able to find out what the official spec is for the gearbox and diff ratios and find out where you stand.
This seems daft. If what you're saying is true, then then they seem to be comparing a road speed from their rollers (presumably calibrated) to an engine speed, via the wheels.
Whilst it's not likely that the gear ratios could have changed from the data that VOSA have, there is no reason why someone could have put a different gearbox in the car. I'd not have thought that would constitute grounds for failure alone, and certainly not simply because it make the the wheels turn at a different rate from that which is stated in their manual.
Furthermore, there is a critical issue with the test you are suggesting that they are doing, in that the tachometer is not calibrated (i.e. your dashboard tacho). It could be that the 2000rpm as measured on the tachometer is incorrect, if so then it will make the wheels rotate at the wrong speed.
What the heck is the manual about???? What exactly are they failing you car on???
It would be valid for them to compare speeds on your speedo, to speeds measured on their calibrated rollers throughout the speed range of the vehicle.
It would be valid for them to compare engine rpms on your dash tacho with those from a calibrated RPM meter pointing at the fanbelt pulley.
It would be valid for them to compare diff and gearbox ratios to those stated in their book.
The only thing common sense dictates would be a problem to them is if your speedo is out of calibration.
Whilst it's not likely that the gear ratios could have changed from the data that VOSA have, there is no reason why someone could have put a different gearbox in the car. I'd not have thought that would constitute grounds for failure alone, and certainly not simply because it make the the wheels turn at a different rate from that which is stated in their manual.
Furthermore, there is a critical issue with the test you are suggesting that they are doing, in that the tachometer is not calibrated (i.e. your dashboard tacho). It could be that the 2000rpm as measured on the tachometer is incorrect, if so then it will make the wheels rotate at the wrong speed.
What the heck is the manual about???? What exactly are they failing you car on???
It would be valid for them to compare speeds on your speedo, to speeds measured on their calibrated rollers throughout the speed range of the vehicle.
It would be valid for them to compare engine rpms on your dash tacho with those from a calibrated RPM meter pointing at the fanbelt pulley.
It would be valid for them to compare diff and gearbox ratios to those stated in their book.
The only thing common sense dictates would be a problem to them is if your speedo is out of calibration.
Dilbert, the actual reason for failure reads;
[quote] The vehicle is not accompanied by satisfactory evidence of compliance with the required standard for (noise, emissions)
(Gear box code different & gear ratios not within 8%)[unquote]
Failure is for incorrect gearing ratios because it apparently produces more noise and emissions. I’m sure that’s debateable but VOSA are a law on to themselves. What they say goes so arguing seems futile.
I agree I can’t see why the gearbox would have been changed to one with different ratios. I doubt it could be from another model, or even if that’s possible? The only logical explanation could be a revision or update to the ratios that Mazda put in during the second year of production. The model report is for a 2003 car where as mine is a 2004. Although having spoken to SVA UK, who also own the model report, they have never had an RX8 fail (regardless of year) before for this reason.
They were adamant that it’s the tyres as apparently even having the wrong tyre pressures can make the difference. The SVA centre and VOSA disagree. The SVA centre suggested the difference in rolling radius is only 4mm which is not significant enough to make the difference in speeds. I agree, however, I’m getting a different measurement. 1 foot = 304.8 millimetres
6.80 feet = 2072.64
6.74 feet = 2054.35
Difference = 18.29 mm
...is that significant enough to make the difference?
The other thing is they don’t read the speedo, they go by the reading off their rollers.
[quote] The vehicle is not accompanied by satisfactory evidence of compliance with the required standard for (noise, emissions)
(Gear box code different & gear ratios not within 8%)[unquote]
Failure is for incorrect gearing ratios because it apparently produces more noise and emissions. I’m sure that’s debateable but VOSA are a law on to themselves. What they say goes so arguing seems futile.
I agree I can’t see why the gearbox would have been changed to one with different ratios. I doubt it could be from another model, or even if that’s possible? The only logical explanation could be a revision or update to the ratios that Mazda put in during the second year of production. The model report is for a 2003 car where as mine is a 2004. Although having spoken to SVA UK, who also own the model report, they have never had an RX8 fail (regardless of year) before for this reason.
They were adamant that it’s the tyres as apparently even having the wrong tyre pressures can make the difference. The SVA centre and VOSA disagree. The SVA centre suggested the difference in rolling radius is only 4mm which is not significant enough to make the difference in speeds. I agree, however, I’m getting a different measurement. 1 foot = 304.8 millimetres
6.80 feet = 2072.64
6.74 feet = 2054.35
Difference = 18.29 mm
...is that significant enough to make the difference?
The other thing is they don’t read the speedo, they go by the reading off their rollers.
I think you're mixing up radius and circumference. The circumference is 2xPi times the radius i.e. roughly 6.3 times the radius. 4mm difference on the radius corresponds to 25mm difference on the circumference.
It seems to me that the basic problem is that you have a different gearbox to the one they're trying to test.
Looking at the numbers it seems unlikely to me that you will resolve this just by dropping the tyre size. You need to drop the gearing by about 15% to meet the spec on first gear which means you're going to be too far the wrong way in 4th gear, even assuming you could get 15% by fitting smaller wheels, which you are unlikely to manage anyway.
I think you need to either get the right 'box ratios installed, or find some way to persuade them that the ratios you currently have are actually OK. If they're failing you on the grounds of noise/emissions, are there any tests that would prove that you're within the noise and emissions specs?
It seems to me that the basic problem is that you have a different gearbox to the one they're trying to test.
Looking at the numbers it seems unlikely to me that you will resolve this just by dropping the tyre size. You need to drop the gearing by about 15% to meet the spec on first gear which means you're going to be too far the wrong way in 4th gear, even assuming you could get 15% by fitting smaller wheels, which you are unlikely to manage anyway.
I think you need to either get the right 'box ratios installed, or find some way to persuade them that the ratios you currently have are actually OK. If they're failing you on the grounds of noise/emissions, are there any tests that would prove that you're within the noise and emissions specs?
May be I am GreenV8S but I’m no expert. I’m trying to work with the logic and information I’ve been given. At the moment though unfortunately I’ve been told different things by everyone and cant work out who to trust.
Anyway, I have some further developments.
Having consulted a Mazda brochure for gearing ratios I’ve found the following.
1st Gear = 2.785
2nd Gear = 1.545
3rd Gear = 1.000
4th Gear = 1.694
Final = 4.3000
Using this calculator...
http://www.gearboxman.co.uk/download/gearspeeds.v2...
And the model report tyre size of 225/55/R16 @ 2000RPM I get the following results.
1st Gear = 13mph
2nd Gear = 23mph
3rd Gear = 36mph
4th Gear = (doesn’t give a reading as the tool works on gear changes)
The results of my car according to SVA guys was;
1st Gear = 14mph
2nd Gear = 24mph
3rd Gear = 37mph
4th Gear = 55mph
Which is within the 8% allowance for the test for the three gears.
However. the model report suggests it should be;
1st Gear = 11mph
2nd Gear = 21mph
3rd Gear = 31mph
4th Gear = 50mph
This is obviously completely different gearing ratios. I’m completely baffled as to how modelreports.com claim no Auto RX8 has ever failed due to this before. Unless the tool in the link is wrong, their figures are way off!
Can anyone advise please?
Anyway, I have some further developments.
Having consulted a Mazda brochure for gearing ratios I’ve found the following.
1st Gear = 2.785
2nd Gear = 1.545
3rd Gear = 1.000
4th Gear = 1.694
Final = 4.3000
Using this calculator...
http://www.gearboxman.co.uk/download/gearspeeds.v2...
And the model report tyre size of 225/55/R16 @ 2000RPM I get the following results.
1st Gear = 13mph
2nd Gear = 23mph
3rd Gear = 36mph
4th Gear = (doesn’t give a reading as the tool works on gear changes)
The results of my car according to SVA guys was;
1st Gear = 14mph
2nd Gear = 24mph
3rd Gear = 37mph
4th Gear = 55mph
Which is within the 8% allowance for the test for the three gears.
However. the model report suggests it should be;
1st Gear = 11mph
2nd Gear = 21mph
3rd Gear = 31mph
4th Gear = 50mph
This is obviously completely different gearing ratios. I’m completely baffled as to how modelreports.com claim no Auto RX8 has ever failed due to this before. Unless the tool in the link is wrong, their figures are way off!
Can anyone advise please?
Right, I’m starting to get somewhere now.
The model report I bought from Modelreports.com was advertised as “Mazda RX-8 (2001-2005) Auto Petrol LA-SE3P” This is the correct model type and since mine is a 2004, the advertisement suggests it’s covered. There also appears to be another company which does model reports for MX5s and RX8s. Their one covers 2004 models onwards.
There has obviously been a revision to gearing ratio for emissions compliance between 2003 and 2004. Either modelreports.com don’t know this or they’re playing dumb. Either way, would you agree its sufficient grounds to demand my money back? As I’ve been sold a model report which effectively isn’t for my car.
The model report I bought from Modelreports.com was advertised as “Mazda RX-8 (2001-2005) Auto Petrol LA-SE3P” This is the correct model type and since mine is a 2004, the advertisement suggests it’s covered. There also appears to be another company which does model reports for MX5s and RX8s. Their one covers 2004 models onwards.
There has obviously been a revision to gearing ratio for emissions compliance between 2003 and 2004. Either modelreports.com don’t know this or they’re playing dumb. Either way, would you agree its sufficient grounds to demand my money back? As I’ve been sold a model report which effectively isn’t for my car.
Aftab said:
The model report I bought from Modelreports.com was advertised as “Mazda RX-8 (2001-2005) Auto Petrol LA-SE3P”
Just from the name I would guess that model has an auto gearbox. Does yours? (I don't understand the logic behind these mph/rpm measurements, but doing them on an auto would seem particularly bizarre.)Edited to add: Yes I see you already mentioned that it is. How the heck can they hope to measure that? Unless they can lock up the fluid coupling, the results surely are going to be all over the place.
Edited by GreenV8S on Tuesday 24th July 20:32
Regardless of your gearbox ratios the SVA people go off what your speedo reads and what the actual speed of the vehicle is travelling at so by the details given your speedo is under reading which is simply not allowed, it can over read by quite a large percent, you can find the SVA regs somewhere on the net, I'm sure someone knows the site, which will give you details of the allowed differences, when i set my electronic speedo for the test I'd to set it up with circumference x diff ratio x four ( pick up points ), gearbox ratios didn't come into it, my own cars an automatic.
GreenV8S said:
Aftab said:
The model report I bought from Modelreports.com was advertised as “Mazda RX-8 (2001-2005) Auto Petrol LA-SE3P”
Just from the name I would guess that model has an auto gearbox. Does yours? (I don't understand the logic behind these mph/rpm measurements, but doing them on an auto would seem particularly bizarre.)Edited to add: Yes I see you already mentioned that it is. How the heck can they hope to measure that? Unless they can lock up the fluid coupling, the results surely are going to be all over the place.
Edited by GreenV8S on Tuesday 24th July 20:32
The irony is my speeds were higher at the 2000 test RPM. The model report, for example in first gear shows at 2000rpm the speed should be 11, mine was 14. So at the same speed my car revs less. This has obviously been a revision for emissions. My car has lower emissions than the one tested on the model report, yet it fails on emission. Where’s the logic in that? Of course they’re all robots and have no discretion so cant make an exception even when provided with fasts.
hal 1 said:
Regardless of your gearbox ratios the SVA people go off what your speedo reads and what the actual speed of the vehicle is travelling at so by the details given your speedo is under reading which is simply not allowed, it can over read by quite a large percent, you can find the SVA regs somewhere on the net, I'm sure someone knows the site, which will give you details of the allowed differences, when i set my electronic speedo for the test I'd to set it up with circumference x diff ratio x four ( pick up points ), gearbox ratios didn't come into it, my own cars an automatic.
That’s not right. They read off their rollers, not off the speedo. If this was a speedo issue my car would be over reading, not under. At 2000rpm in first gear it is supposed to be going 11mph, my speedo reads 14. After hours of searching, I’ve come to the conclusion that is this not the issue anyway;
The model report itself is wrong…
See here; http://www.pistonheads.co.uk/gassing/topic.asp?h=0...
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