Tyre pressures and braking performance

**steveG** · 20th June 2011, 10:50 PM

Had an interesting experience today...
I took my County to the engineer for a final inspection, and one of the things he did was a brake performance test.
Standard front disk, rear drum setup - Isuzu engine - and running 255/85-16 BFG KM2's. About 30kgs of tools/junk in the back, and 1/2 a tank of fuel.

In the first couple of test stops on a dry asphalt road we did he could only manage 0.71G (the acceptable limit is 0.80G IIRC ) as the fronts were locking.
We then did another couple of test stops on a well used clay/stone based road (ie no loose stones and a rough surface due to stones sticking up through the clay). Easily achieved 0.8 with no lockup so the brakes themselves were fine.
Obviously the tyres were getting more grip on the unsealed road, but the low performance on the sealed road wasn't acceptable.

The engineer was confident that we could get the asphalt braking to improve with a slight change in tyre pressure so we headed back and increased the tyre pressures from 38psi to 40.
We then repeated the earlier tests, and repeatedly got 0.83G, with the lockup now happening on the rear tyres. Possibly a bit more experimentation with the pressures would have achieved an even higher result, but that was good enough so we left it at that.

I'm sure those of you that have been involved in racing etc already know, but for me it was quite an eye opener to see just how much difference a couple of PSI made.

Steve

scott oz · 21st June 2011, 07:57 AM

Tyre pressure is interesting.

I went to an insurance sponsored fleet management seminar and they had a rep from a tyre manufacture present.

He stated that of the 20 cars he tested in the parking bay just before the presentation 15 had low tyre pressure.

He stated further that as a manufacture they always recommend that tyres be inflated to the max pressure as indicated on the tyre wall. Stated better wet / dry braking. Drastically improved type wear over under inflated tyres.

My wife’s car comes back from the dealer every time with tyre pressure of 30PSI where the tyre is marked at 45PSI. So I get to pump them up.

**Dougal** · 21st June 2011, 02:39 PM

45psi in a car will be a pretty hard ride.

If the tyre manufacturers truely recommend inflation to the max pressure, then they should have a long hard discussion with the car makers about tyre placards. I beleive there is a lot more to this story.

**steveG** · 21st June 2011, 02:50 PM

Originally Posted by scott oz

Tyre pressure is interesting.

I went to an insurance sponsored fleet management seminar and they had a rep from a tyre manufacture present.

He stated that of the 20 cars he tested in the parking bay just before the presentation 15 had low tyre pressure.

He stated further that as a manufacture they always recommend that tyres be inflated to the max pressure as indicated on the tyre wall. Stated better wet / dry braking. Drastically improved type wear over under inflated tyres.

My wife’s car comes back from the dealer every time with tyre pressure of 30PSI where the tyre is marked at 45PSI. So I get to pump them up.

A blanket statement of always max pressure doesn't sound logical - either that or there was a little more context involved in his statement (such as a particular vehicle/fleet type where they are always fully loaded).
Pretty sure the max pressure on the sidewall of the Maxxis Bighorns (LT rated) that I had on my D1 was about 80psi. That pressure on a unladen D1 would give about a 70mm diameter contact patch I reckon

Steve

Tombie · 21st June 2011, 05:39 PM

A good guide is +10% over placard recommendations.

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