Shift light.

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bigbadbob76
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Re: Shift light.

Post by bigbadbob76 »

937carrera wrote:You could do that too, I did think of that just worried about the diff interfering in some way, but can't argue why it should if one wheel is on the ground.

With one wheel on the ground, the other will do twice the number of revolutions so you just have to half it. :ok

ajsimmo wrote:I've forgotten the point of all this...

To set up my shift light/70mph light on the bench without doing 70MPH while tweaking the adjustment pot.
I live a long way from a motorway or suitable long straight (private :wink: ) road.
'86 1.9 DG, 4 spd, tintop, camper conversion.
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937carrera
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Re: Shift light.

Post by 937carrera »

bigbadbob76 wrote:
With one wheel on the ground, the other will do twice the number of revolutions so you just have to half it.


Let me know when you've done the maths. :wink:
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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bigbadbob76
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Re: Shift light.

Post by bigbadbob76 »

4.13 engine revs=2 wheel revs (if I have a true DU box)
so 1 wheel rev= 2.065 engine revs
or 2 engine revs + 23 crank degrees. :wink:

I'll find out tonight. :lol:
'86 1.9 DG, 4 spd, tintop, camper conversion.
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937carrera
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Re: Shift light.

Post by 937carrera »

4th is 52:61
Final drive is 34:7

1 engine rev * 61/52 * 7 / 34 = 0.2415 wheel revolutions or 86.9 wheel degrees (don't ask me to convert to radians :lol: )

1 wheel revolution = 4.14 engine revolutions

Double check

0.2415 wheel revolutions * 3798 revs * 80.6 inches wheel circumference / 1760 yards / 36 inches in a yard * 60 minutes = 70.007 miles per hour

You don't need to halve the value for only one of two wheels turning, that was my little tease. :oops:

update to change diameter to circumference
Last edited by 937carrera on 09 Aug 2018, 16:26, edited 1 time in total.
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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bigbadbob76
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Re: Shift light.

Post by bigbadbob76 »

937carrera wrote:You don't need to halve the value for only one of two wheels turning, that was my little tease. :oops:

erm.... is that not what the diff does?
If I "cock" one wheel it'll go twice as fast as it would with two wheels on the road.
Your maths works out good for on road speed vs revs but I'll have one wheel in the air and the other not turning. :?
'86 1.9 DG, 4 spd, tintop, camper conversion.
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937carrera
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Re: Shift light.

Post by 937carrera »

Maybe I'm teasing myself then, diff just allows one wheel to rotate at a different speed to the other, I wouldn't expect it to rotate one wheel at 200% while the other is zero %. Both wheels in the air with a non LSD diff one wheel rotates the opposite to the other when turned at the rim.

You'll know for sure later tonight, perhaps something told me that the pushing along the ground in the first place was the right way. It's also the method used in race series where gearbox ratios are not allowed to be changed without stripping boxes down, not my invention.

Let us know how you get on :)
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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bigbadbob76
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Re: Shift light.

Post by bigbadbob76 »

Yes, pushing along the ground gives you rolling radius of the tyre so it's a truer measurement than calculated tyre diameter due to vehicle weight.

Anyway. I counted wheel revs and i get 2.08 engine revs per wheel rev, double it (yes, you do, stop confusing yourself. Haha) gives a total ratio of 4.16:1
Pretty close to a du box at 4.14:1 allowing for backlash etc.
So my tacho must be wrong. Or my speedo is reading low. According to gps it reads 10% high so that leaves the tacho.
I'll whip the cluster out and cal the tacho in shop tomorrow.

Although checking with another instrument-
Image

Image
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937carrera
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Re: Shift light.

Post by 937carrera »

Well, it looks like I'm going to be having a big argument with myself then. :D There was a reason why the old scrutineers rolled the car along the ground then.

Glad you got the data you needed, lets hope the tacho is out and you can get it back into calibration.

I think that also means that your oil pressures are better than thought too

Just seen the pictures, I see you have the same home calibration tool as me so you are even more confused :roll:, maybe time to do the gearing check the hard way ?
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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bigbadbob76
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Re: Shift light.

Post by bigbadbob76 »

Looking at the dwell angle, i get conflicting readings with two different instruments but it seems to go
from 10 degrees at idle to 46 degrees at 4kRPM which sounds about right for an electronic ignition system.
45 degrees is 25% duty cycle so I'll also dial that into the shift light at 127Hz/3.8krpm and see what it does in reallity.

I'm happy the gearing is standard DU.
Tacho cal check against a calibrated sig gen will prove that once and forall.
'86 1.9 DG, 4 spd, tintop, camper conversion.
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bigbadbob76
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Re: Shift light.

Post by bigbadbob76 »

Well THAT was fun....
I got on the airstrip for a few quick sprints. (friends in high places) :D :D

I set the shift light to 70MPH by GPS.
This was at an indicated 4KRPM so my tacho reads slightly high, should be 3800RPM but...
I noticed the Golf tacho is quite well damped and takes a positive change in revs before it responds so I'm not going to mess with it.
That's put this thing to bed. :D
If you want your own shift light there should be enough info here to do it yourself but shout if you have any questions.
'86 1.9 DG, 4 spd, tintop, camper conversion.
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937carrera
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Re: Shift light.

Post by 937carrera »

Does that mean the Gunson reads 5% high too ?

Just wondering as I've got one too
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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bigbadbob76
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Re: Shift light.

Post by bigbadbob76 »

Gunson was all over the place out on the road, reading higher than my tacho. :shock:
It read 5K at one point when my tacho said 4K. :roll:
it seems better sat still but it's not calibrated so yours and mine could be totally different.
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CJH
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Re: Shift light.

Post by CJH »

bigbadbob76 wrote:Well THAT was fun....
I got on the airstrip for a few quick sprints. (friends in high places) :D :D

Great stuff - I thought you were joking when you said you'd need to get to an airstrip.
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bigbadbob76
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Re: Shift light.

Post by bigbadbob76 »

Nah, there's one just down the road, it's pretty short and stacked with wind turbine blades at the moment but long enough to get up to 70 before you have to brake. :lol:
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bigbadbob76
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Re: Shift light.

Post by bigbadbob76 »

Getting back to true rolling radius vs calculated tyre diameter...
This would give me slightly lower gearing than the maths says.
Which might account for my tacho appearing to read slightly high at 70MPH. (4kRPM vs 3800RPM.)
I'm still not going to worry about it but just thought I'd throw it in there. :lol:
Vehicle gauges are not laboratory instruments. :wink:
'86 1.9 DG, 4 spd, tintop, camper conversion.
Split case club member.

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