IVA test and gearbox/speedometer problems
Discussion
Hello all, apologies for the lengthy post.
A few months ago my dad and I took his KVA GT40 for its IVA test in Southampton. It failed on a number of things and he's been working his way through them. One which has stumped us is the speedometer reading. The tester placed the car on the speedometer rollers and ran the car up to an indicated 70mph according to the car's speedometer but the machine showed a speed of 35mph. Whilst we can't be sure the speedo is 100% spot on, we did drive to the test centre and it's certainly not reading 50% too low.
During the test we noticed only one wheel was turning. This is how I understood a differential should work, only one wheel would spin freely when there is no traction. When we asked the tester for his opinion, he suggested the gearbox must have some kind of problem as both wheels should have been spinning. He also said this was the reason for the 50% reading, since the machine takes a total of the 2 rollers' speeds and calculates an average. Obviously 70 + 0 / 2 will be 35 all day long, but since the car's speedometer is electronic and the pickup is located on the one wheel which was spinning, the car will say 70 all day long!
So we're not sure where to go from here. Is the gearbox behaving correctly, or should both wheels be spinning during the test? It's a Renault 21 gearbox.
A few months ago my dad and I took his KVA GT40 for its IVA test in Southampton. It failed on a number of things and he's been working his way through them. One which has stumped us is the speedometer reading. The tester placed the car on the speedometer rollers and ran the car up to an indicated 70mph according to the car's speedometer but the machine showed a speed of 35mph. Whilst we can't be sure the speedo is 100% spot on, we did drive to the test centre and it's certainly not reading 50% too low.
During the test we noticed only one wheel was turning. This is how I understood a differential should work, only one wheel would spin freely when there is no traction. When we asked the tester for his opinion, he suggested the gearbox must have some kind of problem as both wheels should have been spinning. He also said this was the reason for the 50% reading, since the machine takes a total of the 2 rollers' speeds and calculates an average. Obviously 70 + 0 / 2 will be 35 all day long, but since the car's speedometer is electronic and the pickup is located on the one wheel which was spinning, the car will say 70 all day long!
So we're not sure where to go from here. Is the gearbox behaving correctly, or should both wheels be spinning during the test? It's a Renault 21 gearbox.
It may be worth checking to see if you have a brake binding. It can only be something like that or there is a problem with the VOSA kit.
I find it very strange and very frigthening that the vehicle was doing 70 with only one wheel spinning.
If it helps you could bring it in and set the speedo on our rolling road in Fareham. With it booked in for work to be carried out, and insured, you can legally drive it down. This would at the same time give the 'box and diff a chance to free up if there is anything binding.
Steve
I find it very strange and very frigthening that the vehicle was doing 70 with only one wheel spinning.
If it helps you could bring it in and set the speedo on our rolling road in Fareham. With it booked in for work to be carried out, and insured, you can legally drive it down. This would at the same time give the 'box and diff a chance to free up if there is anything binding.
Steve
sicarumba said:
Thanks for the reply Steve.
So both wheels should be spinning? I'll need to find out if there is any binding anywhere in that case. Whereabouts in Fareham are you?
If you have driven it then both sides of the drive must be working or you would have gone nowhere.So both wheels should be spinning? I'll need to find out if there is any binding anywhere in that case. Whereabouts in Fareham are you?
Something has to be preventing one side from driving freely.
If it had been a fault with the VOSA kit I have to think they would have contacted your father by now.
Fort Fareham. Check my profile for full details.
Steve
Try asking on this web site: www.gt40s.com
Lots of very helpfull and knowledgeable people from all over.
Lots of very helpfull and knowledgeable people from all over.
I had a slightly different issue with what was then the SVA since my speedo pickup was off a front wheel, and the VOSA test kit needs a driven wheel to run the roller(s) up to a certain speed. I think the requirement is still the same with the IVA that you have to prove speedo accuracy, but it doesn't actually have to be done using their 'regular' kit/method. In my case I managed to 'prove' the accuracy on the re-test by getting a note from the speedo manufacturer saying that so long as it was configured properly with correct number of pulses, diameter of wheel etc then the readout would be accurate. On the day I then presented the letter and demonstrated the configuration with wheel diameter etc and they accepted it.
But, you do have a problem there! The diff will let each wheel run at an 'average' of the input shaft speed geared down by the crown/pinion ratio, so assuming both wheels are getting the same level of resistance then both wheels should turn at the same speed and so a speedo pick-up from just one side is fine. And the VOSA kit does indeed take the speed of each driven wheel independently so in your case if one wheel wasn't driving it'll 'half' the speed of the driven one.
The fact you had a stopped wheel means there is a problem there, and I think it can only be one of 3 things - namely:
1) the VOSA kit has a problem with one roller giving far more load than the other - enough to stop one wheel. Got to say that's unlikely given the number of cars these things test.
2) there's a problem in the diff not giving drive to one wheel. But no drive (as opposed to a locked drive) on one side means no resistance to force the other wheel to drive, so you'd end up with neither wheel driving...so that's not likely.
3) something on the car is binding on the 'non turning' side. Got to be the most likely. Presumably there's a transaxle on the car so you can't just get underneath and turn the prop-shaft by hand? If you put the car in gear and so 'lock' the drive and jack the back-end up, if you turn one wheel by hand, the other one should turn the opposite way. Maybe not wise to actually start the motor on axle stands, but with the plugs out either manually turn the engine over in gear or use the starter for short bursts in gear and you should be able to hold one wheel still while the other runs - then let go and it should come up to speed while the speed of the previously driven 'other' wheel halves. The force required to hold the wheel to make the other 'double' its speed should be the same on both sides.
But, you do have a problem there! The diff will let each wheel run at an 'average' of the input shaft speed geared down by the crown/pinion ratio, so assuming both wheels are getting the same level of resistance then both wheels should turn at the same speed and so a speedo pick-up from just one side is fine. And the VOSA kit does indeed take the speed of each driven wheel independently so in your case if one wheel wasn't driving it'll 'half' the speed of the driven one.
The fact you had a stopped wheel means there is a problem there, and I think it can only be one of 3 things - namely:
1) the VOSA kit has a problem with one roller giving far more load than the other - enough to stop one wheel. Got to say that's unlikely given the number of cars these things test.
2) there's a problem in the diff not giving drive to one wheel. But no drive (as opposed to a locked drive) on one side means no resistance to force the other wheel to drive, so you'd end up with neither wheel driving...so that's not likely.
3) something on the car is binding on the 'non turning' side. Got to be the most likely. Presumably there's a transaxle on the car so you can't just get underneath and turn the prop-shaft by hand? If you put the car in gear and so 'lock' the drive and jack the back-end up, if you turn one wheel by hand, the other one should turn the opposite way. Maybe not wise to actually start the motor on axle stands, but with the plugs out either manually turn the engine over in gear or use the starter for short bursts in gear and you should be able to hold one wheel still while the other runs - then let go and it should come up to speed while the speed of the previously driven 'other' wheel halves. The force required to hold the wheel to make the other 'double' its speed should be the same on both sides.
Thanks for the replies chaps.
I think we've established there is some drag on one wheel, so some brake adjustment is on the cards before we move on.
Steve - I think my dad will be calling you soon to arrange popping in. Once the wheel is freed off it'll be worth checking the speedo before the retest, just in case.
I think we've established there is some drag on one wheel, so some brake adjustment is on the cards before we move on.
Steve - I think my dad will be calling you soon to arrange popping in. Once the wheel is freed off it'll be worth checking the speedo before the retest, just in case.
@ Sicarumba, One of my customers had the exact same problem. Basically the problem is with the VOSA equipment & they know all about it, I can only assume that the tester you had was new on the job or simply trying to be particularly unhelpful. No diff will ever spin the rear wheels at the exact same speed as each other when in independant rollers, to be accurate the rollers need to be locked together & the Vosa gear doesn't have that facility, so even tho' you may have a dragging brake fixing it won't cure the problem if your pickup is on only one driveshaft.
If the speedo unit is an Acewell (I am a stockist) I can provide a copy of manufacturers letter & further info we have generated ourselves which the test station should accept providing you can show that calcs have been correctly made as per manufacturers instructions. I have personally spoken to the guys (genuinely very helpful) at Vosa head office about this & they have confirmed that if unit can be shown to be working by being driven around test station & unit is correctly setup with correct calcs shown then this is adequate. HTH
If the speedo unit is an Acewell (I am a stockist) I can provide a copy of manufacturers letter & further info we have generated ourselves which the test station should accept providing you can show that calcs have been correctly made as per manufacturers instructions. I have personally spoken to the guys (genuinely very helpful) at Vosa head office about this & they have confirmed that if unit can be shown to be working by being driven around test station & unit is correctly setup with correct calcs shown then this is adequate. HTH
Gassing Station | Kit Cars | Top of Page | What's New | My Stuff