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6 mm Shim? NO!

20K views 142 replies 39 participants last post by  Mayfield07 
#1 ·
2003 'Bird with 20,400 miles. And I will always be thankful to the seller for admitting he had fitted the bike with a 6 mm shim placed above the rear shock. I knew nothing about this mod. He'd heard that it increased handling responsiveness.

If so, it would have been hard to tell during my initial, 3 mile test ride. But I bought the bike. Took it to a local, reputable shop for a full - and I mean FULL - service. New fluids, tires, brakes and a thorough shake-down ride. They blessed it as a solid, low-mileage machine.

Okay. Fast forward; 20 canyon rides and a few thousand freeway miles later.

I didn't like this bike.

It was twitchy, nervous at all speeds - always wanting to fall in to or out of a turn . . . refusing to maintain a steady line through fast sweepers . . . requiring constant correction at the bars. In my memory, the only bikes I owned that ever handled worse were an 82 Kaw LTD with that "spaghetti" frame, and an 84 gPZ that wanted to stand up if the brakes were touched while leaned over.

But this wasn't a Kawasaki. It was a Honda. 30 years of experience has taught me that shade-tree engineers and quick-buck aftermarket supply houses are no match for the engineering acumen and testing resources of a billion dollar manufacturer. There was something better there, waiting for me. I needed to get this bike back to stock. Yesterday.

So the shim came out. Chain, reload and damping re-adjusted. Tires set to spec. Couldn't wait for Sunday.

As the tires warmed and we re-acquainted ourselves with the Angeles Crest, a slight smile turned to a grin. The bike now held its line through the turns, tightening or expanding the radius ONLY in response to my input at the bars. No more nervous, twitchy behaviour. This CBR 1100 xx - this Blackbird is now the bike it always could have been. . . the bike it once was.

I'd offer you a free 6 mm shim for the cost of shipping. Unfortunately, I have no idea where it is at the moment. And I don't care.
 
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#45 ·
Thank you to all for responding.

What I've learned:

1. Many of you actually LIKE the handling difference provided by that 6 mm shim. Some of you don't. (I place myself in the "some of you" group.)
2. The primary reason given for the shim is to quicken handling, particularly in sharp corners. Fair enough and yes, I suppose it does provide that.
3. To me at least, that quickness comes at a price. A slight "nervous" feel at all speeds and particularly on sweepers and straightaways.

Removing the shim provided me with an experience like no other, on a bike like no other. Picture in your mind the entrance to a long, sweeping curve . . . a curve through which you can see to the next straightaway. Begin your turn, as you should, on the far side of the lane (away from the center line). Initiate the turn by pressing on the inside bar. Set your lean angle, roll on the throttle, slowly at first, then harder. As you pass the apex of the turn, closest to the center divider, the bike's 150+ horsepower is foremost in your mind and your soul. Separation between human and machine becomes blurred, as if you're a single moving part.

Through it all, the bike remains calm, centered, maintaining the angle and line you've set. You're relaxed as you exit the turn at 100 - 130 mph - without drama or even thought, really.

Whether at 30 or 130, it just works, and that grin inside your helmet confirms why you are here, on this day, on this bike.

Cheers.
 
#46 ·
Back to Roman times . . .

. . . de gustibus et coloribus . . .

I knew it worked for women, with the above threads I learned it even holds for bikrs.

Silly chaps these romans. . .
 
#47 ·
Perzuki, I agree with you absolutely with your fast bend riding experience - and my bike has the 6mm shim, one of the first things that I modified on the bike, and I thought the mod to be well wort while, though as others have stated it is all a personal experience.
Enjoy your wonderful Blackbird. Cheers, Mike
 
#48 ·
Thanks, Mike. I contacted American Honda yesterday regarding this modification, and will post their response later (it's on my office 'puter). Not surprisingly, they recommend against any alterations to the bike's stock design. It was a canned response. I understand why; a liability thing. Certainly there are mods that DO improve a bike's handling, comfort, even looks. My bike has aftermarket seats, adjustable clutch and brake levers, a slight rise in the handlebars and a slight drop to the pegs. I won't change them. As soon as I can afford it, I'll replace the stock shock with a fully adjustable Ohlins or similar unit.

Thinking out loud (heh), it occurs to me that the PO may have had the front fork springs changed - possibly shortened - from stock. When combined with the 6mm shim, could this have caused the hunting, nervous "too steep" feel in the front?

When I took it to the shop for a full service prior to riding it, the mechanic noted some slight cupping in the front tire tread. I replaced both tires and both were balanced. But the (slightly) nervous handling remained until the rear shim was removed. Suddenly all better.
 
#49 ·
Cupping usually is a.result of low tire pressure. This is probably a very commen issue as alot of riders rarely check and. A shorter spring will a absolutely affect rake rebound and a host of other issues as well as handling. Probably a nervous bird as you described earlier in this thread.
 
#50 ·
Just out of interest, have you measured the sag on your bike Perzuki?
Static and with you on board? The shim puts more weight on the front and if the sag is outside of normal levels the handling will be nasty.

FYI a quick check for the fork springs is a nosecone to front fender minimum dimension of approx. 4-4.5 inches* (about the size of a 'normal' fist with thumb up) ...... any less than this and either the springs are shot/cut down or the forks have been dropped in the yokes.

*measured with the bike standing on its wheels vertical and with no rider on board.
 
#54 ·
I've too done a jack up on the back years ago and agree it is definitely better. But has anyone else experienced that when the bike is on the centre stand the rear tyre is just on the floor and is hard to turn to inspect the tyres etc? Also has anyone had the bike fall over while on the centre stand due to this?


Problem exists here also.

Solution is the same!:thumb::thumb::thumb::thumb:
 
#51 ·
I've too done a jack up on the back years ago and agree it is definitely better. But has anyone else experienced that when the bike is on the centre stand the rear tyre is just on the floor and is hard to turn to inspect the tyres etc? Also has anyone had the bike fall over while on the centre stand due to this?
 
#56 ·
Hey SRAD... You should not, but you can try. Since I run sport tyres instead of touring rubbers I experienced "nervous" behavior at higher speed and "fall into curvs"tendance.
Now I ended up by 2 mm shim which is perfect for ME.
To make it easy I will attach a scheme from my shim solution. With this spacers you don't need to detach the rear shock from the mounting pin only unwind but not take off the mounting pin nut which is between the fuel tank holder screws. Than underneath the frame you can slide the spacer between the frame and mounting pin. I had 1mm aluminum sheet metal at home, so I have made 6 spacers and did the trial/test lifting the back millimeter by millimeter - up to you...
Be careful, the frame underneath the tank, where the mounting pin is sitting, it has a lip! You will feel it with your fingers, be careful, the spacer should not seat on this lip.
I will try to come back with more detailed drawing, honestly I do not remember the exact dimensions, but for sure the cut out and the width dimensions are correct.

I am hungarian so I hope my description is understanable.
Cheers
Robert Text Line Diagram Font Parallel
 
#58 ·
Well, i've just gone from 5.5mm down to a 3mm, and have gained a degree of balance between 'flickability' and useability. The thicker shim seemed to me to be introducing a confusion between leaning more or countersteering with more wrist work as a result. Now it seems the two methods are in balance, and still i have some cornering agility which i suppose is what the shim is all about. It just seems to be at one now, at least for me, my weight, my height, my current suspension and what toothpaste i use! So 3mm for me is :thumb:
let the discussion continue!
 
#60 ·
This next line of thinking maybe should have it's own thread but...

Has anyone considered the tire profile as being part of the mix when they think of turn in rate and cornering and handling concerns?

I've run a set of Dunlop something or others on my then new to me XX back in 2007 when I first got the bike. They wore rather poorly as I recall, being cooked in a season and flat spotted down the middle by the end of our rather short summer riding season. Next I went to Michelin and the Pilot Road series. This thinking based on the "touring" aspect of my intended use. I liked them very much and managed a season and a half out of them as I recall. But, as I've stated before, after wanting a more performance based tire as opposed to a touring tire I changed up to the Pilot Power 2CT skins. I found a marked increase in the ability to get the bike into a corner in a flick-a-bility sense. The handling, in a general sense, I've thought, became lighter to a degree. The shape (profile) of the Powers cross section is more triangular in shape as compared to the Roads flattish oval-ish cross section. This is one of the reasons I continue to use the Pilot Power 2CT's, and have not gone the 6mm shim route as others. I also like the idea that the stock attitude of the bike is not altered in a center stand/rear wheel clearance or side stand sense.

In short: I've found the tire cross-sectional profile to make a noted difference in the handling to such a degree that I don't need the shim mod. I agree my approach is very much one sided in that I've got a shim and just refuse to install it even for a test ride. I just like what I've got already is all.


T.
 
#126 ·
This. A review of the stock bike's rake, trail an wheelbase place it in the sportbike - not touring or dragbike - category. As I recall, someone suggested that the 'bird was designed to be stable at 180 mph, and that that required kicking out the front end. But that's not true at all. Rake and trail are near the "stable" end of the sportbike genre, but definitely NOT in the touring or drag bike field Thanks.
 
#61 ·
I'll be able to see that difference when i change from a pr2 that i have never liked to an angel gt later this year. Will be interesting.
 
#62 ·
You are very correct in the statement that Honda Engineers knew exactly what they were doing when they built this Bike,

They built it to be the fastest straight line performer in the world, and thats what it was at the time, and very successfully at that,

They didnt build it for tite twistys, That was not their intention, So was not part of the design specifics of the bike, It was built to go 300 KPH, In a near dead straight line,

If it was designed for tite twistys, they would have made the Rake steeper for it, So it would go around corners easier, and a few other mods as well,

I have a stock suspension with the 6 mm shim, I love it, I only ride tite twisys, I only slow down for very tite Hair Pins as the long wheel base of the Bird is not designed for tite corners,

People put on lower foot pegs, I couldnt get around corners with the lower pegs, My feet and Fairings drag now with out them being lowered,

I cant go over $2-40 as my Pack rack on the back effects the air flow and I get a wobble up, Take the pack rack off and I am unlimited in speed again,

All bikes need to be changed from manufacturers specs to suit the individuals riding habits, Unless your four feet tall, shaped like a bullet and ride inside the petrol tank,

60,000 identical bikes, Dont fit 60,000 individual riders, We are all different in size, weight, Height, Experience, Riding style, Etc Etc Etc, No bike, Fits all,

Honda didnt make the Blackbird for roads like these,
 

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#63 ·
"Deadman," your post made me smile. And it's a pretty good example of the many riders here who defend the notion of lifting this bike's back end to make it steer "more quickly."
Let me say right off the bat that I'm not the expert racer that you are. And I won't deny or (publicly) question your ability to navigate "tite" [sic] corners at impossible angles and speeds. I've only owned and ridden 14 motorcycles, of various style, size, power and origin.

I WILL share my own experience with this particular bike, 6 months and some thousands of miles after returning it to stock by REMOVING the 6mm shim.

Here is what it DOES do: Steering response is immediate. When I initiate a turn, the bike leans and turns instantly, as it should. Slight pressure on the bar is enough to keep it heeled over in a turn at a steady and constant lean angle. Increasing the pressure on the bar increases the lean and tightens the radius. Decreasing the pressure eases the bike, predictably, to a more vertical stance. As it should be. In short, the bike handles quite well for a 590 lb. machine.

Here is what it no LONGER does: It no longer requires constant correction at the bars ("hunting").

I simply don't enjoy great, big bikes that twitch. Sorry. This one was designed to eat miles and curves rapidly, efficiently, enjoyably. Returned to stock, it does that now, and it does it very well indeed. I've got a 400 lb. boxer twin for the twisties - locally we have Mulholland, Latigo Canyon and surroundings . Heh. And the BMW isn't twitchy EITHER.

I'm happy that your own engineering expertise has solved a problem that Honda never identified, with a modification they recommend against. I suspect it's your superior riding expertise that makes it all work so well for you. Good job. Take care.
 
#66 ·
I've taken my 6mm shim out recently and although I'm yet to ride the bike on my favourite tight n twistie road (Springbrook Mountain Gold Coast Australia), I prefer the balanced feel of the bike without the shim. It doesn't tip in as quick (I just crank on the bars and push on the pegs a little harder) but I do prefer the feel midcorner and beyond. Further R&D to be done after the festive season :smilebig:. Rear sag is 35mm loaded.
 
#67 ·
I am in favour of the shim and anything else that helps with suspension and control.
Honda and most other manufacturers only bring out bikes stock, I mean wheels frame suspension that's all you get.
If you want the bike to handle properly modifications need to be done , as in lowering the front shocks through the triple tree, rear shim,and work on your front shocks done etc.
Your bike will handle better in tight corners , and absorb road bumps at a higher rate, you will need to mod.
 
#81 ·
The shim does not help with suspension OR control. Although the bike is slightly higher in the rear with the shim installed, spring, damping rates and travel are not altered by the shim.

Control IS changed, however. To me the change was significant and not in a pleasant way. With the shim installed, the bike (a) required frequent, slight corrective inputs at the bars to maintain a straight line, and (b) had a slight tendency to "fall in" at the onset of a turn, requiring, again, a countering slight push on the outside bar to maintain the desired line. Together, these two traits lessened the enjoyment of an otherwise proficient machine. It took some research and some thought to identify the problem and the source. Once corrected, the bike became exactly what I'd hoped for; a sport tourer with intuitive controls, good looks, stellar fit and finish, precise handling and

. . . that

. . . . motor. :--)
 
#73 ·
Deadman sounds like you need to get your arse off the seat. LOL

I couldnt get around corners with the lower pegs, My feet and Fairings drag now with out them being lowered,

Wet or dry, I choof through these roads at a great pace, Its fun, Hahahahaha
 

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#70 ·
Its your bike, You do what you want with it, We are all different, The Birds are clones, We arent,
 
#76 ·
6mm shim to high for me, 5.7inches and vertically challenged. I live in the Dorset sticks and needed it to take the muddy twisties, so this will fek up the discussion.
I made a 3.5mm shim and it's bleddy marvelous, right on the button.
Been on 4 years now and would never revert to standard, after all I'm not by any means.
Get on and fekkin ride it, if you don't suit, or it don't suit, mess with it. It's a soddin machine, ask Sammy Miller, or Guy Martin. Bet they don't ride anything standard.
Add a pillion, panniers, straight bars and it's as different as Friday nite at my local. Another story!
 
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