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traction ratings
 
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tires404
Frequent Contributor
Frequent Contributor
343 Points

Canada
PostYou have posted in this forum: Sun Oct 29, 2006 10:09 am   Post subject:  traction ratings Back to top 

I got lost there when I was following a discussion about someone rating the traction of tires. Where was it?. I have used a method for years now by checking the distance a pair of tire will pull up a ramp covered in ice.
As a mechanic I have always been surprised by the difference in predictability in handeling of various cars. Here is one of the articles the ministry of transport accepted.Let us look at 3 cars:

We will assume that each car has a weight of 1,000kg
It will have front traction (FT) and rear traction (RT)
Traction is calculated as weight x the adhesion of a tire and we will give a standard all season radial a value of 1.

CAR 1 1000kg 49% of weight on front, 51% on rear

The theoretical perfect car: It exists as sports cars and race cars. It has a split of 49% weight on the front and 51% weight on the back. (FT=490 RT=510) It has very predictable behaviour and if it starts to get out of control it will be felt on the front end first and will tend to go straight ahead and braking or slowing down will bring it back under control.
If I put radial tires on the front axle (A=1)and belted tires on the rear axle (A=0.6) then FT=490 and RT=306. At this point I have broken an MTO law and made the car susceptible to oversteer and if it goes out of control it tends to go sideways and roll over or veer into other lanes of traffic.

CAR 2 1,000kg
Front wheel drive 65% of weight on front, 35% of weight on rear

FT=650 RT=350 with new tires
After driving it 10,000km I rotate the tires; now FT=650 RT=315
Or I put snows tires on the front and partially worn all seasons on the back; now FT=975 RT=210
This car is very unpredictable and given to roll-overs and lane changes and even an experienced driver can easily over correct and loose control. see further information attached


CAR 3 1,000KG
Front wheel drive 65% of weight on front, 35% of weight on rear

4 new tires FT=650 RT=350
At this point this is the same as Car 2, however for winter driving if we put front tires on that feel fair on pure ice at about 50kph, which I have found to be about 6 months old all season tires with a value of about 0.8 so FT=520 and on the rear, softer ice-snow radial tires which will give the rear a value of RT=525 (FT=520 RT=525)
This car is very predictable and can out perform even an all wheel drive in some conditions. If it starts to get out of control you just brake, it slows down and you regain control.

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jwernerny
Site Admin
Site Admin
3123 Points

USA US New York
PostYou have posted in this forum: Sun Oct 29, 2006 11:32 am   Post subject:  Re: traction ratings - RWD vs. FWD on hills Back to top 

That's a very interesting excerpt. It also points to a problem my father complains about fairly regularly. The drive up to his house is fairly steep.

When I was living there, it was straight up the side of the hill -- about 100 feet of 50+ % grade. At the time he was driving a couple of Mk I VW Rabbits and a Mk I Golf (FWD). I had a pair of old Volvo 122s (RWD). He found that he always had a harder time getting up the drive in the winter in the Rabbits then I did in the Volvo.

Being a physics major, he realized it was because the front end was getting too light. As you lift a car, more of the weight will shift to the rear wheels. It might be hard to thing of at first, but if you consider two limits (0 and 90 degrees of tilt), it makes sense.

Assuming you have a car of 50/50 weight distribution. At 0 degrees of fore/aft tilt, both front and rear axles have 50% of the weight on them. At 90 degrees tilt (with the front in the air) all of the weight of the car is on the rear axle, so the car has a 0/100% Front/Rear weight distribution.

If we take tract to be weight x adhesion, then in the first case, the 90 degree case, the front wheels will have no traction. Thus, in a front wheel drive car, you will not go anywhere.

[I cut some corners in my explanation, but I think it is enough for a brief overview.]

_________________
John Werner - Driving Innovative, Out of the Box Solutions
Editor: The Snow Tire FAQ (http://www.snowtire.info)
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tires404
Frequent Contributor
Frequent Contributor
343 Points

Canada
PostYou have posted in this forum: Mon Oct 30, 2006 9:40 pm   Post subject:   Back to top 

Actually the vw has a weight ratio of about 58% on the front 42% on the rear. This gives it more predictable handling than say a neon at 65%/35%. It also gives it a fatality rate of 1/4 the rate of the neon. You can put 4 snows on a vw to give it more traction but doing so on a 65/35 will not decrease the fatality rate.

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