quattro Digest, Vol 7, Issue 102 - hydroplaning

[l.leung at juno.com]

Au contraire, 

The more open tread of, say, snow tires, end up with
MORE individual tread blocks in contact with the road.
So the TOTAL contact patch remains constant, however,
since each tread block is comparatively small compared
to say, a summer tire, the contact pressure is higher
ON A PER BLOCK basis. This is needed to help the snow
tire penetrate through the snow surface. TOTAL pressure
and TOTAL area remain constant, it's simply the distribution
that changes. AND as stated before by Huw, Doug and others,
the higher the pressure in the tire, the smaller the patch,
the higher the contact patch pressure. It's basic statics
in physics and engineering. All forces must balance, or the
car will either fall through the pavement, or fly. Basically,
hydroplaning occurs when the dynamic water surface pressure equals 
the weight of the car. Therefore, no road contact.

LL - NY


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Date: Fri, 28 May 2004 06:44:32 -0400
From: George Harris <harchris at smokesignal.net>
Subject: Re: hydroplaning
To: audi at humanspeakers.com
Cc: quattro at audifans.com
Message-ID: <40B71810.1070800 at smokesignal.net>
Content-Type: text/plain; charset=us-ascii; format=flowed

I think it's the 'contact patch' thingie where the theory falls apart. 
With an aggressive tread, less of the rubber makes contact at the same 
pressures. Take the snow tire as an example. The contact patch is 
relatively small compared to a touring tire at the same pressure. The 
rubber in the tread is supporting the snow tire on the knife edge of the 
tread allowing it to cut through the puddle and squeeze the water out 
through the valleys between the tread.

If the contact patch were the same for all tread designs at all tire 
pressures then I would agree with the theory, but I don't believe it is. 
With an aggressive tread, less rubber actually meets the road at the 
same tire pressures. The pressure per sq inch is greater on that rubber 
due to the support given by the rubber in the tread. The tires in 
question, a belted touring tire, and a belted snow tire or rain tire, 
deflect the same amount presenting the same 'contact patch' but on the 
rain tire with aggressive tread, the actual rubber meeting the road is 
smaller.

If this is not true, why does every manufacturer have special designs 
for wet road handling such as the Michelin HydroEdge?

Huw Powell wrote:
> 
> I think this is the way that rule of thumb formula works.  At those 
> speeds, as long as there is a layer of water, the tire can no longer 
> penetrate it fast enough to make any contact - tread pattern does not 
> matter because the "contact patch" is doing the hydroplaning.
> 
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