R65 Yokes on a R100?

[steve hawkins]

One of the better things on an R65 as opposed to other BMW models of the same period is the upper and lower fork yokes (triple trees - to the Americans) - which are nice sturdy thick aluminium items.

This is opposed to the weak lower yoke and stamped metal plate that is an excuse for a top yoke that can be found on my R100 (1982).

I only had time for a cursory glance at both bikes this morning, but as long as the fork legs are of the same diameter, which I could not check, I do not see why you should not be able to fit the yokes off of an r65 onto a R100?  They do not seem to be much different as far as reach and width are concerned - but I have not yet had a very close look to confirm.

Anyone put me out of my misery?

No doubt there will be other things to mess around with - headlight ears and clock mounts, etc.  But the bearing are the same, I wonder if the stem length is different?

[Justin B.]

I'll go measure, but I think the R65 tubes are a hair smaller...

They are the same diameter, at 36 mm, but the distance between tubes is greater on the R100.  I believe the steering head bearings are the same on all Airheads so maybe you could put a complete fork assembly on, lower the rear shocks, change the rear subframe, and make a low-slung short wheelbase R100?  The early /5 models were the shorter wheelbase and some loved them for being "nimble" and others thought they were twitchy...

[steve hawkins]

Justin,

I was sure that was not the case?

I also need to check the distance between the middle of the steering post to the middle of the fork leg, both laterally and for and aft, as someone on Boxerworks reckons I am going to increase my trail.......hmm

Can't be that difficult, but there must be a reason why nobody else seems to have bothered.

[nhmaf]

I also thought that the fork tube spacing was closer together on the R65 versus the R100.  But, I am not sure, and the only R100 that I owned for comparison was
a monolever models with the stouter K bike forks..

[Justin B.]

I didn't take an exact measuremenr but just used calipers for comparison and the R65 are narrower by about 1/2" or so but I didn't have my glasses with me to read exactly...  

[steve hawkins]

No, on this particular bike the fork legs are the same distance apart as on my R65.  And the legs are the same diameter.  So I will not need to change anything lower down.  

However, the issue is that the R65 fork yoke puts the R100 fork legs 1cm further forward the current yoke on the R100.  And we have to take into account that the R100 wheel spindle is already in front of the fork leg - 3.5 cms aprox.

The question is, what affect on handling will this increase in wheel base have?

If the effect is negative, can it be reduced by allowing the sliders to come up by 1cm?  As I am not using this bike off road - a minimal reduction in ground clearance would be no great loss.  And it is the only adjustment I have to pay with - other going the opposite way and putting a spacer in the fork to preload the spring and effectively make the front of the bike sit higher at normal attitudes with the weight of the rider taken into consideration - this is limited and does depend on how tired your springs are.

I suppose I could also vary the length of the rear shocks as well

However letting the sliders up through yoke will shift the C of G forward slightly.......

My head hurts -  :-/

I needs some detailed specs.  Or I need to just go and do it and find out for myself.

Thanks for listening.

[steve hawkins]

All,

I am following up on this as I have finally done some more measuring and have got much better access to the R100 top yoke.

The forks legs on an R65 are actually further apart, 5mm each from the centre line.

This raises all sorts of issues with regard mounting the wheel centrally on the R100.  Machining the axle and getting bigger wheel spacers, mounting the discs on 5mm spacers and not forgetting the fender.  Nothing insurmountable, however the there is also another fly in the ointment - the fork offset is larger - closer to 0.6" (15mm).

That might be a step too far. :-/

Hmmm....Time to go away and have a think.

Steve H

[MrRiden]

There is a bolt on solution made by an Air Head here in occupied Mexico
http://www.pbase.com/toastertan/top_braces
Rich

[steve hawkins]

Here we go then:

Some experiements on rake and trail:

http://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm

Tony Foale is a respected motorcycle suspension/frame expert.

Explore the site for some interesting reading.

Cheers

[nhmaf]

Thanks, Steve - that is a very interesting experiment and a good read !
I had always considered that a larger rake angle would improve high speed stability, but this testing tends to disprove that.
I did know that reducing trail does make the steering much lighter, this is where things can perhaps reach the point of twitchiness
if taken too far.   It definitely does speak to the advantages of the hub center steering design like appeared on the limited production
Yamaha sport tourer and some exotic race bikes a number of years ago.  Also, a setup like the Telelever on the modern boxers can
allow for steeper angles and lighter steering, while maintaining greater structural rigidity to avoid braking issues.