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Old 02-08-2015, 08:21 PM   #1
thetubespoke OP
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Thumb Airheads, monolevers, paralevers, and handling

Hi everyone,

I notice on my GSPD that it's pretty sensitive to ridges in the pavement. It feels like it flexes over them (ridges in a straight line with the bike). I've ridden a completely different bike over the terrain and it just felt like a bump that it could climb/drop over. Is that possibly due to the paralever flexing?

On that note, do monolevers, paralevers, or twinshocks have more lateral stiffness? I can't figure out what the paralever lateral stiffness comes from, but it looks as if it would not have much. My understanding as well, is that the R65 has a thicker drive arm than some of the others, so perhaps the twinshocks vary in lateral flex. I wonder if the paralever only fixes shaft jacking and the monolever is better in the twisties due to less flex.

In slow turning, the GSPD is a piece of cake, but, turning left is easier than right. Is this the nature of the shaft being on the right side, or is the frame likely misaligned? I know the right is more difficult because of the rider discussion, but it really does seem to fall over at a lower speed than it would going left. I have noticed that the (right?) steering catch is partly broken. I'm sure that was from some kind of a crash.

When riding without hands on the bars, is it normal for the shaft bike to pull to the right and need to lean to the left to keep it straight? This is not due to pavement slope, either. Tires are fresh and ran at high pressure, too.

Thank you,
TheTubeSpoke
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Old 02-08-2015, 08:58 PM   #2
Rucksta
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Airheads are sensitive to minor changes in the setup.
This is a good thing if you are onto it and can be a minefield of confusion until you are.

Groove tracking is often due to tread pattern, wear & pressure and sometimes steering head bearing condition & tension.

I notice you have fresh tyres at "high" pressure.
On my mono I think 28/30 psi is high even with road tyres. Other's experience may differ.

With the bike on the centre stand spin the front tyre.
Does the resultant pattern when viewed from the front look like a bunch of radial lines?
The more district the "radial" pattern the more groove tracking seems to occur.

Most airheads I've ridden pull right with hands off bars. It's only slight and can be corrected with a minor weight shift.
My mono has displayed pull left as a result of impact damage.

No information about paralever vs. monolever regards stiffness.


The bikes turn left better due to a number of reasons.
One is the offset of the rear wheel to the left of the front wheel.
Essentially the bike is already turning when the lean starts (initiated by whatever means you choose) as the rear wheel is describing an arc inside the line of the front.
Right turn needs the wheel to cross over the "hump" created by the offset and at slow speed the bike tends to fall into the turn off the top of the hump.

At high speed the behavior due to wheel offset is different.
Get the bike up to speed say 25-30mph in a safe area and try flicking left/right/left using handlebar pressure.
Lay into it hard.
The front wheel will lighten up or leave the ground at the crossover point and will often produce a flick of the bars at touchdown.
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Old 02-09-2015, 01:55 AM   #3
chasbmw
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Some tyres are very sensitive to ridges and tar snakes, they "white line". Back in the day on twinshocks the Metzler Lazer was horrible in the wet, modern versions may well be better.

I find that with modern tyres on twinshocks, tyre pressures should be quite a lot higher than indicated in the handbook, think about 33-36 front, 36-38 rear.. Twinshocks are also sensitive to front headstock bearing preload requiring more preload than you think.

Generally airhead frames got stiffer as time went on and suspension was improved.
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Old 02-09-2015, 02:06 AM   #4
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In my opinion, the Paralever is streets ahead of the the twin shock models. I love both but the GS paralever is just better and feels more planted than my R100 twin shock.
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Old 02-09-2015, 10:19 AM   #5
walkingbear
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airheads

I own now a 89 R100GS and a R100RS (twin shock)

I think both are good if you pay attention to the suspension
and the type of tires you are using.

My GS has a stuffier frame adding gussets to them.

The 77 I just made sure all the fasteners were tight and I think the
fairing adds a bit of rigidity to the frame.
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Old 02-09-2015, 03:01 PM   #6
thetubespoke OP
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Thank you for the replies!

Is the offset of the front and rear wheel something done from the factory? Is that because of flex in the swingarm if single sided, where it tends to straighten out under load? If it's not necessary, how can I correct the offset so it's balanced?

I wouldn't be surprised if in most ways, the single-sided swingarms are more planted, but I think that may be more due to improved performance over bumpy surfaces due to the shock mount location/forced applied to it than actual stiffness.
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Old 02-09-2015, 08:30 PM   #7
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Quote:
Originally Posted by thetubespoke View Post
Thank you for the replies!

Is the offset of the front and rear wheel something done from the factory? Is that because of flex in the swingarm if single sided, where it tends to straighten out under load? If it's not necessary, how can I correct the offset so it's balanced?

I wouldn't be surprised if in most ways, the single-sided swingarms are more planted, but I think that may be more due to improved performance over bumpy surfaces due to the shock mount location/forced applied to it than actual stiffness.
The offset has some adjustment at the swing arm bearing though it is minor.
Offset seems to vary from one bike to another even within the same model.
I had my straight frame corrected on a frame jig to produce zero offset.

BMW made claims of improved stiffness and reduced unsprung weight when the Monolever was introduced.

Moving the shock top mount off the sub frame onto the mainframe is an improvement although it does introduce other issues with the load on one side of the frame. (not an issue with twin shock forward mount)

Increasing the size of the shock mount/swing arm pivot triangle reduces the loads with a trade off in suspension performance.
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Old 02-09-2015, 09:30 PM   #8
dazzadm
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Quote:
Originally Posted by thetubespoke View Post
Hi everyone,

I notice on my GSPD that it's pretty sensitive to ridges in the pavement. It feels like it flexes over them (ridges in a straight line with the bike). I've ridden a completely different bike over the terrain and it just felt like a bump that it could climb/drop over. Is that possibly due to the paralever flexing?
Hi, My GSPD used to do the same thing very noticeably.

I upgraded the forks to a pair of USD off a YZ250. The fork travel was cut down to 230mm. Overall length of the fork is about 20mm longer than original, which has slightly added length to the wheel base. I then put the same tyre from the old front wheel onto the new Yamaha wheel and suddenly realized that the same ridges were no longer noticeable. I suspect the fork flex is the problem, as the stiffer forks have improved the ride immensely.
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Old 02-09-2015, 11:52 PM   #9
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I recall a thread re 'fork braces' recently, and all the hullaballo (mostly mine ) about them.
Most of all the perfection fitment required.
Then I saw a 81 G/S with what looked like the same forks as mine. WITHOUT any fork brace at all...
I ain't gonna make no steenkin' strengthened brace if no other GS has one. I'll look like a pansy!
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Old 02-10-2015, 03:50 PM   #10
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I had an ohlins rear and racetech emulaters in my 93 PD, and it was very stable (actually, was stable on pavement before emulaters, they just helped immensely on off road terrain). As far as I know, nothing about paralevers that make them inherrently unstable.
I had many a day of California twisties on that bike and I loved how composed it was, even at the edge of the tires (TKC's and Mefos).
I'd start with tires and pressure, then suspension settings.
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Old 02-10-2015, 05:05 PM   #11
Solo Lobo
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Quote:
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I had an ohlins rear and racetech emulaters in my 93 PD, and it was very stable (actually, was stable on pavement before emulaters, they just helped immensely on off road terrain). As far as I know, nothing about paralevers that make them inherrently unstable.
I had many a day of California twisties on that bike and I loved how composed it was, even at the edge of the tires (TKC's and Mefos).
I'd start with tires and pressure, then suspension settings.
+1 I had my old GS dialed in with Ohlins, emulators, custom springs and new head bearings and it was a dream to ride.

I highly doubt the failure is in the frame or design, and can be found with worn parts or poor set-up
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your bike is suitably dirty. Well done.
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Old 02-15-2015, 02:31 PM   #12
thetubespoke OP
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Thank you again for the helpful replies and feedback.

Quote:
Originally Posted by Rucksta View Post
The offset has some adjustment at the swing arm bearing though it is minor.
Offset seems to vary from one bike to another even within the same model.
I had my straight frame corrected on a frame jig to produce zero offset.

BMW made claims of improved stiffness and reduced unsprung weight when the Monolever was introduced.

Moving the shock top mount off the sub frame onto the mainframe is an improvement although it does introduce other issues with the load on one side of the frame. (not an issue with twin shock forward mount)

Increasing the size of the shock mount/swing arm pivot triangle reduces the loads with a trade off in suspension performance.
Wow, I didn't realize the offset was so common. Do you know of any documentation on setting the offset at the swingarm? I can try to set it there and see if it's enough. When you set alignment in the frame jig, did you bend the frame or just adjust the swingarm from there?

Regarding fork flex on ridges; I think it's the rear, but maybe it is the front. The front has a pretty stiff fork brace, but it was put on wrong. The brake line holder is under the brace and the aluminium cracked. I should find a replacement, but I think it is more or less working for now.

Thank you,
TheTubeSpoke
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Old 02-15-2015, 03:15 PM   #13
Solo Lobo
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Factory spec for the swing arm is centered in the frame
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Quote:
Originally Posted by Stagehand
your bike is suitably dirty. Well done.
R65LazaruS
So I sold my GS and went shopping for a G/S!
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Old 02-15-2015, 03:29 PM   #14
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Quote:
Originally Posted by SOLO LOBO View Post
Factory spec for the swing arm is centered in the frame
Yes, but when the swingarm is centered in the frame, the front and rear wheels to not track together. I just had my 92 RS measured on a GMD Computrack machine and it was 6mm offset (rear wheel to the left of the front wheel) and the rear wheel was out-of-plumb by 0.1 degrees. The front fork triples were out-of-plumb by 0.5 degrees which required correction. This is an unscarred bike with 15,000 miles that has spent most of its life in a static collection. I am sure the out of plumb is just sloppy German craftsmanship, but I think the offset was intentional for some reason.

Before you "correct" it, make sure you heed the old cowboy wisdom to "never tear down a fence until you know why it was put there in the first place."
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Old 02-16-2015, 08:22 AM   #15
Rucksta
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Quote:
Originally Posted by thetubespoke View Post

When you set alignment in the frame jig, did you bend the frame or just adjust the swingarm from there?

TheTubeSpoke
Both frame and swing arm need alignment.to get the fit I want.
Extra swing arms get aligned at the same time.
(repeat offender at the bike chiropractor)
Takes Dave Kellet about 3.5 hours once the frame is in the jig.

Technique uses the steering head as a reference point and bend everything else to fit with the engine in place.

I think BMW put the fence where they did because there was a rock in the way of one of the fence posts.
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