Ohlins rear shock length

[moose]

I just received my Ohlins rear shock with adjustable length. After searching a bunch of threads, I couldn't find a target length that equates to the BMW sport rear shock. Would someone please publish the stock shock length and the sport shock length, and the measurement reference (for example, mounting hole center).

Thanks

[pmc847]

I measured the stock sport shocks the best I could ('03 BCR).
Here's what I got.

Stock Ohlins Difference
Front 330 mm 318 mm 12 mm shorter
Rear 305 mm 313 mm 8 mm longer

The rear Ohlins was measured (mounting hole center) with the hight adjuster bottomed out (shortest length) so you can add about 10mm more to length.

[moose]

Thanks! I didn't realize those were the measurements for the sport shock.

[DavidSoine]

Here's what I have written down:

standard front/rear: 319/314 mm (385mm torque arm)

early sport (until sep 2000): 330/318 mm (385mm torque arm)

later sport: 330(?)/310 mm (with 365mm torque arm)

Dave Hopkins has a great post about this in the torque arm thread. I plan to fiddle with this a little in the future, since my Fox shocks are height adjustable and I now have both long and short torque arms at my disposal.

FWIW, if you don't mind mailordering parts, the GS 365mm torque arm can be had from Chicago BMW for about $60.

best,

Dave

[Rapid Dog]

How do you like your Fox, David? I think they're great.
Just had my rear revalved for high speed compression (less damping), and will test it at length this weekend.
I adjusted my front length to be exact to the stocker.
Adjusted the rear to be about 8mm longer than stock, or, the Fox max allowed of 11mm and adjuster threads.
I have the stock paralever link.
I want that GS link tho'.
So, where are your lengths set at?
Can't recall the MM's at the moment.

[Rapid Dog]

O.K., I give up. I can't seem to find the thread where Dave Hopkins made an excellent analysis on shock height and paralever combo's.
Something along the lines of, "stock front, long rear, short link-magic"...
Anyone got a clue?

[DavidSoine]

Page 2 of the "More Custom Torque Arms!" thread.

Right now I have lengths of 324/318 mm (f/r), with the standard torque arm.

324 is as long as I could go with the front - I would have to get a longer lower clevis to to get the front up to "sport" spec, which is available from Fox.

Honestly I have not done enough riding on the Fox units to give a good evaluation. Ride is vastly improved, though I do think the valving could be developed to make it even better. I cannot just tune for handling, since the mean streets in the Detroit area are brutal from a ride perspective.

best,

Dave

[Rapid Dog]

Thanks D, I'll check that out.
I rode with my rear Fox undersprung until they figured out they installed the wrong spring. Alas, vast improvement over stock as said. Really turned in well at all speeds.
Now that I have the new spring, and the re-valve, on the fwy a short jaunt, showed to be a possible improvement in smoothing ot the joints.
More this weekend after Superbike at Fontana...

[fredman]

the "sport shock " is 4mm longer than the stocker (rear). I started with a Wp rear at +10mm, and also having a 365 torque arm, found this a little too much.It made the front qutie nervous as the trail is reduce to that extent.Now running at +7mm, and love it.It turns really well, holds lines well, and is not so twitchy in the front.Plenty of ground clearance-run off the tyres without decking anything.(admittedly, most parts that normally drag have been removed/altered.)

[Dave Hopkins]

Here are the two related article I have written:

TRAIL

The ride height issue is all about “Trail”!
To understand trail, first we will analyze a Telescopic fork bike, visualize the bike laid out on the draftsman’s table at the factory. Now visualize an imaginary line true the steering stem down and forward along the imaginary line which angle is called “Rake” until it contacts the ground. This contact point is forward of the tires contact patch by what is called the “Trail”, commonly about 95 to 100mm. If it where zero the riders arms would be required to keep the front end straight, the “trail” makes the front self straightening just like the castor type wheel on the front of the grocery cart always pivots around so the wheel “trails” behind the pivot! Home modifications to a bike that change “Rake” change the bikes steering characteristics* making people think “Rake” is a big deal but all “Rake” changes bring about a change in “trail” that is what changed the bikes characteristics!

Next lets visualize a motocross rider landing on his front wheel, the rear of the bike is up say 15 degrees, so that imaginary lines angle (Rake) which was about 25 degrees is now at 10 and the tires actual contact point is actually forward of where it was on the drawing board, then the front fork compresses say 8 to 12” and that “Rake” line gets to vertical and the tires actual contact patch is moved even further forward of the point it was on the drawing board. His “trail” has gone from about 100mm positive “Trail” well into the minus range (or push?). In this mode it is strictly relying on the rider’s arms to keep the front wheel pointed ahead. This move is occasionally followed by an abrupt rotation of the fork around the steering head followed by the popular only on TV, face plant!

Now while we are in visualization mode lets think about our front tire in a normal, smooth bend in the road! We can see where this “imaginary point” was say 95mm forward of the bottom center of the tire when the bike was straight up, but now we are leaned over lets say 45 degrees. Our tires contact patch has moved to the side and is now very crooked in addition to being more than that normal 95mm! I can’t explain all of what happens here but I can tell you more trail makes the steering vague & wishy washy, less makes it more precise!

Also while we are in visualization mode lets think about our front tire as it passes thru a chuck hole. We can see where this “imaginary point” is say 95mm forward of the bottom center of the tire but what if the tire is not centered in the “chuck hole”? In this case it is way off to the side! The edge of the chuck hole grabs the tire say 150mm forward and 50mm to the side of that contact patch that they designers showed on the drawing board. Now for a brief moment the chuck hole tries to take control of the bike by rotating the front end around the steering head.

Now the chuck hole tries to twist the front end from your hand, the fork, triple clamp, wire wheel if you gotem, etc will flex! If you are on an old “wimpy” bike with flexy steering components the problem can be huge. If you have an opportunity, just walk up to a 25 year old bike, pinch the front wheel with your knees and push one handlebar end while pulling the other. This can get you thrown out of the Guggenheim but on the Husqvarna’s I rode in the 70’s I could move either end of the bar 3 inches easy! The early Japanese dirt bikes with the steel (exhaust pipe) lower fork sliders I could do 4 inches per side without breaking a sweat! Some tried to control this with wider bars! About 1970 I bought a set of “hot setup” MX bars from England that where a full 36” across, that had to have been conceived in fear! First time out I clipped the end on a pole I thought I was clearing and the brake lever nailed the back of my fingers! Never again! After that my bar widths where reduced to about 32” and my levers pointed down no less than 45 degrees.
When the front wheel twists around the steering head more than the handle bar as all the components flex it stores up kinetic energy, then springs back the other way unloading that kinetic energy into what in the dark ages (of wimpy chassis) was called a “tank slapper” where the front end shakes side to side. Ask any rider who was competing in those dark ages about “tank slappers” and you will see their eyes get big!
Now that the manufacturers understand this, modern bikes have much more rigidity between the handlebar and the wheel, the cast aluminum wheels are much more ridged than spokes, axels are bigger, fork tubes and triple clamps have near doubled in 3 decades and when I do that same knee test on a modern bike the flex is almost undetectable! When you look at the big time racer’s bike be it MX or Road Race the front end components are huge. If the part can not flex, it can not store this kinetic energy! You read a road test it is “last years 42mm fork has been replaced with a 48mm”, the 15mm front axel is how 22MM tubular design with four pinch bolts on the bottom yoke in place of two, yada yada yada.
More mass, equals less flex equals the ability to get away with less trail, equals more precise steering!

Now while we are visualizing, picture both the “chuck hole” and the leaned over scenarios first with a real wide front tire, then with a real skinny front tire! With the wide tire there is all kinds of things trying to take the steering out of your control that are not evident with a skinny tire. That is why dirt bikes have skinny front tires, they run on more irregular terrain and always have something tugging on the side of the tire, and is why no mater what you ride your front tire is narrower than your rear!
At every motorcycle manufacturer there is a “tug a war” going on! The Race Team and the R&D guys want less “Trail” in their pursuit of lighter, quicker and more precise the steering.
Because of the ever present “face plant” scenario above, which is occasionally followed by a law suit, the corporate pin stripe suits are pulling the other way dictating that the production bikes have more “trail” and our production bikes become a compromise! The compromise is almost always give it a little more “Trail” to make the handling just OK at the sacrifice of excellence!

Now on to the Telelever!
When BMW got their hands on the Telelever design one of the things they realized was they could move the rear “A” arm pivot up or down and map out how, or if the trail declined like that Telescopic fork. This was well illustrated at
http://www.mecc.unipd.it/~cos/DINAMOTO/Telelever%20or%20forks/Telelever.html
(if it is operating?)
The way they laid it out on the production bikes with the swing arm near flat front to rear the trail actually increases slightly as the front suspension compresses! The effect of this is they could get away with less “Trail” to start with. However BMW feared that the Telelever would not be well received if it steered differently than conventional bikes so the left the straight up, flat ground, suspension extended shall we say starting point of the trail at a normal spec of about 95mm. The same number conventional telescopic fork bikes have knowing they are going to decline! Thus it appears quite safe for a BMW rider to reduce their “Trail” with confidence while reductions in trail are done on Telescopic front ends with much fear and trepidation. The race guys, dirt or pavement with Telescopic forks, buy a standard bike, then raise the back to reduce the trail, it improves most every bike so much that they raise until the front becomes unstable, add a steering damper to achieve stability and raise again.
The way I have explained trail over the years is “if you are in a corner, leaned over and do not feel that you can tighten your line, you have too much trail! When it is right you can carve to the inside of the bend with confidence! The production bikes that will do this are few & far between!

Thus a BMW Telelever owner can decrease the trail, improve the steering and improve the bikes overall performance with reductions in trail that are very much within tolerable levels. BMW has provided us with 3 levels of height in the rear,
standard with the 314mm rear shock,
Up about an inch (due to leverage) with the 318mm “sport” shock
Up about an inch with the “GS” link, or
Up about two inches with both,
All of the bikes they have produced with the rear up about 2” have had the longer “Sport” front shock so a combo that exists but has not seen production is the “up 2” in the rear with the short front shock! Additionally some aftermarket shocks may be available with ride height adjustments, some owners have trimmed down the rubber bushing on the top of the front shock and tire size changes come into play as well, a 65 series front and the 180 series rear can each decrease the trail.
There is also an issue with the “U joint” and its tolerances of angularity which is a separate article!

*In Tony Foales book, (available thru Motorcycle Industry News) he built a series of front ends for an old “air head” all having the same trail, with rakes from zero to 25 degrees, all handled the same! The lesson was that rake changes leave the world thinking it has an effect because a rake change alone makes a trail change.

[Dave Hopkins]

“U” JOINTS

“U joints” and there tolerances of angularity! Sorry this is long, the angles I measured with a protractor and it is difficult to line up the parts perfectly so I may e off a degree?

In a perfect world a “U joint” would have;
No less than 2 degrees angle, (if they are run straight the needles pound notches in the trunions), and
the front and rear being equal angle and opposite direction due to what is called “velocity fluctuations”*, and
the rear “phased” so that it’s cross is parallel to the rear*, and the max angle is very debatable, about 10 degrees being the preferred upper limits and the more angle it has the more frequently they need to be greased or replaced. On the “S”, BMW used a “tall” rear drive gear ratio of 2.75 to 1 which keeps the driveline speed fairly modest so they get away with not following these rules to the letter.
Standard “S”:
With the 314mm rear shock, has approximately 2 to 3 degrees front and 8 to 9 degrees rear, opposite but no where near equal, not proper and the rear approaching that 10 degree preferred max.
“Sport rear shock: (Shock length effects front U joint angle, not rear)
This 318mm rear shock available thru BMW accessories as a “Sport” option, standard on the 2001 “Lite”, the 2002 “Boxer Cup” and “Boxer Cup Replica” models. This increases the front angle to about 8-9 degrees. Still opposite and certainly more extreme.
“GS” or “Sport” “Paralever Link”: (Link length effects rear U joint angle, not front)
Commonly used to tune up the steering on standard models and used on the “Boxer Cup” and the “Boxer Cup Replica” models. Its length center to center is 265 mm as opposed to 285 for the standard link. Its shorter length rolls the rear gear case down thus raising the rear of the bike at a ratio very slightly above 1 to 1 so about 21 -22mm. The purpose of raising the rear ride height is to decrease the amount of “Trail” and the subject of anther article but note, the improvement is steering is significant! The fact that the “GS” or “Sport” “Paralever Link” brings the rear U joint closer to the fronts angularity has always made me believe that is the way the bike was designed, and someone at the factory whined about seat height so they put the longer link on it for production & sales purposes? This decreases the rear U joint angle to about 3-4 degrees, the front remains the same. Still opposite and closer to equal. This shorter link with the standard length shock would appear to have the best combination of angles however the fact that BMW have produced all three rear height combos would appear to prove that all are reasonably within tolerances. In any event more angle is always going to equal more wear.
Thus Front Rear
Standard “S” 3 9
Standard shock with short link 3 3
Sport shock with long link 9 9
Sport shock with short link 9 3

*When a U-joint is operating at an angle the drive shaft is off center from the shaft that drives it so due to the leverage its speed changes at different points of rotation called “velocity fluctuations” in a manual on U-joints published by Spicer Dana Corp. The effect is say the transmission is turning 100 RPM, the drive line may turn one ¼ turn at 100, then 101, then 100, then 99. If the rear U-joint is equal and opposite its “velocity fluctuation” will nullify the effect back to an even 100 RPM. There are tolerances and low driveline speed help. Back in the ‘60’s/“muscle car” era my brother and I had put together a lowered 49 Merc with a small block Chev, 3 speed overdrive trans and a 5.38 rear end. It had a vibration that would not go away, new tires, re-balance the tires, borrow the wheels of another car, balance the drive line etc. finally I got my hands on that Spice Dana Corp book. We realized the angles it had where closer to 0 in front & 15 rear = bad and the 5.38 rear made the driveline speed fast! We un-lowered the car to put angle back into the front U-joint and changed the pinion angle to get close to out 4 down & 4 up goal, instant smooth!