Project Red Update – Final Drive Replaced

In the midst of working on Project Blue, welcoming a new R18 Classic to the garage, and all my work over the summer, one thing kept bugging me – the final drive…

I wrapped up Project Red at the end of last summer. It was a full rebuild with the vast majority of the work being done in my basement. Some components which clearly needed more expertise were outsourced. This included adding the circlip groove to the output shaft, rebuilding the heads, paint, and “rebuilding” the final drive. I’ve written about all that before, but here’s a short summary.

The original final drive had a wobble. If you watch the Boxer2Valve videos covering a very similar conversion to Red, you might see that project needed a new final drive as well. The wobble that William Plam found on that unit was essentially identical to what mine was doing. I sent it off to a fairly well-known shop with an owner who claimed to have had the same model as his first BMW. He assured me that he knew the bike inside and out. After a couple conversations, I felt good about working with him and sent the final drive off for service.

A couple months later, I got the unit back along with a bill for service and parts. I also got a call from him where he stated that the drive was in excellent condition and only needed a reseal. Great, right? Not so much.

Much later in the project, as I was finally getting the bike reassembled, I noticed the brake cam sleeve had been damaged when it was reinstalled and the brake cam would no longer fit into the unit. This was the first sign of a major red flag. That damaged part caused a side-quest to have a drift manufactured to remove the sleeve and install a replacement.

Later, when I had more of the bike assembled, I found that the wobble was still there. In a call to the shop, I was told that it was normal for this model and I should not worry about it. I decided to proceed with caution.

After a few hundred miles, I never felt quite right on the bike. It has plenty of power and runs excellent. Other airhead friends declared it to be exceptionally well-well dialed for performance. But in regular riding, I noticed a regular unsettled feeling with the bike. Sometimes, a bit of an unbalanced wheel vibration. Sometimes a bit of a shimmy entering and exiting a turn. It just never felt confident.

So, I began calling other experts for advice. A great call with Anton Largiader convinced me that the final drive needed another look. Another few months and I got the word. It’s a goner. Anton was unable to see a way to correct it properly, so it now lives in his shop for some limited salvage opportunities. Fortunately, I picked up another final drive as a spare when the opportunity arose. The spare was from a 90s R100 and had a better gear ratio (33/11 instead of 37/11), and a reported 45,000 miles.

On the down side, it was from a bike that lived outside, so the aluminum was stained and oxidized. Mechanically, it was sound, but I could not renew it enough to install as is. Rather than tearing it down and vapor blasting it, I opted to painting it with black Motor Coater. It turned out looking great and I got it installed on Red. I used the 1985 brake cam and the still new brake shoes I already installed, so I am giving up a little bit of leverage in the rear brake. Down the line, I will be replacing the rear wheel with one that has more life left in the brake liner and will update the shoes and cam to the 90’s versions then.

Today, we had a nice 55 degrees and I had a perfect opportunity to get Red out on the road for a bit. What a difference. The new(er) final drive is in great condition with no wobble at all. The bike feels planted and confident. After only a few miles, I knew just how much the old unit was causing uncertainty in handling. Now, there is none of the old effects and the bike feels much more predictable. Now, Red rides like I hoped. It’s a magnificent bike.

What remains, is the issue of different gear ratios though. This means the speedometer is now way off. It only registered 45mph when I was easily going 60mph. The current speedo has a 1.244 gear ratio and it needs a 1.112 ratio. I anticipated this and already have what I need. As an aside, there is precious little information about the design and configuration of airhead speedometers. While there is a good amount of information about the various ratios and the pared final drives out there, good luck finding much on the actual servicing of speedometers. Essentially nothing on the bits that designate the gear ratios.

I have a few speedos that I’ve nabbed and was lucky enough to already have a spare with the correct gearing. In my next post on Red, I’ll cover changing these over to a new gear ratio. That’s on deck for the next week, if I keep to the plan.

More to come.