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The 81- forks are as Rob says very different. They are essentially the same in principle as the the forks first introduced in 78 on the R45/R65. Though by 81 they had adopted the shortened sprung valve body to reduced noise, a mod which was also applied to the R45/R65 forks somewhere around that time.

barryh wrote: ↑Fri Dec 26, 2025 4:22 amThe 81- forks are as Rob says very different. They are essentially the same in principle as the the forks first introduced in 78 on the R45/R65.

I think you may be thinking of the Monolever forks there, i.e. when the axles were moved to the bottom of the slider, instead of using a leading axle, and the 18" wheel.

I'm thinking about the intermediate ones that shared the leading axle design, the same length and diameter of tube, same width and diameter of axle, same triple tree/yokes, same springs, same 19" wheel ... but moved to Brembo brakes.

I was wondering whether those sliders could be attached to the older fork legs, with the older dampener.

I have no doubt the later dampeners were better, I just wanted to know if the sliders would fit. A budget brake upgrade.

Todd wrote: ↑Mon Feb 23, 2026 1:00 pm

barryh wrote: ↑Fri Dec 26, 2025 4:22 amThe 81- forks are as Rob says very different. They are essentially the same in principle as the the forks first introduced in 78 on the R45/R65.

I think you may be thinking of the Monolever forks there, i.e. when the axles were moved to the bottom of the slider, instead of using a leading axle, and the 18" wheel.

I'm thinking about the intermediate ones that shared the leading axle design, the same length and diameter of tube, same width and diameter of axle, same triple tree/yokes, same springs, same 19" wheel ... but moved to Brembo brakes.

I was wondering whether those sliders could be attached to the older fork legs, with the older dampener.

I have no doubt the later dampeners were better, I just wanted to know if the sliders would fit. A budget brake upgrade.

I'm certainly not thinking of the Mono legs... As I've said before, I'm pretty sure the answer is no. Yes the tubes are the same diameter, yes, they are the same length, yes, they fit in the same yokes, yes the axles are the same length and diameter... and fit the same wheels. In fact they are the same axles.The difference is in the internal arrangements of the forks, the places where the damper assemblies mate with the tubes and the places where they are secured to the sliders. It may be possible, with a bit of bodging and machining to make the various parts work together but you specify a 'bolt on' solution. By far the best solution would be to source a complete pair of post '80 fork legs, if possible complete with calipers, which will fit in the yokes and will work with only a very few minor additions and adjustments.

Rob

Rob Frankham wrote: ↑Tue Feb 24, 2026 5:49 amI'm certainly not thinking of the Mono legs...

You weren't, but barryh mentioned the R45/R65 fork which looks like the R80/R100 monolever fork (1985/1986), i.e. axle directly at the bottom of the slider. You and I are discussing the 1981 to 1984 twin shock/Brembo forks. Incidentally, why did BM change the forks/front end design on the big bikes to the 18" set up, or why did it take them so long to so? Was it for performance, or just product line rationalisation, i.e. only one basic fork design?

Has anyone stuck an 18" front end on the older models, and how did it effect handling? It certainly has other benefits, e.g. the brace etc.

Yeah, without having an actual BM engineer chime in, it's impossible for me to tell whether the changes were for performance, or cost cutting; and without have two complete examples laid out beside themselves, I can't tell.

It seems that my biggest problem in understanding was the misleading information on the RealOem parts fiches, i.e. where it listed other models as using the same parts ... which it wasn't actually the other models, but just the other models' parts fiches that listed the same items, i.e. used the same drawings. I need to find some original contemporaneous parts books.

Motorworks in the UK list an ATE to Brembo but it looks like it costs about £1,000 to do. Either way one turns, it seems the easiest or more cost effective way to do it ... is just sell the bike, and buy one with them pre-installed by the factory when it was sold. May be own two bikes for a while and mix and match the best parts, then sell the other.

Mine has had its paint work done, and frame powder coated, and so I thought it was worth investing in updates. But to do it well simply isn't cost effective. I found a narrow hub, which you need for wire wheels ... but, same again, for what it would cost to build up just one wheel to suit, I could buy two alloys. The economics of upgrades never really make sense, unless you just approach it as entertainment and paying to enjoy the process, which I don't really. Especially when you compare it with what bikes can buy second hand at the present.

Same with forks. Most that I've seen require new legs or rechroming, and would benefit from bead blasting or coasting. By the time you add it all up, you could afford a complete. low mileage 2005+ bike for the same price as the parts to upgrade.

It seems the cost of "essential" replacement parts is increasing exponetially, while the cost of complete bikes is plummeting.

I can see why some people are just turning to breaking them up for the sake of business ...

The reasoning surrounding BMW decisions is often complex and difficult to follow but here is my understanding of the way things went...

Firstly, they have a history of trying out innovations on a 'minor' model change before introducing them over the whole range...

By the late 70s, the front fork design (now around ten years old) was becoming dated and they went for an upgrade. This involved a complete redesign of the damper system and was introduced for the '81 model year and included the change from ATE to Brembo front calipers. The new design was made to use as many of the older parts as possible (wheels, brake rotors, yokes etc.) but was otherwise completely different. The '81 model year also introduced a large number of other changes throughout the model range and you could almost say that post '80 models are almost a different design.

The R45/R65 models had previously come out in 1978 and had a number of innovations that were not carried forward to the 'main stream' bikes at that date. These included the 18" front wheel and some fork changes that weren't but also things like twin piston front brakes which were.

Moving on to 1984, BMW were intending to run airhead production down to a minimum in favour of the recently introduced 'K' series retaining only the 'GS' models and for some reason, (probably because there was no equivalents yet in the 'K' range) the R80RT and the R65RT. They redesigned the frame without greatly changing the dimensions, and used front suspension components, wheels & etc. based on those used for the early 'K' series. Some of the innovations had been first seen on the R45/R65 range.

The plan didn't go - well - according to plan because there was an outcry among BMW users. They liked the Boxer designs and many didn't like the 'K' series so they backtracked and introduced new (old) models based on the new rolling frame. This must have been acutely embarrasing as they already produced 'last ever' specials for the R100RS and RT models.

Anyway, that's a potted version of my take, there are other variations and the development of the BMW 246/247/248 series airhead was a continuous process to the extent that it is claimed that the actual parts commonality between the machine introduced in 1969 and that last produced in 1995, apart from standard fasteners, was reduced to one part... the connecting rod. Not sure if thats exactly true but it sounds about right.

Rob

Rob Frankham wrote: ↑Wed Feb 25, 2026 4:09 amThey redesigned the frame without greatly changing the dimensions, and used front suspension components, wheels & etc. based on those used for the early 'K' series. Some of the innovations had been first seen on the R45/R65 range.

Presumably to 'de-rubberise' the old frames a bit?

It seems the headstock and bearings all seem to be similar or identical (making allowances for RealOEM being incorrect again), so which of the later fork assemblies fit and how do they handle? Can one move up to 38mm/41mm forks?

If the headstock angle remained the same, presumably the new forks and effectively 1/2" smaller tyre (radius) would tighten up the rake angle a little, leading it to turn in more quickly, but at the cost of a little high-speed stability? Offset by the increased rigidity of the new set up.

I've never sat down to try and work out rake and trail but, back of a napkin calculation, I'd guess it could be correct by something as minor as a lower profile rear tyre.

The 'Gummikuh' description was more a comment on the soft, long stroke suspension favoured by BMW which was )is) very different to the sometimes harsh suspension favoured by a lot of the opposition. The frame itself is at least as rigid as the contemporary opposition in the late 60s when it was designed. If anything, the later frame was less robust in that the twin walled main frame spine was abandoned in favour of a single walled version.

Yes, all of the headstocks are basically similar. You can fit the later forks to an early frame although there are other adaptions that would have to take place depending on the model you are using and what you are trying to achieve. Obviously, the change would have to include the yokes, wheel and mudguard assembly.

I understand (never sat down myself and worked it out) that the steering geometry on the Mono shocks is almost identical to that on the earlier models. The slight decrease in whee size is compensated for by a slight increase in the length of the fork so all of the angles are retained.

One of the biggest issues when swapping to the Mono forks is matching the wheels. Spoked wheels for the Mono bikes are available but few and far between and probably unavailable new. The 'Snowflake' wheels used on later twinshock bikes were never made to fit the Monoshock forks.

There are 18" snowflakes from the R45/R65 models but the axle dimensions are different and more importantly, there would be problems matching brake discs to the Mono calipers or fork legs.

'Y' spoke rears wheels as used on most of the Monos won't fit the earlier rear drives for obvious reasons although there was a version of the 'Y' spoke wheel fitted to the R65LS which will fit

On a more positive note, perhaps it might be worth looking at the front fork assembly from a R45/R65 complete with 18" snowflake, wheel and brakes. I believe these would fit with only minor work arounds and might achiove your ends...

Rob

The 'Gummikuh' description was more a comment on the soft, long stroke suspension favoured by BMW which was )is) very different to the sometimes harsh suspension favoured by a lot of the opposition. The frame itself is at least as rigid as the contemporary opposition in the late 60s when it was designed. If anything, the later frame was less robust in that the twin walled main frame spine was abandoned in favour of a single walled version.

Yes, all of the headstocks are basically similar. You can fit the later forks to an early frame although there are other adaptions that would have to take place depending on the model you are using and what you are trying to achieve. Obviously, the change would have to include the yokes, wheel and mudguard assembly.

I understand (never sat down myself and worked it out) that the steering geometry on the Mono shocks is almost identical to that on the earlier models. The slight decrease in whee size is compensated for by a slight increase in the length of the fork so all of the angles are retained.

One of the biggest issues when swapping to the Mono forks is matching the wheels. Spoked wheels for the Mono bikes are available but few and far between and probably unavailable new. The 'Snowflake' wheels used on later twinshock bikes were never made to fit the Monoshock forks.

There are 18" snowflakes from the R45/R65 models but the axle dimensions are different and more importantly, there would be problems matching brake discs to the Mono calipers or fork legs.

'Y' spoke rears wheels as used on most of the Monos won't fit the earlier rear drives for obvious reasons although there was a version of the 'Y' spoke wheel fitted to the R65LS which will fit

On a more positive note, perhaps it might be worth looking at the front fork assembly from a R45/R65 complete with 18" snowflake, wheel and brakes. I believe these would fit with only minor work arounds and might achiove your ends...

Rob