A modern fork upgrade for 17,18 and 19" wheels

[John Kat]

Having installed the GSXR 11 K front fork on my GS 1000 ST, I wondered to what extent the rake and trail had been modified?
The OEM specs for the GS 1000 are:
Rake: 27°
Trail: 116mm
First, I'll assume that the bike's ride height remains constant by adjusting the fork tubes in the triples.
This means that the rake will remain constant at 27°.
The formula that links the different variables is:
a=R*sin(rake angle)-d
a is the trail
R is the radius of the front wheel
d is the offset between the fork tubes and the steering axle
So with the 19" front wheel that has a radius with tire of 344 mm, and the GSXR 11K triple that has an offset of 32 mm, the trail comes out at 124 mm.
That's a lot and the bike will feel heavier to turn.
Second, if we lower the front end by 2.5 cm the rake will decrease by 1°.
With this new rake of 26° the formula will give us a new trail of 118 mm
This is almost the same as the OEM spec!
Is this achievable with the GSXR 11K fork legs?
The answer is yes as the loaded length of the fork tubes goes from 745 mm on the GS 1000 to 715 mm for the GSXR 11K resulting in a shortening of 30 mm that translates in a reduction in height of 26.7 mm
In other words a GSXR 11K fork will give you the same trail as the OEM bike while the rake will decrease by 1° which is good.
You loose of course 26.7 mm of clearance in the front that should be negligeable.

[John Kat]

I can't help comparing with my GS 1000 XP equipped with 1st gen GSXR components.
Again here the front fork is way superior to the previous model despite the "gold valve" and endless tuning with different oil viscosities and springs
It's very strange, when you pump the front suspension up and down with the cartridge type fork you get the feeling it's way too stiff compared to the 1st gen fork but when you ride the bike you feel the front wheel is under control and it's still comfortable?
[/IMG]

Since the weather improved, I had more time to test my two GS 1000's back to back and I discovered a few important facts
1) My GSXR 11K fork is way overdamped and while it works OK on perfect pavement it unsettles the bike when the road is in poor condition.
2) My GSXR 11G/H fork now works perfectly

GSXR 11 K fork
I decided to pull the fork apart to understand why it was so stiff despite using 2.5w oil at the recommended level.
As I had another fork laying around, I determined the springs were identical.
Next, I changed the bushings to make sure there was no stiction.
Last but not least, I ended up opening the cartridges to see if anything was wrong?
Despite 20 years of accumulated muck everything looked OK?
See the pictures below.
After putting everything back together with Kayaba 1M fork oil ( the lowest available viscosity I believe) the fork is still very stiff.
I just don't understand why

GSXR 11 G/H fork
At last some good news!
I'm running with 10w oil at the recommended level with .95kg/mm springs and RaceTech Gold Valves.
My local mechanic ( proclaimed ex-racer) had recommended the equivalent of 5 turns of spring preload!
Again the fork was very stiff!
In the end, I went back to RaceTech's recommendation i.e. two turns of preload on the Gold Valves.
Believe it or not it works beautifully
The chicken zone on my tires has reduced drastically since the change!

The internals of the GSXR 11K cartridges:

[John Kat]

In despair to get the appropriate damping from my GSXR 11K forks, I managed to get an extra set of fork tubes...
On the bench, they seemed a little less damped than my original ones but it wasn't really obvious.
In any case, I decided to put them on the bike and an hour later I went for a test ride.
Guess what? The suspension works just fine.
It absorbs the road irregularities very nicely and when the pace gets ( much) faster the bike tracks perfectly with no wiggle whatsoever
If I compare the way this fork works in comparaison with the GSXR 11 G/H, I would say that the damping is stronger on the later model making it more suited for the very fast courses while still beeing comfortable on the poor roads.
One difference that is immediately obvious is the extra force needed on the bars to flip from right to left or vice versa.
No wonder that the 19" wheels were later abandonned to reduce the trail they generate ( all other parameters beeing equal)
This leaves me with one unknown: why was my original GSXR 11K fork so harsh?
My guess at this point in time is : could it be the oil seals???
Any experience out there?

[John Kat]

I might have an answer to the firm damping on my GSXR 11K forks
As I mentionned in a previous post, the fork oil I use is the Kayaba 1M.
The viscosity of this oil is indicated nowhere so I searched the net and found this interesting table that compares the viscosity of most fork oils on the market: http://www.qemsc.com.au/documents/suspensionoils.pdf
The table shows that the Kayaba fork oil has a viscosity of around 17 ctSt at 40°C that is somewhat higher than most 5W grade oils
The good news is that BelRay markets a 2.5W grade oil with a viscosity of 9.2 ctSt
Worth a try in my opinion!

[John Kat]

I decided to move on with my front end modifications since I got a brand new ( well almost ) GSXR 1100 J wheel from Posplayr ( thanks Jim)
The wheel is an 18" by 2.75" 3 spoke wheel that comes with a 17 mm spindle.
I had to decide to eitheir go down to a 15 mm spindle to use the current adapters I have built for the 19" wheel or move up to the OEM 20 mm spindle used on the GSXR 11K fork.
After looking up the bearings available for the 47 mm housing, it became clear that the only option available was to move up to the 20 mm I.D. bearing.
It's good news on one hand as it will strengthen the front fork but...it requires a new spacer to fit between the wheel bearings and....the spacer will not fit inside the wheel hub unless it's enlarged.
Nothing a machinist can't fix but clearly not an off the shelf solution.
Beyond that the OEM disks for the J model should fit nicely into the OEM K model calipers.
One remaining potential issue will be the speedometer pick-up that's of course different between the J and the K model.
I'll meet one of my pals tomorrow to determine if they are compatible or not...

[John Kat]

The project has moved on as I had the GSXR 11 J wheel hub bored to 30 mm to take the new bearing spacer.
The wheel bearing spacer had to be made to order as the required length is 75.8 mm vs the std GSXR 11 K that comes at 75 mm
Next came the new wheel bearings measuring 47*20*14.
The left side bearing has to go in first then the spacer then the right side bearing.
So far so good except that the GSXR 11K speedo drive doesn't come flush with the LH bearing
There is a lip on the wheel hub that protects the OEM speedo drive that could probably be ground down to fix that?
More info next week

[John Kat]

I completed the 18" wheel project today
To center the wheel in the front fork, I had to mill 1 mm away from the speedo drive.
As the disks are mounted further apart on the 11R88 wheel, I also had to mill .75 mm of the calipers brackets.
Nothing comes easy when you start modifying a bike.
First ride programmed for tomorrow if the weather allows?
Here's the result so far: