Mole Posted July 30, 2002 Share Posted July 30, 2002 I gather that the SVA includes a measurement of the accuracy of the speedometer. The speedo reading is taken from the drive at the gearbox, but for a given rpm at the final drive, the actual road speed will be dependent on the size of wheels fitted. Can anyone tell me what wheel size gives the least error. I have 15" wheels - what degree of error is there likely to be between the speedo reading and the actual road speed? Is there any way of calibrating the speedo for the size of the wheels fitted? Keep flying low The Mole @ the molehill Link to comment Share on other sites More sharing options...
F355GTS Posted July 30, 2002 Share Posted July 30, 2002 Mole The SVA error variation is quite huge as long as it over reads, under reading is not allowed eg at true 26 speedo can read up to 35 Calculate other speeds as follows true speed divided by 10 plus 6.25 MPH gives you max error over eg 40 mph real speed divided by 10 = 4MPH plus 6.25 so Speedo can read up to 40+4+6.25 =50.25 Not all test stations have suitable equipment Don't think there's anyway to calibrate the speedo but different size drives are available ofr gearbox to get it 'more accurate' Seem to recall seeing that the 'standard' drive is most accurate with 15" wheels Mark Link to comment Share on other sites More sharing options...
Mole Posted July 30, 2002 Author Share Posted July 30, 2002 Thanks Mark - just what I needed to know Keep flying low The Mole @ the molehill Link to comment Share on other sites More sharing options...
Chelspeed Posted July 31, 2002 Share Posted July 31, 2002 When Caterham used to use cable driven speedos there were different speedos supplied depending on which size wheels and tyres you went for. You could tell which one you had by a sticker on the side, if I recall correctly there were actually only two types, W=1.00 for 13 and 14 inch wheels and W=0.9 something for 15 and 16 inch wheels. So you can change the calibration of a speedo, not sure how, it's a specialist job, changing sets of gears internally presumably. Speedy Cables is a name that springs to mind but never had it done myself. Or you can change the gears in the gearbox, the drive gear is 6 or 7 teeth but thats internal so hard to change, the driven gear is fitted from outside so can be changed, a range of teeth numbers is available, each one a different colour. But as previously advised the permissable error is so large you probably don't need to bother. Now Caterham use electronic speedos there'll be a calibration number that you enter, by menu on the LCD panel presumably. There'll be a different number for each size of wheels / tyres. Same with stack dashes or push bike speedos. Link to comment Share on other sites More sharing options...
Mole Posted August 1, 2002 Author Share Posted August 1, 2002 Since the permissible error in over-reading is so large I'm not so worried now, but logically I can't see how 15" could be the most accurate because if this was the case, speedos with cars on 16" wheels would under-read (by about 1.5% if my maths is correct). Or is my logic more inaccurate than my odometer (see later thread)? Keep flying low The Mole @ the molehill Link to comment Share on other sites More sharing options...
F355GTS Posted August 1, 2002 Share Posted August 1, 2002 Mole Larger wheels will under read because in simplistic terms they need to go round fewer times to cover a set distance therefore the speedo gets fewer pulses and thus reads lower. Mark Link to comment Share on other sites More sharing options...
Mole Posted August 1, 2002 Author Share Posted August 1, 2002 Sorry, but some of us yokels are a bit slow on the uptake - surely the speedo reading is taken at the gearbox not at the wheels which means that for each revolution of the propshaft larger wheels will cover more distance and will thus over-read? Keep flying low The Mole @ the molehill Link to comment Share on other sites More sharing options...
Mole Posted August 1, 2002 Author Share Posted August 1, 2002 Think I may have got the over-reading / under-reading bit the wrong way round in may last contribution, but you get my drift (I hope). For a given number of revolutions of the propshaft (which is effectively what the speedo indication is based on) you will have travelled further on the 16" wheels than on the 15 inchers, which means if 15" wheels give the most accurate reading, you are under-reading with 16 inchers. It also means that I'm getting dreadfully confused now and since AFAICT no-one seems to have failed SVA on speedo accuracy I think I'll stop worrying now ..... Keep flying low The Mole @ the molehill Link to comment Share on other sites More sharing options...
F355GTS Posted August 1, 2002 Share Posted August 1, 2002 Mole Exagerated example!! Assume a propshaft has to turn 10 times to make a wheel with a 1' circumference turn once now take the same propshaft and turn it 10 times with a wheel that has a 2' circumference untill it completes a full circle the 2' wheel will cover twice the distance for the number of propshaft revolutions. Assuming the speedo hasn't been recalibrated it will only show 1/2 the speed and 1/2 the distance actually covered because the propshaft still only turned 10 times but the ground covered was double in the same time as the 1' wheeled item god it's like bloody Blue Peter, now get an egg box carton, some sticky tape and a fairy liquid bottle......sorry lost the plot!!!!! (enter smily face here!!!) Mark Link to comment Share on other sites More sharing options...
F355GTS Posted August 1, 2002 Share Posted August 1, 2002 It just took me 10 minutes to write the above reply and you've got it in the meantime!!! Don't know the answer to your other problem though, sure your missus hasn't been taking it for a spin whilst your down the pub??? Mark Link to comment Share on other sites More sharing options...
Trevor Phillips Posted August 1, 2002 Share Posted August 1, 2002 If your worried disconnect the drive and fit a Sigma computer speedo and light at £27. Easy to calibrate and you can put it to the front wheel where they can't test it on their rollers. They do have the right to take it out on the road with a second vehicle but this is unlikely. Anyway, they are simple to calibrate. Link to comment Share on other sites More sharing options...
Mole Posted August 1, 2002 Author Share Posted August 1, 2002 Any suggestions where to get a sigma - do Halfords do them? Keep flying low The Mole @ the molehill Link to comment Share on other sites More sharing options...
Trevor Phillips Posted August 1, 2002 Share Posted August 1, 2002 I got mine here Edited by - Trevor on 1 Aug 2002 18:55:33 Link to comment Share on other sites More sharing options...
Mole Posted August 1, 2002 Author Share Posted August 1, 2002 Excellent - thanks Keep flying low The Mole @ the molehill Link to comment Share on other sites More sharing options...
DohNut Posted August 2, 2002 Share Posted August 2, 2002 You may also want to try www.evanscycles.com Search for computers there is a VDO one (same people as car instruments) that is wireless. I am not sure weather the wireless versions would work with all the electrical noise in a car but I am thinking of getting one anyway and putting it on my bike if it doesnt work. Nick Link to comment Share on other sites More sharing options...
JAG Posted August 2, 2002 Share Posted August 2, 2002 I was interested in fitting a bike speed computer instead of a standard (expensive) electronic speedo. I was told that the re-settable odometer wouldn't pass SVA. Anyone got any comments on that?? Link to comment Share on other sites More sharing options...
jf_renard Posted August 2, 2002 Share Posted August 2, 2002 Hello Mole. I remember having a speedometer calibration problem on one of my first cars (a Peugeot 404). It was a Jaeger unit. I dismanteled it and found it quite easy to calibrate: The needle is secured to a round magnet, with a spring which tends to take it back to zero. The cable from the gearbox is terminated by a small bell, around the magnet (without touching the magnet). When the bell turns, it tends to take the magnet, so the needle, to vmax. The needle actually stabilises when the spring force and the magnetic force are equal. There was an adjustin screw which was acting on the distance between magnet and bell. Don't know if all mechanical speedometers work like that, but it's worth having a try. Have a good week-end. Jeff Cosworth 1600 BDR Link to comment Share on other sites More sharing options...
Mole Posted August 30, 2002 Author Share Posted August 30, 2002 Just thought I'd close the loop on this one: SVA last Tuesday - speedo was spot on! At 35mph it read 35mph! The maximum error recorded by the tester was 2mph (I assume that the speedo was over by 2 mph but he didn't say). All my fears were unfounded (clearly) Keep flying low Mole @ the animal with attitude (well almost!) Link to comment Share on other sites More sharing options...
Area Representative Nick Chan Posted August 30, 2002 Area Representative Share Posted August 30, 2002 I'm having fun and games with the speedo in my car with the needle bouncing between 20mph and 90mph for virtually any speed I happen to be travelling at! Odometer reading seems perfectly fine (as much as I can tell anyway) but the needle has taken on a mind of its own. After reading a website found through Blatchat (see here) the idea of a cycle computer seems an excellent and cost-effective option i.e. cheap! The wireless option is not suggested as suitable due to interference so I have ordered the same Sigma BC1200 model as used in Strangely's car for £21.49 inclusive P&P from a shop in Congleton. They do mail order and can be found at here. The only difference I will make in installing the computer is to mount the speedo somewhere else than the steering wheel (as mine is QR) and I will mount the magnet on the brake disc rather than the wheel to allow the wheels to be rotated around the car at a later stage - a bit difficult if one is dedicated to the front offside for the speedo use! Otherwise it all seems very straightforward. In the archives someone has written a thread on using the disc rather than the wheel as magnet base so I'll have to dig this out... HTH Nick Yellow HPC - A 2.0 VX - 😬 Link to comment Share on other sites More sharing options...
Trevor Phillips Posted September 2, 2002 Share Posted September 2, 2002 My Sigma computer failed the Sva on Friday [sensor front wheel mounted] as Mitcham didn't have equipment to check it's accuracy. The Manual talks about "with the vehicle positioned on, or fitted with a speed measurement device...." Their equipment was for driven wheel rollers only. They read the Manual and we discussed 5th wheel attachments. They said only certain centres had these. I am now trying to get clarification from Swansea. Link to comment Share on other sites More sharing options...
JAG Posted September 2, 2002 Share Posted September 2, 2002 Trevor, I was hoping to fit a bike speedo to my car but was concerned that it would fail SVA test because; a) the odometer can be easily re-set, b) it's not backlit. What did your testers have to say on these points? I hadn't foreseen the problem you mentioned, I would be very interested in how you get on with this. JAG Link to comment Share on other sites More sharing options...
Trevor Phillips Posted September 2, 2002 Share Posted September 2, 2002 Sigma have a neat light attachment for £7 that illuminates from the side. They thought it was great! They asked about it having an odometer - I said yes and that it was showing zero miles as we had not yet driven it on the road and they were happy with that. Link to comment Share on other sites More sharing options...
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