Fo­cus on

Not your typ­i­cal man­ual gear­box, but not a stan­dard au­to­matic shifter ei­ther, the dual-clutch trans­mis­sion (DCT) is sup­posed to the pro­vide ad­van­tages of both. So, why is it los­ing favour?

Leisure Wheels (South Africa) - - CONTENTS - Text: GG van Rooyen

Dual-clutch trans­mis­sions

In its out­ward op­er­a­tion, the du­al­clutch trans­mis­sion (DCT) is so sim­i­lar to a stan­dard au­to­matic that it is vir­tu­ally in­dis­tin­guish­able from it. There is no clutch. You sim­ply con­trol the throt­tle and the brake, and the car fig­ures out the rest. How­ever, a DCT dif­fers from your stan­dard au­to­matic gear­box in a num­ber of im­por­tant ways. In fact, in some ways it is more sim­i­lar to a man­ual shifter than an auto ‘box; more of a ‘clutch­less man­ual’ than any­thing else. In the world of pro­duc­tion cars, the DCT is a rel­a­tively new phe­nom­e­non, but in the rac­ing world it has been around far longer as the semi­au­to­matic trans­mis­sion or the se­quen­tial man­ual gear­box.

THE MAN­UAL GEAR­BOX

Be­fore we look at the DCT, it’s prob­a­bly a good idea to briefly ex­am­ine the in­ner work­ings of your stan­dard man­ual gear­box. When you de­cide to change gears with a man­ual gear­box, you press down on the clutch pedal. The pedal ac­ti­vates a clutch, which dis­con­nects the en­gine from the gear­box. Power flow be­tween the two is thus in­ter­rupted. You then se­lect the de­sired gear with the shifter, which moves a toothed col­lar from one gear to the next.

Im­por­tantly, grind­ing would oc­cur if th­ese two weren’t mov­ing at the same speed, so a man­ual gear­box has syn­chro­nis­ers (from here the word ‘syn­chro’) in it that help match th­ese two speeds. Typ­i­cally, there is a cone on the gear that slots into a groove on the col­lar. The gear and the col­lar then syn­chro­nise their speeds through the fric­tion cre­ated where the cone and col­lar meet, and once they move at the same speed, the col­lar slides out of the way so that the ac­tual teeth can be en­gaged.

THE DCT GEAR­BOX

As should be ob­vi­ous to any­one who has ever driven a ve­hi­cle with a DCT, there is no clutch pedal, but that should not lead you to be­lieve that there is no clutch. As its name sug­gests, a dual-clutch trans­mis­sion ac­tu­ally has two. As with a typ­i­cal au­to­matic gear­box, clever hy­draulics and elec­tron­ics man­age the swap­ping of cogs, but in­stead of your usual torque con­verter and

bell hous­ing, you have two clutches that make the gear­box more like a man­ual one than your typ­i­cal auto trans­mis­sion.

So, why two clutches? The first clutch is re­spon­si­ble for the odd gears (first, third, fifth), while the sec­ond clutch takes care of the even gears (sec­ond, fourth, sixth). While the ve­hi­cle sets off in first gear, the sec­ond clutch has al­ready en­gaged sec­ond gear. When the ve­hi­cle shifts from first to sec­ond gear, it is ac­tu­ally swap­ping from one gear­box to another. When shift­ing from sec­ond gear to third gear, the ve­hi­cle switches back to the orig­i­nal gear­box.

A DCT trans­mis­sion is sort of like hav­ing two gear­boxes in one. The ad­van­tage is that the up­com­ing gear can be pre-se­lected, mak­ing shifts smooth and vir­tu­ally in­stan­ta­neous. Also, a DCT al­lows the driver to con­trol up­shifts, usu­ally with the help of pad­dles be­hind the steer­ing wheel. Sim­ply tap the pad­dle, and the trans­mis­sion in­stantly swaps cogs. A dual-clutch trans­mis­sion can gen­er­ally also per­form some­thing called a matched-rev down­shift. When the driver se­lects a lower gear via the pad­dleshift, the clutches are dis­en­gaged and the en­gine is revved to pro­vide the ex­act speed needed for the gear. When you down­shift, you get a smooth shift with­out any lurch­ing. Some DCT gear­boxes al­low you to skip gears when down­shift­ing, and once again, the gear and en­gine speed is matched to pro­vide a smooth and ef­fi­cient shift.

AD­VAN­TAGES OF THE DCT

The man­ual gear­box is quickly go­ing ex­tinct. They’re still used in ve­hi­cles be­cause of af­ford­abil­ity, but many pre­mium man­u­fac­tur­ers have done away with them. It’s ba­si­cally im­pos­si­ble to buy a Ger­man lux­ury ve­hi­cle with a man­ual trans­mis­sion th­ese days. Many peo­ple have no prob­lem with this, since an au­to­matic gear­box is eas­ier to live with, es­pe­cially if you’re sit­ting in traf­fic ev­ery day. How­ever, for true driv­ing en­thu­si­asts, the dis­ap­pear­ance of the man­ual shifter is a tragedy, since an au­to­matic gear­box doesn’t of­fer the same thrill and con­trol. The DCT is sup­posed to be a sat­is­fy­ing ‘au­to­matic’ al­ter­na­tive to the old which school man­ual gear­box. With its quick shifts and fa­cil­ity for the driver to take con­trol, a DCT of­fers the con­ve­nience of an au­to­matic trans­mis­sion, while still pro­vid­ing a fun and in­volv­ing driv­ing ex­pe­ri­ence for those who care about that sort of thing.

Sadly, the DCT may be go­ing the way of the man­ual gear­box, too. While no one is deny­ing the ex­cel­lent per­for­mance of­fered by a dual-clutch trans­mis­sion; cost, com­plex­ity and weight is an is­sue. As men­tioned, a DCT is es­sen­tially two gear­boxes in one, means they are in­cred­i­bly com­plex, and there­fore pricey to build. Some man­u­fac­tur­ers, like BMW, for ex­am­ple, feel that stan­dard au­to­matic gear­boxes, like those pro­duced by ZF, are so good that they are as good as a DCT ver­sion. Peter Quin­tus, vice pres­i­dent for sales and mar­ket­ing at BMW M, said ear­lier this year that he be­lieved DCT gear­boxes, at least within BMW, would be gone in a few years. Ac­cord­ing to him, ZF’s eight-speed au­to­matic trans­mis­sions are now as slight and fast as the DCT ones.

Main: With man­ual gear­boxes dis­ap­pear­ing from high-end per­for­mance ve­hi­cles, du­al­clutch trans­mis­sions are con­sid­ered the best al­ter­na­tive. But even DCTs are now be­ing ig­nored in fa­vor of stan­dard au­to­matic gear­boxes. Op­po­site page, top: Some ex­am­ples of du­al­clutch trans­mis­sions. Th­ese gear­boxes are large and com­plex. Op­po­site page, bot­tom: Here a BMW DCT has been fit­ted with a shifter that’s aimed at re­cre­at­ing that man­ual trans­mis­sion feel.

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