Barksavon
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| posted on 23/2/09 at 08:26 PM |
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SVA Fail--Speedo Reading Out
My Tiger avon has failed sva today, would like advice on 2 issue's if possible
1. My speedo was 1mph below the bottom tolerance level on the 60 & 70mph accuracy test. The tester suggested it could be low tyre pressure or the
gearbox drive gear could need replacing....got to admit forgetting to check tyre pressure's (basic schoolboy error i know!!) and lo and behold
they were well down, any views on whether correcting the pressure's will rectify this or do i need to change the drive gear?
2. Steering didnt self centre on left lock....whats the likely cause...tracking, camber, castor....none of which have been professionally set up?
Any help greatly appreciated, thanks
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jacko
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| posted on 23/2/09 at 08:32 PM |
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Is your steering rack tight ?
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Barksavon
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| posted on 23/2/09 at 08:44 PM |
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yes think so i'll check it though....Ta
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andrew.carwithen
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| posted on 23/2/09 at 08:44 PM |
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How did he test yor self-centreing?
Did he do several rapid figure-of - eights?
You cannot adjust the castor on an Avon (which is a main consideration wrt self-centreing).
But you can set camber, toe and tyre pressures.
I rigged up a heath robinson contraption at home. To act as a datum for the above measurements.
Its basically a rectangular frame (made out of aluminium angle) which surrounds the car. This is supported on 4 axle stands at hub centre height. It
is moved until it is square to the car by ensuring it sits equidistant from the wheel centres on both sides.
You can now take measurements along the frame to assess/adjust toe on both front and rear wheels. (I set rears to parallel and fronts to 1-2mm toe-in
both sides. I then bought a camber gauge on ebay (crop up regularly for about a tenner) and set rears to zero camber and fronts to 1 degree negative
camber.
Set tyre pressures to 18psi all round and adjusted shocks so that bottom front bones are parallel to floor and similar ride height either side.
Had plenty of self-centreing during figure-of-eight test. (which, I must admit, I was really surprised at.!)
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Paul TigerB6
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| posted on 23/2/09 at 08:45 PM |
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You should be fine with the tyres pumped up (in fact make sure by over-inflating a little for the test) which will increase the rolling circumference.
1mph at 60 mph means you need less than a 2% change in circumference so not much at all.
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Barksavon
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| posted on 23/2/09 at 08:51 PM |
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thanks for that....very comprehensive, are you running all 4 tyres at 18 psi....mine show a max pressure of 36psi....i was hoping that setting the
rears at that would rectify my low speedo reading
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Barksavon
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| posted on 23/2/09 at 08:53 PM |
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Paul, thanks for that...re assuring...
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Paul TigerB6
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| posted on 23/2/09 at 09:44 PM |
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17 - 18psi should be fine for your Avon on the pressures all round. You have cortina front uprights?? Try a small amount of toe out at the front and
see how that goes - you should be able to rig something up with a couple of straight edges to get it at neutral, and then add a little toe out each
side. Camber should be around 1deg negative (top of the wheels slightly in) each side.
[Edited on 23/2/09 by Paul TigerB6]
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britishtrident
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| posted on 23/2/09 at 11:10 PM |
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Soft tyres will make the speedo read fast not slow.
If the speedo is within tolerance at 30 but too slow higher up the speed range then the most likely cause is the spiral spring inside the speedo has
lost a little of its tension -- if you are brave you can fix this yourself one of the Triumph owners web sites has instruction to do this.
Personaly I would either get speedy cables to fix it for you or change the tyre size.
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britishtrident
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| posted on 23/2/09 at 11:13 PM |
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Asymetric self centereing problem -- assuming the track rods are the same lengty and tyre pressures equal next step check the corner weights.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Paul TigerB6
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| posted on 23/2/09 at 11:35 PM |
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quote:
Originally posted by britishtrident
Soft tyres will make the speedo read fast not slow.
If the speedo is within tolerance at 30 but too slow higher up the speed range then the most likely cause is the spiral spring inside the speedo has
lost a little of its tension -- if you are brave you can fix this yourself one of the Triumph owners web sites has instruction to do this.
Good point on the tyre pressures - thinking about it more, of course thats the case!! The rollers give a true 60mph etc and low tyre pressures give
more wheel turns and hence more on the speedo too so show a higher speed with lower pressure.
Is your tacho a new mechanical one from Tiger (ETB)?? Another option to the above and one that may be a whole lot cheaper (but maybe not simpler due
to access) is to change the speedo drive in the gearbox. These are available from Burton Power for about £15 (and no doubt cheaper direct from Ford) -
you would obviously have to find out what you have currently though before ordering.
linky - 3/4 of the way down
Probably well worth speaking to Tiger and ETB about calibration
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Phil.J
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| posted on 24/2/09 at 08:27 AM |
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Get an old speedo that will work with your installation, open it up and take out the dial, spray it matt black and put new 'letraset, white
digits on in the required places to give correct reading. (I've done this with rev counters in the past to get through static noise tests at
hill climbs when I was running a particularly noisey rotary engine!)
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