Hey Guys,
Is there a way to identify what the diff ratio is with it in the car. The car I have currently runs 13" wheels. It does not appear to be 3.4:1

Thanks.
Ian

it's a bit hard because you need to count the turns of the motor, either by marking the flywheel pulley or with a pointer down a spark plug hole. Basically, take all of the spark plugs out, jack one wheel up, put the car in 4th gear and turn the wheel that's in the air until the piston with your pointer in it is at the top of the bore, then turn the wheel one full revolution and work out how many turns the engine did.

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Thanks. That's sounds simple enough. Will give it a try.

Cheers

If you test this with 1 wheel jacked up it with throw the numbers out by 100% due to the diff action. So allow for this in your calculation.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

This is the method I used
viewtopic.php?f=2&t=89197&hilit=+diff+ratio

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Heya Kev - what am I missing?

Thats not how a diff works, I even had to make one out of lego to check - stop one axle from turning and the crownwheel turns 1:1 with the other axle...

compfussed?

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Stop 1 axle turning, if no LSD fitted, the other axle drives the spider gears and the diff ratio calc is out by 100%. Try it... I did.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

nup, I'm still confused, the spider gears are the same size... if you have a car up on stands with the engine going and in gear, the wheels spin away merrily, stop one of the wheels with your boot and the other one doesn't change speed.... we'd have really weird effects going from grippy surfaces to non-grippy

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The way I see this...

If the crownwheel is turning 1:1 with the rotating road wheel, then the spider gears aren't rotating, so the other axle must be turning as well.

Hold the crownwheel (and thus spider carrier) still and turn one axle forwards. The other axle is driven backwards at the same speed.
Now rotate that whole system so one axle is stationary - the relative motion within the system is the same, with the free axle turning at twice crownwheel speed.

With split surfaces, it's not too weird. The gripping wheel turns at car speed, the other wheel can spin or lock, and the crownwheel turns at the average of those speeds.

I just tried this with the mini on the hoist, slow one wheel and the other wheel definately increases in speed.

Makes sense as when both wheels are rotating at the same speed the spider gears are not rotating (except with the diff, imagine a welded diff) stop one wheel and the output meshing with the spider stops forcing the spider to rotate so the rotating side is seeing the rotation from the crown wheel plus the rotation forced on it by the spider gear now rotating.

The animations I looked up show this much but don't really explain well how much the wheels speeds can differ, however it would make sense that it doubles the spinning wheel as this is the amount of movement that was lost from the stationary wheel and as there is no significant loss of power in heat or noise it has to be going to the other wheel.

https://www.youtube.com/watch?v=SOgoejxzF8c

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Roads need more corners
A Deluxe(CG13DE), 2 Clubbies(998 and 1275) and 2 Morris 1100Ss

I was thinking about this on the way down to check the mail.... (as you do)

If the spinning wheel has to double it's speed in response to low traction this may explain why an open diff works as well as it does in a low traction situation.
(earlier explanations of 1 wheel spinning on an open diff 4wd had me wondering how they got anywhere)

If the spinning wheel very suddenly needs to double it's speed, well that's going to take some time right? there's a fair bit of weight in brakes driveshafts etc and doubling is a big jump..
So until it is able to double it's speed there is still some torque and effort going to be imparted to the stationary wheel so you get a window in which to find more traction.

Kind of explains why as long as you can keep a 2wd open diff moving you've still got a chance.

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Roads need more corners
A Deluxe(CG13DE), 2 Clubbies(998 and 1275) and 2 Morris 1100Ss

It is probably easiest to think about the tracks taken by the left and right wheels while cornering, and the difference in distances they need to travel i.e. further on the outside and less on the inside. This is why it is called a differential!

OK... lets say a vehicle is travelling in a straight line at a speed that requires the wheels to do 50 RPM.

If it stays at the same speed and turns a corner with a radius that happens to require the outer wheel to travel 50% further than at the same vehicle speed in a straight line, and the inner wheel to travel 50% less distance, then the outer wheel will need to do 75 RPM and the inner wheel 25 RPM. The vehicle speed (i.e. engine or tailshaft speed) is still the same so the crown wheel is still turning at 50 RPM.

Whatever RPM comes off the inner wheel needs to be taken up by the other wheel. So, 25% greater distance further on the outer = 25% shorter on the inner for the same constant vehicle speed.

Extend this the other way to a turn so tight that the inner wheel does not need to travel any distance (pivots on the spot) and the outer wheel needs to travel 100% further than it would in a straight line.

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lol, nup, I'm going back to my lego...

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mind = blown.... redid it with lego, hold one wheel and the other does rotate at double the input speed

it's amazing the things I learn from this website (and Dr Mini)

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Glad to hear it Simon, otherwise my only other explanation was physics must be broken!

Lego - helping people learn since 1932!

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