300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

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zippyonline
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by zippyonline » Sat Feb 09, 2019 9:27 pm

delta0 wrote:
Sat Feb 09, 2019 6:09 pm
Looks like Felix from Living with a Rotary is in a club of its own. Over 400 miles in an S1! :shock:
Cabby who is an ex member/owner also managed it. Also notice how it had the older non Milltek/petit racing exhaust on it, they did that a while back!
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by warpc0il » Sat Feb 09, 2019 9:55 pm

Nickp47 wrote:
Sat Feb 09, 2019 7:06 pm
Fair play to him, wonder what 42psi tyres do to the handling!?
As an aside, I did some data analysis with the Ford Powertrain engineers a few years ago, looking at the correlation between rolling resistance (and therefore fuel consumption) and tyre pressures.

On a perfectly smooth track the rolling resistance does reduce slightly if you increase the pressure, up to about 20-25% over the "normal" running pressure. beyond that there's no advantage.

However, on a test surface that better represents a real road, the benefit disappears at around plus-15% and then the rolling resistance goes up significantly as the pressure is increased further.

The actual numbers were slightly different with a variety of test vehicles but the pattern was much the same.

Once the tyre gets too hard, any imperfection in the road surface is no longer able to locally deform the tyre and the whole wheel has to rise to roll over. The energy required to lift the wheel is much greater than the scrubbing and sidewall deflection associated with a softer tyre.

What they did find is that an under-inflation by as little as 10% would cause a measurable increase in fuel consumption.

A similar increased could also be caused by have the steering geometry "off", especially front or rear toe, such that the wheels are trying to send the car in different directions, again a surprisingly small error was required to have a significant impact on fuel consumption - e.g. much less than can commonly be caused by bumping a kerb.
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by Dr. FrankenRex » Mon Feb 11, 2019 2:24 pm

As Zippy said, there's another S1 (ex)owner who managed it a while back too - although his was a 192.

As for the exhaust and wheel notice, this was done after the rebuild, and after the initial tuning session where the found the fuel pump to be struggling at high RPM, but before they replaced the fuel pump and before they finalised the tune. As such it has been rebuilt and 'run in' by then, as well as tuned across the rev range they were using :)

That puts me count at 3 owners in the 400 club, with each one owning a different variant of RX8 :lol:
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by PeteH » Mon Feb 11, 2019 3:19 pm

warpc0il wrote:
Sat Feb 09, 2019 9:55 pm
On a perfectly smooth track the rolling resistance does reduce slightly if you increase the pressure, up to about 20-25% over the "normal" running pressure
Rolling Resistance.jpg
Rolling Resistance.jpg (28.63 KiB) Viewed 108 times
You can see that the rolling resistance at 20psi is around double what it is at 50psi. And it doubles again if you lower it to 10psi. All that rolling resistance gets turned into heat. That heat is the direct consequence of the extra petrol you are using (you are burning petrol to heat the tyres). It is also why low tyre pressures will cause your tyre to fail. The extra heat causes parts of the tyres to exceed their temperature limits, and the bonds break down, so it delaminates, degrades, and blisters.
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by zippyonline » Mon Feb 11, 2019 4:34 pm

Have you got the corresponding "grip" chart vs psi... (yes I know this is waaaay more complicated, but there must be some rules of thumb?).
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by PeteH » Mon Feb 11, 2019 5:45 pm

Nice try!!

The rolling resistance versus pressure is a fairly universal trend, regardless of the tyre type and size. So this plot applies reasonably well to all road car tyres. The grip versus pressure plot is tyre dependent, temperature dependent, load dependent, and has many other dependencies. There is no universal answer, and each tyre has to be independently tested. Even then, it's one of the most difficult metrics to extract.

You know my job, so you know the answer is probably yes, but you also know that I'm not likely to be posting it here.... Sorry. :-#
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by warpc0il » Mon Feb 11, 2019 5:55 pm

I was aware of that chart, or at one's like it, and so were the engineers that were doing the testing.

However, the curve changes as soon as the road surface isn't smooth and the sort of ruts you see on real roads changes it dramatically.

The guys were surprised by how much, as were Bridgestone, that had test-rig results that suggested much less difference.
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by Dr. FrankenRex » Mon Feb 11, 2019 6:06 pm

I'd imagine that the 'rough roads' principle - as I have dubbed it - wouldn't be as impactful on the motorways though?
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by wiggles » Mon Feb 11, 2019 6:28 pm

warpc0il wrote:
Mon Feb 11, 2019 5:55 pm
I was aware of that chart, or at one's like it, and so were the engineers that were doing the testing.

However, the curve changes as soon as the road surface isn't smooth and the sort of ruts you see on real roads changes it dramatically.

The guys were surprised by how much, as were Bridgestone, that had test-rig results that suggested much less difference.


There has been similar tests for road bikes (pedal variety) where everyone used to think skinniest tyre possible at highest pressure would offer the least rolling resistance but more recently manufacturers have tested and realised a wider tyre at lower pressure actually offers lower resistance in "real world" conditions eg. Not perfectly smooth roads. Not as initiative as you might assume but makes sense.
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by PeteH » Mon Feb 11, 2019 7:23 pm

I agree with the concept that a stiff tyre can cause an increase in rolling resistance but, as with all engineering, it depends! Here's a couple of extreme examples:

When the pneumatic tyre was in the initial stages of invention / development, an experiment was conducted where the force to pull a push bike was measured. A "standard" solid rubber tyre was measured against a new-fangled pneumatic tyre. The pneumatic tyre won hands down, and became the dominant technology. On a perfectly smooth surface a solid tyre would have much low rolling resistance, but on a real surface the way a pneumatic tyre "flows" around the surface means it has much lower rolling resistance.

Trains have almost totally rigid "tyres" (i.e. steel wheels). They have these to minimise the rolling resistance. But they only work because of two factors; 1) they run an very smooth steel tracks. 2) The bogies have very compliant suspension, allowing the wheels to traverse over the joints and imperfections without resistance.

Basically, if you have a good suspension system, or a very smooth surface, you can afford to have more rigid tyres. For any given vehicle, suspension system, and road there will always come a point when the tyre is too rigid, and the rolling resistance will start to rise again.

Because this is a function of the car and the road, we tyre engineers tend to ignore it! ;) :D
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