VCUs - The sweet spot

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Hippo said:
In theory the vcu should be able to take out any differences (absorb if you like) in normal driving conditions.
That would include straight line driving to tight circle driving.
Tight circle driving is where the prop speeds differ the greatest.
Hence peeps first suspect something is wrong when turning starts to feel different.
It's my view that as the vcu starts to "stiffen" up over time (or whatever factor) straight line driving which only puts a small stress on the vcu with minor prop speed difference isn't effected to such a condition where it's noticeable.
But tuning I tight circles or reversing round a corner does feel different as the vcu has stiffened and fights more against the props needing to turn at the greater end of the expected speed difference.
The mechanicals can't lose the additional force not absorbed by a good vcu so it's either down to tyres slipping or something mechanical braking. Hence ird and/or rear diff cracking.
Click to expand...
rrc and p38 v/cs stiffen over time, though it takes longer from my experience but then rr are equally driven front and rear. lr supplied us with test prodecure for v/c for their recon borgwarners but it was a very commonly replaced part because you needed to think of the length of life of the recon unit
 
That's all true - but my Series11A LTWT never broke when turning tight in 4WD - is the damage not more likely to be caused by prolonged overloading by a tight VCU not soaking up the smaller differences in normal running - only finally manifesting itself in a breakage in the tight situations - reverse turns etc - so comparative loading of good and bad vcus at known slippage is really relevant - and that's what we are all moving towards here.
 
Well, the intention of my post was not to start a race or whatever. It was more of a case of floating an idea I've been thinking about for a while, and seeing if those more knowledgable in the mechanics had any thoughts or experience on if it was a worthwhile effort or not. The two RPM method is probably the quickest and most straightforward approach though.

Interesting thought the temperature sensors too...

I'll keep you posted how this goes.

Jim.
 
We can take a stab at temperature reasonably well by stopping and looking at the VCU with a handheld infra-red temperature gun - it does not change that fast in normal running
 
If you look up the Turnip Test we've monitored heat before. Still on going for thems who want to measure the heat of their vcu.
 
dfossil said:
Got any numbers to share?
Click to expand...
Unfortunately it's all in log files so not easy to transfer/translate. Still trying to analyse it but put it to one side due to trying other stuff first. The thing I found was due to the rear diff being 3.214 ratio, the prop spins that many times for every 1 wheel turn. If only having 1 pulse per rev of the prop, you don't get enough pulses oft the props to be able to get good measurements as there int enough granularity. Hence you need more pulses per turn. abs sensors are easier as there's loads of pulses per wheel turn but you can't upset the sensor signal to the abs or it won't like it. The quicker option is to film yer vcu/props turning at slow speed to see what's going on.
 
Hippo said:
Unfortunately it's all in log files so not easy to transfer/translate. Still trying to analyse it but put it to one side due to trying other stuff first. The thing I found was due to the rear diff being 3.214 ratio, the prop spins that many times for every 1 wheel turn. If only having 1 pulse per rev of the prop, you don't get enough pulses oft the props to be able to get good measurements as there int enough granularity. Hence you need more pulses per turn. abs sensors are easier as there's loads of pulses per wheel turn but you can't upset the sensor signal to the abs or it won't like it. The quicker option is to film yer vcu/props turning at slow speed to see what's going on.
Click to expand...
have yu thought of fitting a couple of reluctor rings to the vcu/props?
 
The Mad Hat Man said:
have yu thought of fitting a couple of reluctor rings to the vcu/props?
Click to expand...
I used nails sticking out of a thing to make several electrical contacts as it spun round. Much easier with electrics/switch than messing with pick up signal from rings I think.
 
Well the units I have on order have a display resolution of plus or minus 2 at 5000 revs - the props will be doing circa 2700 revs at 100K/h with a theoretical difference in speed of 57 revs on tyre circumference alone (if typically 2% difference in loaded rolling radius front to back) - so if I get a resolution of hopefully plus or minus 1 rev that's a resolution of about 3.5% (for £6 each that'll do for a first attempt) First thing is to see if that is anywhere near what actually happens and take it from there. By adding magnets I could sample up to 10,000 pulses at plus or minus 3 if that gave any better info.
 
edit: just seen the /h - sorry

At 60mph the rear wheels are turning at 96.56km
That's 96,560 meters per hour
Divide by the typical circumference of a tyre at approx 2.15meter = 44,912 approx
44,912 is the revs of the rear drive shaft per hour at 60mph (assume driving in a straight line)
44,912 x rear diff ratio of (45 / 14) = 144,360
144,360 is the revs of the rear prop per hour at 60mph
Divide by 60 give revs per minute = 2406
Divide again by 60 gives revs per second = 40.1
 
Last edited:
OK - your 60mph 2406 prop revs - Grumpygels 100kph 2720 prop revs - same order of magnitude - haven't checked the figures but assume you are both about right with similar assumptions
 
Hmm, some interesting figures and discussion. I'm no VCU expert, but from what I've picked up in these threads is that I suspect that hard turning puts a bad strain with the goo stiffening quicker than it should as it ages. Mk1 of the measuring device will just send the speeds/difference to a little LCD matrix. So I can get the prop back on without a further delay. But... what could be very interesting is to use the accelerometers/gyros/compass in the phone, when the data is fed into the phone, to store the movement data alongside the prop's speed data, and see if we can spot the degradation over time.

Jim
 
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