Open post

Explaining the Slashes

Can someone please explain the difference between a /2, /5, /6, /7 (is there a /8?), and what exactly is an “oil-head” and a “k” bike? And what are these Earle forks that everyone speaks so reverently about?

OK, this is off the top of my head, so it may contain some errors.

2 = second version of some of the Earles fork bikes. The designation is often used incorrectly for all Earles fork twins. Not all were designated /2. Updated normal 500cc bikes beginning ca. 1960 were designated /2. The Earles fork is a swinging arm design, superior for sidecar rigs and off road applications, very plush riding, but a little heavy steering as a solo setup. They do not nosedive under braking, in fact they tend to rise in the front as the bike comes to a stop under heavy braking. These machines have very stout frames with sidecar bosses built in. The fork has adjustable geometry to accomodate the sidehack. The bikes were produced from 1955 – 1969. They came in normal and high performance versions of the horizontally opposed twin displacing either 500 or 600 cc. R50 and R60 designated normal performance. R50S, R69 and R69S are high performance models, the R69S making 42 HP and exceeding 110 mph. Despite the nomenclature, R69 models still displaced 600 cc, go figure…. 1969 models were shipped with either the Earles or a telescopic fork the same fork as the /5 series built from 1970-73. They were designated “US”. So an R69 US was a high perfomance 600 cc with a telescopic fork. There is also a 250 cc bike, this was a vertical single cylinder engine, uncommon in the US due to its lack of performance.

The remainder of this article is only available for Airheads Beemer Club members. Please login or purchase a membership here to join the Airheads Beemer Club, after which you will enjoy access this article and all members only content of the ABC web site.

Continue readingMore Tag

Re-Gearing an R80/7

Charles Bachmann, writes:

Hi Airheads,

I am a fairly new owner of a 1978 R80/7. It is a great, fun bike. However, I find that at speeds over about 60 mph, the thing turns higher rpm’s than I would like. I am considering going to a higher gearing.

Thanks for any feedback/advice!

Charles Bachmann
Raleigh, NC

I know how you feel. I’ll bet you’re always looking for 6th gear.

A 37/11 drive is the same that came on my ’74 R60/6. At 60 mph I’m showing at least 4000 rpm. Kinda high. But it’s only a R60. On the other hand, it’s the same drive that came on my ’83 R80RT. This was not acceptable to me. I replaced the 37/11(3.36:1) with the next higher-up 32/10 (3.20:1) from an R75/5. The drive housing has a different look & design, but everything fits fine. It was better, but still too revvy for me. Unlike you though, the ’83 R80 has lower compression than your ’78, therefore less horsepower too. I dare not try a higher drive because I have already lost a fair chunk of power.

You must be a member to view complete articles on this website. If you are already a member, you can log in here. If you aren’t a member yet, you can purchase a membership here.

Continue readingMore Tag
Scroll to top