Snow tyres - emergency braking and cornering performance comparison with conventional tyres
The use of snow tyres in New Zealand is becoming more common due to lower priced late model second hand passenger cars being imported fitted with older half worn snow tyres. Inappropriate use of snow tyres has been anecdotally linked to a number of serious and fatal crashes nationally. A primary consideration for crash investigators is to establish whether or not the use of the snow tyres contributed to the cause of those crashes. This paper describes comparative testing of snow tyres and conventional tyres for both wet and dry road surfaces. Testing was undertaken for both cornering and straight line emergency braking. The tests were performed by NZ Police Serious Crash Unit staff. Preliminary test results indicated a significant reduction in traction for snow tyres when compared to conventional car tyres during cornering manoeuvres, and a considerable increase in stopping distances on wet surfaces under emergency braking situations. An analysis of the results from initial testing showed a reduction in cornering (lateral) traction of around 6.0% on a dry road and 9.2% in the wet. Regarding stopping distances under emergency braking, in the dry there was a reduction in stopping distance of around 13.1%, but an increase in stopping distance in the wet of 16.0%. This equates to an increase in stopping distance on a wet road surface of about 2.6 metres at 50km/h and about 10.6 metres at a speed of 100km/h. The research, although limited in extent, show that snow tyres do not offer as much grip as can be achieved with conventional tyres, particularly in wet conditions and also during cornering in wet or dry road conditions. Further testing is required to determine if the age of the snow tyres and/or the road surface type will have an effect on the results (asphalt, bitumen chip etc). This testing should clarify the relationship between tyre type, tread depth and friction.