With win­ter tires, it’s all about the grip

Winnipeg Free Press - Section F - - AUTOS - JIM KERR

QUES­TION: Last week somebody was ask­ing about stud­ded tires, and you wrote how you feel ice tires are su­pe­rior. I was won­der­ing if all win­ter tires are ice tires and if not, how can you tell the dif­fer­ence? An­swer: Great ques­tion. The Cana­dian Rub­ber As­so­ci­a­tion says “tires dis­play­ing the moun­tain snowflake sym­bol meet or ex­ceed in­dus­try-es­tab­lished snow trac­tion per­for­mance re­quire­ments and have been de­signed specif­i­cally for use in cold weather and se­vere weather con­di­tions.” Th­ese are clas­si­fied as “win­ter” tires, with a moun­tain sym­bol (which looks like a jagged tri­an­gle) on the side­wall of the tire and a snowflake inside it. How­ever, not all “win­ter” tires are cre­ated equal. Some are bet­ter on icy roads than oth­ers. Grip on ice is de­rived by two meth­ods. The first is achieved by the rub­ber com­pound it­self. If the rub­ber re­mains more flex­i­ble as tem­per­a­tures drop, then it can con­form to the icy road sur­face and pro­vide bet­ter grip. Of­ten, spe­cial ad­di­tives are in­cluded in the rub­ber com­pound, in­clud­ing things such as wal­nut-shell par­ti­cles, to help with the grip. The sec­ond method of grip­ping icy roads is the me­chan­i­cal de­sign of the tread. When you put pres­sure on ice, a thin layer of wa­ter forms on the sur­face and makes it even more slip­pery. Any­one who curls knows this prin­ci­ple well — sweep hard and the rocks travel faster and fur­ther. Part of the tread’s me­chan­i­cal de­sign is to pro­vide voids in the tread sur­face so the wa­ter can be dis­placed from the sur­face and held in the tread un­til the tread is no longer in con­tact with the ground. This al­lows the rub­ber to get bet­ter con­tact with the ice and more trac­tion. The other me­chan­i­cal method used is to build thou­sands of sipes (fine cuts) into the tread sur­face. The edges of each sipe cut can grip for bet­ter trac­tion. Gen­er­ally speak­ing, tires with more sipes on them will work bet­ter on ice, but there are more fac­tors in­volved than just the num­ber of sipes. The tire man­u­fac­turer’s de­scrip­tion of the tire will in­di­cate if it is bet­ter suited for ice trac­tion than other win­ter tires of the same brand, so this is the best way to pick the best “ice” trac­tion tire from a par­tic­u­lar brand. Also, don’t con­fuse some of the lat­est “foursea­son tires” with win­ter tires. Four-sea­son tires may be bet­ter in cold weather than allsea­son tires, but they are still not as good as win­ter tires. Ques­tion: I have a 2008 Chevro­let Sil­ver­ado 4x4 truck, and in the lat­est cold snap, it wouldn’t go into gear. The en­gine started fine, but a warn­ing mes­sage came up on the in­stru­ment clus­ter to “Ser­vice the 4x4 sys­tem.” Ev­ery­thing seems OK, but if I put the trans­mis­sion in gear, the ve­hi­cle doesn’t move. It has an au­to­matic trans­fer case, and the small in­di­ca­tor light on the con­trol is flash­ing. Is this a prob­lem with the trans­mis­sion or has some­thing in the 4x4 sys­tem failed? An­swer: The prob­lem is in the trans­fer case con­trols, and there is likely noth­ing wrong with the trans­mis­sion. The trans­fer case uses an elec­tric mo­tor and gear pack assem­bly on the side of the trans­fer case to switch be­tween two-wheel drive, 4-high and 4-low ranges. In au­to­matic mode, it is au­to­mat­i­cally shift­ing be­tween two-wheel drive and 4-high mode based on the amount of speed dif­fer­ence in the front and rear drive­shafts, which rep­re­sent tire slip. There is also a “Neu­tral” mode in the trans­fer case, and I think your trans­fer case is stuck in this neu­tral mode. Neu­tral mode is re­quired to al­low the trans­fer case to be able to shift be­tween 4-high and 4-low modes. It can also be used when the ve­hi­cle needs to be towed with the tires on the ground. The elec­tric mo­tor on the trans­fer case (GM calls it the en­coder mo­tor) likely needs to be re­placed. Be­fore do­ing that, check the wiring to the trans­fer case en­coder mo­tor for dam­age from mud or ice. If the wiring looks good, re­move the en­coder mo­tor assem­bly. The trans­fer case can be man­u­ally shifted into gear with a wrench when the en­coder mo­tor is re­moved, so you should be able to drive the truck to a re­pair shop if nec­es­sary.

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