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Spring Length Vs. Seat Height

OK-

I've just faxed in an order for Ohlin's with Mr. Ed, but now wondering if I have made a mistake. Ed said that the Ohlins can be ordered 1/2-3/4 in shorter than stock. I am assuming this would require a shorter spring, but it could be a ride height adjuster too. Either way, my question is the same. I went for the 3/4 shorter...cause I'm short.

Now I was figuring that a 3/4in change in the spring length would actually result in a SMALLER height reduction at the seat because the spring is at an angle to the seat. For example:Let's say the angle is 66 degrees and the length reduction is .75in...then the overall height reduction would be something like .5in.

Makes sense...even if my geometry is suffering from 17 years of disuse.

However, in shopping around with shock manufacturers, one replied as such:

"Keep in mind that 3/4 of an inch at the shock is a tremendous amount at the axles."

I've already written back asking for clarification, but I can't see how a 3/4in reduction in the spring/shock length could lower the seat MORE than 3/4in. Nor do I see how it has any effect at the axle as axle height is dictated by overall wheel diameter. The angle of the paralever may change, and the trail on the front fork should be increased, but the angle of the telelever fork tubes should NOT change. (trail calculated from the line of the front shock to the ground vs the plum line from the axle to the ground)

Thoughts on this from any engineers (or wannabe engineers) on the board?
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Last edited by 1967 R50/2; 04-10-2002 at 07:10 AM..
Old 04-10-2002, 05:03 AM
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A 1/2 inch change in shock length will make approx. a 1" change in the front and a 2" change in the rear. This is mostly because the shock is attached to the front control arm about half way out from the pivot (which does not move in relation to the chassis) and the ball joint (which has the most movement of any part of the arm in relation to the chassis). The rear shock is 1/4-1/3 of the way out from the pivot.

As the shock is angled away for 90 degrees from the arm the travel of the shock in relation to the arm actualy decreases. This means that a larger wheel movement occures for the same inch of shock travel if the shock is angled away from 90 degrees to the arm.

To confuse matters even more... If you shorten a spring without changing the wire diameter or number of coils it will get stiffer and not compress as far when a riders weight is applied to the saddle. A short spring can be made to work like a longer spring by changing the wire diameter and number of coils, if that is the case then both springs will have the same "spring rate".

Also some or all of the change in spring length can be adjusted for in setting the preload of the spring (sag). By adjusting the spring preload or sag, you are simply adjusting the point in the suspension travel where the bike will sit with no dynamic motion (bumps, acceleretion, braking, etc).

The fork angle must change in relation to the chassis as the front suspension moves since it pivots around the upper ball joint which is attached to the chassis. On a telelever it changes less than a conventional fork in relation to the road since as the front end compresses normaly steepenging the rake angle the telelever increases the angle compensating for the change in chassis angle. Also the telelever eliminates most of the brake dive that causes a majority of the chassis pitching that most riders experience. There are two photos showing how the fork tube angle changes here:
http://www.pelicanparts.com/gallery/eron

I hope this answers your question(s).

Eron
Old 04-10-2002, 07:11 AM
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Now THAT is the kind of answer I was looking for!!!

So a 3/4 in reduction in shock length will result in a in a 1.5 reduction at the front and a 2.25 reduction at the rear assuming an attachment point on the paralever of 1/3 of the total length from pivot to axle.

The total effective wheel base would also be increased, so the bike should be more stable on the straights but slightly harder to turn in...or...or....is it that the trail on the front end would be decreased, but the rear would be increased? I gotta go home and consult my books....
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Old 04-10-2002, 07:31 AM
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Which is why you then put the shorter Paralever Arm on it to confuse the issue one step more.
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Did you get both front and rear Ohlins? Sorry it isn't clear to me in your post.

Assuming you have, you lowered the rear more than the front your turning will also be slower because the effective "rake" is now more.

The effect on the front end itself is somewhat more complicated than with a front fork, because of the more complex geometry of the telelever. It doesn't follow the simple rules that a fork does, nor does it behave linearly. The first 1/2" of movement may cause an increase in rake and trail, but the second 1/2" may bring it back to the at rest rake and trail, and the third 1/2" increment may reduce the rake/trail below the "at rest" value... Ppart of the "anti-dive" behaviour of the telelever is based on the geometry of the system at rest. If you change the shock length your bike may not perform the same.

Be aware of this when changing suspension lengths.
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Old 04-10-2002, 09:33 AM
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By the "1st 1/2in of movement" are you refering to the movement in the telever in action or are your refering to merely the changeing rake/trails of the bike at rest caused by various length shocks?

And yes, I went with both front and back Ohlins, and have a call into Don at Mr. Ed's to see if the shortened shock is through the use of a shorter spring or a ride height adjuster.
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Old 04-10-2002, 09:44 AM
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1967 R50/2

I didn't think Ohlins offered different length or ride hight adj. for our "S" Make sure he's not selling you shocks for a different model.
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Old 04-10-2002, 10:04 AM
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Joe-

I did a search on "Ohlins Ride Height Adjuster" and turned up this thread which seems to indicate that it is available by special request.

"Ohlins Installation: HELP PLEASE!!"

That would seem to indicate that ride height adjusters are available.

Still have heard from Don at Ed's.
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Old 04-10-2002, 10:27 AM
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When I say first 1/2" or whatever, I'm saying both/either using the stock suspension "at rest" position as a reference.

Shortening the front shock so its at rest position is 1/2" shorter than stock is like the stock shockwith 1/2 of compression. Similarly with 1" total (second 1/2") etc...

If you think about it, the fork angle changes as the telelever swings in an arc. The precise dimensions and the current position are what determines the behaviour of the front end geometry.

Not simple like forks. There was a good link on the web at one point that illustrated all this quite well.
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Old 04-10-2002, 10:42 AM
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Hi,

I believe a height adjuster can be made custom, but shorter shocks for the S are not a catalog item.

Eron brings up a slew of good points. Not that the shock
to axle height change ratio is even greater than 2:1
Actually 2.6:1 (50mm shock stroke yields 130mm wheel travel)
So, 1/2 inch is going is going to net a bit more than a 1-1/4 inch change in rear height.

I'm curious what they're shorter shock is too. I doubt it uses a height adjuster, or they'd have stated that. To me, the real question is is the shock dimenionally shorter (a turn or so less) and thus actually stiffer, as Eron points out is possible, or is it both shorter, and a lower rate.

The point here is that the Íhlins suppliers are relatively sharp, and if you're replacing both at once, you ought to be just fine, albeit with somewhat reduced cornering clearance.

Note, throwing a short paralever arm on a short shock is NOT the thing to do. Long-short, or short-long works. short-short or long-long will cause severe driveline radial loads towards one end of the suspension's travel.

hth
later
roger
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Old 04-10-2002, 11:15 AM
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Rapt-

Yes, but the link that demonstrated it is now gone.

In anycase, I'm not sure if your proposition is 100% true although alot of it makes really good sense.

At rest the telelever will still be flat...not pulled up into a 1/2 inch higher arc by the shorter shock. The fork sliders will still have the same angle as well, merely the inner tubes will be somewhat more retracted. What is changed is the angle of the shock. The upper attachment "Eye" and the rest of the bike will be lower, which is the whole point of me blowing out all this cash. Because this angle is different, the rake and trail will be definately be different at rest.

However by comparison, with the stock shock under compression, the telelever WILL be angled. The angle of the shock will change, but the lower EYE not the upper should change position. This is because of the telelever's swing. Could the movement of the two eyes result in the same angle? I suppose it could, but under compression the lower eye should ROTATE to it's new position, whereas with the lowering shock it should be a fairly linear relationship between the new upper eye position and the old one. At this point, it gets to complex for me to visualize in my head.

But I'm thinking the net net will be a decreased rake/trail value resulting in quicker steering simply because the upper eye, and the rest of the bike will be lowered at the same angle as the forks meaning that the angle of the shock will have to be increased...more perpendicular to the ground.


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You're mixing reference points thats why it seems different. But trust me putting in a shock thats 1/2" shorter will have the telelever geometry effectively in the same position (*assuming the bike remains level) as if the stock shock was compressed 1/2". I can't say whether it will increase/decrease or remain neutral in rake over that travel because I don't have the actual geometry to look at. My presented examples were merely hypothetical examples not statements of fact.

However, if you were just putting a shorter shock in the front then the effect would likely be a slight increase in rake at all positions because the rear would be higher relative to the front. (Similarly if you lower the front more than the back, this will be the case.)

Lowering the front and rear together puts your bike back at the same base angle as before but with the rake now at some resultant determined by the change in front geometry due to suspension.

Lowering just the rear increases rake (less steep) throughout its travel. (Again the same is true if you lower the rear more than the front.)
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Old 04-10-2002, 11:32 AM
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Question

Here's some info on the BMW Telelever


Also if they are adding a height adj. that will take up travel length, and at a 2.6 to 1 ratio you might lose 20%-%30% of your rear wheel travel that might be ok if you race but if you like to tour and load up you want all the travel you can get. Just something to think about.
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Old 04-10-2002, 05:01 PM
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Joe-

You said: "Also if they are adding a height adj. that will take up travel length, and at a 2.6 to 1 ratio you might lose 20%-%30% of your rear wheel travel "

Surely some rear travel is going to be lost. But the beauty of a ride height adjuster is that you don't have to shorten it the entire way. Anywho....on this very board I have seen bikes that have been lowered by two inches via Works Shocks and yet are claimed to perform admirably. I must wonder if the rear shocks are so tall to increase the rake of the bike..with a side effect being that rear wheel travel is over engineered. Even as I say it, I doubt the very words. Not much escapes the designers in the "Four Cylinder" . I don't think it is necessarily a side effect, but perhaps it is intentionally over engineered for travel.

Curiously, in that great animation you posted, the missing element is the angle of the shock as it is compressed.

I have consulted my tome on the subject "Motorcycle Design and Technology" By Gaetano Cocco. Great book. Has a special section on alternative front ends like the Earles Fork and the Telelever. Sadly...in the only visible BIG mistake I've been able to find..the Telelever section is called "BMW PARALEVER"!!!!! Figures. The book was published by APRILIA!!!
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Quote:
Originally posted by 1967 R50/2
Joe-

You said: "Also if they are adding a height adj. that will take up travel length, and at a 2.6 to 1 ratio you might lose 20%-%30% of your rear wheel travel "

Surely some rear travel is going to be lost. But the beauty of a ride height adjuster is that you don't have to shorten it the entire way.
Click on blue lettering above drawing for more Televeler info.

Since the stock Ohlins shock body is being used you only have so much shaft travel length to work with some of the travel length is needed for the adjustment hardware since their not changing the shock body. Figure about .5"- .75" for adj system which means you lose about 1.5" wheel travel.

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I completely understand and agree with what you are saying...but what if the adjuster replaces the FIXED silent eye and not the one on the end of the shaft? (Granted , it is on the shaft in the pic.) The shaft would still be of normal length and therefore of have normal travel. The eye at the other end could be extended or retracted as desired.

Some travel would still be sucked up by the change in the angle of the shock caused by the slightly shorter overall body.

I don't have an Ohlin in front of me, nor have I heard back from Don, which is beginning to make me antsy, but as soon as I do I will query him and post answers as to how this setup works on the board.

When other people lowered their bikes, didn't anyone ask these types of questions? Searches have provided suprisingly little info, which is quite suprising to me.

Also the close up pic you provided suprises me a little. I'm guessing it is an Ohlin because of the yellow spring. What suprises me is that the silent eye is not a ball bearing race.

Guess I get suprised alot!!! heheheh

Overall, I think 1.5in of travel traded would probably be acceptable for a lower set height and an Ohlin quality ride. But the proof is in the puddin'
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Old 04-11-2002, 05:02 AM
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Since they only offer one rear shock body for our bike there is no easy way to put adjuster on fixed end.

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Old 04-11-2002, 05:28 AM
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Hi guys.........I've substantually lowered the suspension on my S, I designed it that way initally as I was building the bike for my wife to ride. Well so much for that, the project got out of hand, (as they always do), I feel in love with the beeemer and decide to set the SS up for myself.
The fear is down 2 1/2", the front is down 1". The bike feels wonderful with a more flat set-up. There is no effect on handling whatsoever. Remember this is a street showbike bike that will never see the wrecking yard, I mean the track! I never ride a motorcycle on the street at more than 6/10's, hense I do not require the ground clearance that the bike had in it's stock profile. Frankly, I feel the stock set-up is over kill for most, a lowered 1100S looks cooler and is much more comfortable if done with a bias to the rear. When was the last time you dragged a head on the street?
I was provided with Works Performance custom shocks, for which I thank Sandy ands his team very much, they are exceptional. You can now buy them as standard issue, just make sure you tell them how low you wish to go.
FYI - I have an Ohlins/RaceTech complete suspension on my Muzzy Kawasaki ZX1270, I feel the WP sytem is of equal quaility. I'm an ex-roadracer and hold a license here in Canada.
Cheers,
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Old 04-11-2002, 12:18 PM
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Just hung up with Don at Mr. Eds. He says what Ohlins will supply is a set of shocks that will reduce OVERALL RIDE HEIGHT by 1/2in or 3/4in depending upon your desire. No ride height adjuster, rather it is done during the build. Probably by means of a shorter shaft, but this point wasn't very clear.

What WAS CLEAR is that Ohlins in not merely lopping length of the shock without regard to the rake of the bike. The angles of the shocks are taken into account to uniformly lower the bike by the desired amount. Rake should therfore remain the same.

Trail and travel, as discussed extensively above, should be changed.

When the shocks get here in two months I'll try and post some pics.
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