Fork Rake and Trail on 1974 R90/6?

[Major Softie]

(for comparison: R1200S rake = 24º, trail = 86.36 mm)

For comparison with what? That thing's rear wheel is, what, 2-1/2 times as wide as an early R bike?

[KenHawk]

Theres more than one way to skin that cat!

Try fitting a set of 13.5 inch shocks to your bike and your airhead's handling will really wake up.

I first noticed this on my RS and I was shocked (No pun intended.)at the improvement in my bikes handling after I swapped in a set of standard Progressive 412 shocks. Progressives standard length for Airheads is 13.5" and my turn -in response and the ability of the bike to hold it's line in a turn improved drastically. Then I installed a set on the Hot Rod with the same results. When I installed new IKONS (13" standard length for Airheads and no custom lengths.) on the Hot Rod, the bike went back to sleep.

I'm saving my pennies for a pair of Olin's. They come a 13.31" length for Airheads and then I'll have the mounts modified to raise the rear end a little, to give me my 13.5" and maybe a tad more. It's the same thing as installing an 18" front wheel except the bike looks stock.

Try it. You'll like it!

[vanzen]

(for comparison: R1200S rake = 24º, trail = 86.36 mm)

For comparison with what?

For comparison of rake & trail #s of the R12S vs the range of #s used on airheads through it's years ...

That thing's rear wheel is, what, 2-1/2 times as wide as an early R bike?

For comparison:
The R12S rear tire is usually a 180-55-17,
4.00 H-19 is book spec for an early air-head, while the later R100R was fitted with a 140-80-17.

[Scot]

4.00 H-19?

You sure?

[vanzen]

4.00 H-19? :?

You sure?

Well ... I'm sure that 19 was a typo !!
4.00-18 might be a better fit, eh ?

... BTW: straight line measured width of a 180 series tire is @ 7.25".
[4" x 2.5] = 10

[Major Softie]

I may have exaggerated the difference a bit, but I believe you understood my point, and you haven't addressed it: The rake and trail numbers of a modern bike running super wide very flat section tires on 17 inch rims has little to do with the numbers for an early airhead with round, or sometimes triangular, section tires, so I still don't understand what "for comparison" meant. They don't compare, because the other factors (besides rake and trail) determining stability and quickness are not comparable.

[Jean]

Go shopping for shorter gaiters. I put Kaw gaiters on my R100s.
No one knows the diff.
(PLEASE don't tell!!)

[vanzen]

I may have exaggerated the difference a bit, but I believe you understood my point, and you haven't addressed it: The rake and trail numbers of a modern bike running super wide very flat section tires on 17 inch rims has little to do with the numbers for an early airhead with round, or sometimes triangular, section tires, so I still don't understand what "for comparison" meant. They don't compare, because the other factors (besides rake and trail) determining stability and quickness are not comparable.

But I do understand the point, Major.
And nothing so much to do with rear tire width or profile (although, yes, still a factor to consider).
Chassis design will be a paramount consideration – and we have apples and oranges here – air-head vs R12S.
Although the reference has already been made to Tony Foale's experiments with rake and trail on an R75/5 ...
I might suggest that you click the link and go back and read that article.

Of particular note in that article:
Regarding rake –
"... it seems there is nothing magical in the conventional rake angle of 27 to 28 degrees. Indeed, balance, stability and lightness of steering were all enhanced by steepening the angle."
And trail –
"... there seemed no obvious optimum value. Results were satisfactory throughout the full test range, so making personal preference the decisive factor."

This will be with factory spec'd tires as available during the time of the experiment in 1982/3.

In any event, Sam is asking about rake and trail #s on an air-head chassis using a 1991 K-model fork.
I have some experience with these forks as used on an air-head chassis.

After reading your posts, Major, I'll ask you to consider:

1) Neither Sam nor I have ever mentioned what size, model, or even make tires were being used
on the front or the back of a K-fork equipped airhead chassis.

2) If you will recall, I suggested a trail # of about (@) or greater than (>) 93.7 mm,
as per my post, my preference, and based upon my experiences with this specific fork / chassis combination.
I know that Sam has ridden his R-bike with 1991 K-forks for some time, is experimenting with less trail,
and that his experiences might better my knowledge ...
Likewise, Major, if your experiences at establishing a working relationship with this particular set-up differs –
Give it up, man ! I'd love to hear what you have to say !

3) Did I even suggest that the rake & trail #s of an R12S are suitable for an airhead chassis with a K-fork ?
I think not, Major !
When Sam proposed running his bike with an 88.6 trail – What was my response ?
I'll quote it to you verbatim:

What #s had you been running on that beast ?
88.60 will be (close to) the stock trail of an R75 or an R100RS (both at 88.9) –
Should be good handling without shaking getting on the throttle (hopefully) ...
but then the stockers had a 19" wheel and a tad more rake.

and:

... #s as the R12S may be pushing the frame / chassis beyond it's abilities to keep up ... be cautious ...
In any event, let me know how it behaves !

Please note that I am trying to learn something here, and not simply argue irrelevant details ...

4) A dictionary definition, since you seem to be having some problem with the term:
Comparison – an examination of two or more items to establish similarities and dissimilarities.*
* my emphasis, Merriam-Webster's definition.

5) Have there been any triangular profile tires on the market since the mid '70s ?
They were a disaster !

[Sam LP]

This discussion is really interesting.

As Vanzen said, I have had these forks for a while.

Before making some recent alterations, I had found my optimum set-up as follows:

1: Establish what the full suspension travel is and go no lower than that
(e.g. make sure to leave that much travel when front end is jacked up: 130mm)

2: drop the forks through the yokes a bit. Look at the bottom frame rail as an eyeball guide to general attitude (angle) of the bike

3: ride it.

4. decide how the handling was affected and whether to raise or drop the front end further

My final set up was to have the forks up through the yokes as far as possible. The bike turned in really nicely, felt balanced and still remained stable.

During the time the bike has been off the road I changed the swing arm to a post '81R65 model*. This is conjunction with adding the gaiters (and having to raise the forks in the yokes to account for the gaiter clamp and compressed concertina sections) made the bike much taller and the angle of the bottom frame rail look very similar to stock BMW geometry. The reason I posted the rake and trail question is that I needed a baseline to refer to.

Last night, after reading Vanzen's post where he says:
"Cut those damn gaiters to make them fit what handles well, brother !"
I pulled the gaiter bottoms further down the fork leg, it looks a little weird, but allows me to drop the forks through the yokes some more. I'll measure and photograph this set up, as well as tyre data etc. tonight and post it online.

Thanks, Sam


*To shorten the wheelbase to something like a SWB R75/5 (the first BMW I rode)

[gspd]

There is no OPTIMUM setup.
Tuners have been experimenting with rake/trail variations since the first motorcycle was built.
All are a compromise between stability and flickability.