Gear Removal

Hi Andre

I am a part of formula student team, and as one of the major objective of it is weight reduction we are planning on removing 5th and 6th gear from the KTM 500 exc engine. From the past year data at Silverstone we confirmed that the length of straight in the track are not long enough for engagement of 5th or 6th gear with our final drive ratio. But my major concern is the dynamic balancing of the gear box. So I just wanted to confirm whether dynamic balancing will be a problem?

Be most surprised but, if you are really concerned, you could make up a test fixture - might be worth some points as well.

Actually, you can probably reduce the gears' 'webing' and maybe the gear widths as well.

If you - the team - haven't already got them, I would suggest checking out past issues of RaceCar Engineering as they have frequent articles on Formula Student and, if I have the name correct, the earlier Formula First.

Thanks Gordon!!

Can someone else also share their views please!

I want to be sure before proceeding forward!

Removing a balanced component should not change the rotational balance. I assume the shaft the gear sits on have bearings at both ends.

Yes the shaft sits on bearing at both ends. I plan on removing 5th and 6th idler gear. This will help me remove 580 gm.

Thanks David and Gord for your help.

Don't forget you do have to add a spacer the width of the gear (on the splined side) -- I assume the side with the bearing will act as the necessary spacer. You probably also want to limit your shift mechanism so it doesn't go into the now neutral 5th if you inadvertently upshift from 4th. This is usually done with a tapped hole and a socked-head-cap-screw in the groove of the shift barrel on sequential gearboxes I've used (I configure it as a 5 or 6 speed depending upon the course and ratios needed).

David since all the gears are separated by the circlip so I don't think that there is a need for spacer. And since we are using electronic shift mechanism so I am planning to restricting upshift electronically with the help of gear pot intput.

Good luck!

Really really keen to hear how you get on with this Guru. We used a 450cc Husquvarna single when I was in FSAE, with a turbo, and had great results :-).

Thanks David!!!

Zac.Perston I will post on this thread as soon as I finish this project.

Thanks everyone for Help!!!

Could I suggest instead of taking 5th and 6th out, take 1st and 2nd out and change your final drive to suit. I would have to check my math but you may save more weight that way - you also have less inertia to overcome in the gearbox as the remaining gears have smaller diameters.

You can also look at changing over to an SX (the 450 SX gearset will fit in the 500 casing) gearbox which is a closer ratio gearbox and you could then possibly run all 6 speeds with a different final drive ratio.

For example, on the 2017 models, the enduro (EXC) ratios are:

1st - 14:36

2nd - 17:32

3rd - 19:28

4th - 22:25

5th - 23:24

6th - 26:21

Whilst the MX (SX) bikes are:

1st - 16:32

2nd - 18:30

3rd - 20:28

4th - 22:26

5th - 24:24

No 6th - you could keep the overdrive 6th from the enduro (EXC) box or just leave 6th out.

I will look into it. Thanks Ben!!