Gear-change trou­bles

Motorcycle Sport & Leisure - - Classified - Kevin Cameron

There are three lo­ca­tions for these gear-chang­ing prob­lems: 1) ex­ter­nal to the en­gine, 2) in­side the clutch cover (which means ser­vice does not re­quire en­gine re­moval), and 3) in­side the gear­box proper, ser­vice for which re­quires re­mov­ing the en­gine from the frame and split­ting the cases (ex­cep­tion: en­gines built with a so-called ‘cassette’ gear­box that can be re­moved from the en­gine in a few min­utes).

In the first cat­e­gory, I al­ways ask the rider if he/she has re­cently ad­justed the height of the gear-change pedal. It can hap­pen that a shifter op­er­at­ing through link­age jams against it­self, pre­vent­ing the mech­a­nism from achiev­ing full stroke. An­other kind of prob­lem arises if the change pedal can hit the pave­ment, in ef­fect shift­ing it­self. And yet an­other (I’ve only seen this once) is the bike whose change pedal is so heavy that hit­ting a hard bump can cause the in­er­tia of the pedal to change gear.

Take hold of the pedal and move it slightly up and down against its cen­tring spring. The move­ment should be easy and fluid, with no per­cep­ti­ble fric­tion. Why would there be fric­tion? Be­cause at some time in the bike’s life it has fallen over on its change pedal, bend­ing the shift shaft, cre­at­ing heavy fric­tion be­tween the shaft and what sup­ports it. A bent shift shaft can be re­placed with­out en­gine re­moval.

The clas­sic kinds of shift­ing prob­lems are 1) half­shift­ing, in which you end up in a neu­tral be­tween the gear you were in and the gear you wanted, 2) over-shift­ing, in which a shift some­times puts you in a neu­tral that is beyond the gear you wanted, 3) re­fused shifts, in which move­ment of the shift pedal pro­duces jerky mo­tion from the bike, a grind­ing feel­ing, and in­abil­ity to com­plete the shift, and 4), jump­ing out of gear.

The usual cause of half-shift­ing is that if the driv­ing dogs in the trans­mis­sion hit face-to-face rather than drop­ping neatly into en­gage­ment, sur­face rough­ness on the dog faces can pre­vent the dogs on one gear from slip­ping off and into en­gage­ment, with the re­sult that the change pedal does not go full stroke and a be­tween-gears neu­tral is found. This one is en­gi­ne­out-and-split to fix, and in­volves smooth­ing the dog faces against a piece of abra­sive pa­per.

Over-shift­ing is caused by in­cor­rectly set lim­its of shift-shaft ro­ta­tion, in­side the clutch cover (easy fix). Nor­mally the shift shaft’s ro­ta­tion is lim­ited by some­thing akin to an ec­cen­tric pin se­cured by a nut. It is de­sir­able that the mo­tion of the shift shaft be stopped by the pin about one mil­lime­tre be­fore the shift drum drops into the de­tented po­si­tion for the next gear. This is be­cause oth­er­wise, a fast-mov­ing shift-shaft can con­tinue driv­ing the drum all the way to its de­tented po­si­tion for the next gear, al­low­ing its in­er­tia to carry it past the de­tent and into a neu­tral beyond. Stop­ping the shift mech­a­nism a mil­lime­tre be­fore the de­tent al­lows the shift drum to be drawn the last mil­lime­tre into its next de­tent po­si­tion by the de­tent spring.

Re­fused shifts are the worst be­cause they can re­quire re­place­ment of gears and pos­si­bly shift forks. Let’s say you are a rider in a hurry, up­shift­ing with­out the clutch be­cause it’s racy good fun. De­pend­ing on how the dogs come to­gether, they can hit face-to-face, they can slip sweetly into en­gage­ment, or they can hit cor­ner to cor­ner.

The prob­lem comes with hit­ting cor­ner-to-cor­ner, be­cause the en­gag­ing dogs, with en­gine torque be­hind them, will if this is oft re­peated ham­mer what was well-de­fined dog cor­ner into a rounded shape. And as the dogs be­come rounded, one dog set be­gins to glance off the other, pro­duc­ing a pow­er­ful wedg­ing force that tries to kick the dog sets apart. This is shift re­fusal or jump­ing out. The rider’s foot mean­while is urg­ing the dogs to­gether, so the ham­mer­ing be­comes in­tense and great pres­sure is ap­plied to the shift fork that is try­ing to push the one gear’s dogs to en­gage those of the gear ad­ja­cent on the shaft. What we then see is not only rounded-off dogs but also a heat-blued and pos­si­bly bent shift fork. Re­place in­ter­nal parts. Pay bill.

Be­cause the rpm dif­fer­ence be­tween first and sec­ond gear is the great­est in the gear­box, there is the least time avail­able for the dogs to en­gage nor­mally, so shift re­fusal is com­mon­est on the first-to-sec­ond up­shift. All I can rec­om­mend is that rid­ers have a de­gree of com­pas­sion for ma­chin­ery. In the long his­tory of the TT races, it has mainly been those with such com­pas­sion who fin­ish.

Wait! If you were about to leaf past this page, ex­pect­ing it to be an­other of my te­dious so­cial trea­tises – don’t. My sub­ject this time is pure me­chan­ics: poor gear-chang­ing and its many causes.

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.