72 hub drive speedo

[paul]

Hey POGers, why are the speedo drives on the 125 (Sachs)6 day on the engine and would it hurt the value of my 72 to install a drive for the speedo like my 250 has on the front wheel?. I have the stuff to do it(VDO) but would have to drill 2 holes so the drive will go into the hub. The holes would not show but I wanted to know why the factory choose to do it the way they did. My old boss used to say "the factory did that for a reason" and before changing something I find its best to ask someone who knows. Thanks,TTP

[SouthRider]

I believe that the engine drives were an old relic from the use of that engine for European street bikes. The problem is that they were not very accurate on dirt bikes due to rear wheel spin.

Most serious enduro racers did away with the engine drive & adopted the VDO front wheel drive just like you suggest doing.

I have been contemplating whether to put an old VDO on my Berkshire project bike or to mount a newer digital speedo. I will probably mount neither until & if I race the bike somewhere that I need a speedo.

The VDO's really were a pain because of their size & weight, were hard to keep mounted correctly, and suffered a lot of damage from limbs & spills. The front wheel drives weren't extremely reliable either.

It all depends on what you are doing with the bike - Is it a "corrrect" restoration? Will you race it in Enduros where a working speedo is critical?

A front wheel drive VDO would certainly be considered a "period modification".

It really comes down to how correct a restoration you are doing, and to what purpose are you building the bike.


Clark




Clark A Gristina

1979 SERA Louisiana State Champ

[john durrill]

Paul,
 The VDO speedometer that is used with the front wheel drive gear box is a different ratio from the speedometer used with the counter shaft drive unit. Look on the bottom or sides of your VDO and see what #s are engraved on it. Front wheel drive speedo should have a 1.42 ratio i think. Mark Anna did an article on how to figure which ratio speedo you need for the different drive systems. I think i can still post that article if no one else has it handy.
John D.