Blocking first gear out on a Sachs for mx?

[Mike Lenz]

I brought up this discussion because I may build a Penton/sachs 100 for mx, and if so would like to block out first making it a 5 speed to be able to properly gear it for mx without the opportunity to downshift into first by mistake, and also take advantage of the closer ratios between 2nd thru 5th, as I have done with my 125 KTM motors. Donny Smith mentioned what sounds like a relatively simple way to do that below. My question is if doing that mod would you not have a neutral? Or would neutral be at the bottom, below what would now be first? I dont know much about Sachs trannys, so please inform.
If I can do this I plan on running 13/66 or 12/61 gearing to get "first" gear about at a 31 overall ratio. If a 30 ratio is right for a 125 (13/64 or 12/59), then I would expect a 100 needs geared a little lower?
TY

[Mike Lenz]

Another thing on the subject immediately below. The 006 100/125 parts manual shows a different Sachs gear selector boss and a bracket, etc that says its "for gear lock". Alan Buehner has this gear selector boss on his web parts Sachs engine parts. Al/anybody know what this is for?

[Mike Lenz]

I have never seen this, but back in the early 70's there was a wrench at Stan's Honda in Duncan OK, who also sold Monarks, that told me he took a 6 sp Sachs engine for a Monark he was building and used the Gear Lock selector assy from a 5 sp and adjusted it so that 2nd gear in the engine was actually 1st gear on the Gear Lock, thus 6th gear was 5th on the Gear Lock. This effect make the 6 sp gearbox a 5 sp gear box without the use of 1st gear. Like I said, I've never seen this or attempted it, but he swore it worked well for his rider. I will do some research and see if I can even track the guy down.

Donny Smith
Paragould, AR

[Mike Lenz]

Found out some interesting stuff from John's numbers using the gear ratio site I have previously mentioned at //www.nightrider.com.
The Sachs engine final drive ratio to the rear wheel is as following:
13/60 sprockets=28.35 2nd gear overall ratio
11/54 sprockets=30.16 2nd
75 RM 125 1st gear with stock gearing 31.08
76 RM 125 1st with stock gearing 35.15
98 SX 125 1st with stock gearing 30.04
07 SX 144 1st with stock gearing 30.03
Interesting. Around 30 seems to be the magic number for MX low gear for a 125. However, the post I read about Mettco probebly is the most accurate. Gear it down as much as possible just not to run out of top gear on the fastest straight...unless your like me and will shift down to far if 1st is too low. FYI, all the newer 250's are around 20 for first.

Some other interesting things are that the gear spacing on a Sachs and KTM are very similer 2nd thru 5th. Sixth is spaced out more on the 125 KTM, but not 250/400? Even more amazing is this ratio seems to work ok for 125 thru 400! However, these ratios are wider than a more modern close ratio 125, and even a modern close ratio 250 a bit. Also, first to second is wider on the Sachs! Second to third is much closer together than first to second, and from second to fifth the ratios get closer together. As stated, some trannys do space out sixth quite abit for top speed.

Verdict: For a Sachs and 70's KTM engine 2nd thru 5th gearing is what you want to focus getting the correct overall gearing ratio on for MX. For me this also means I have to block out first on mine or I will inevitably shift down too far, inevitably at just the wrong time in the race!

I included this cuz it seemed relevant to what were talking about here

[Carl Hill]

I'm just happy when I shift a Sachs transmission and it goes into ANY gear lol

[Mike Lenz]

Im also going to do this, from a old JP Morgen post:
For those of you with Sachs five and six speed engined bikes, you may or may not know of a modification I developed to greatly improve the shifting quality of these bikes. I have the openings for the selector key in each gear made wider to give the key more time to engage the next gear. I don't do the cutting because I don't have the specialized machinery required: wire feed EDM machine.
He has them opened up 4 mm.