Lets dispell some myths about linkages, An aftermarket swingarm for the most part will have its own linkage because of the difference in physical makeup of the arm and an aftermarket linkage for a stock swingarm will be designed to provide a different rate, that basically sums it up.
Sportbikes and bikes in general have progressive linkages and as such are not really suited to competitive motorcycle racing, the designers dont think that joe bloggs is gonna track his bike, the have to consider he may carry luggage and/or pillion and thus design a progressive link. The progression rate of the linkage will vary greatly between brands and models as there is no set rule, the early R1 had a linkage progression of 3.5% and a link ratio of 2.09:1.
The link ratio basically means that 1mm of movement at the shock was 2.09mm of movement at the wheel, the wheel rate got progressively stiffer by 3.5% over 100mm of travel. We tend to look at wheel rate over 100mm and not over the full travel of the shock in a competition motorcycle because the bike should rarely even see full compression (except for certain circuits).
Comparison of 2 linkages below:
2011 Kwak ZX10R 2.37:1 with 7.3% progression
2015 Yamaha R1 2.12:1 with 6% progression
You can tune the linkage to the demands of the circuit, however for the most part race teams just want a linkage that is more linear because it will provide more edge grip at the point that matters, which is mid corner. MSS produce a link that has a 3.5% Progression, which is half the progression of the stock link.
How much progression do you want, it depends as it factors into other areas such as anti squat, but try not to overthink it.
The link ratio will also affect the spring rate, a lower ratio will use a softer spring for the same setting and a higer ratio will use a stiffer spring. Some old bikes used to use ratios as high as 3.5:1, but modern bike link ratios have evolved and reduced.
As for valving strategies, most bikes with a motion ratio in the 2:1 region will not need a revalve for a change in link. Now, If you purchase an aftermarket shock is the valving perfect? I could show you data that would argue not! The TTX shock for example has more compression damping than it does rebound damping! A strange strategy when rebound damping are normally 2-3 times stronger than compression, also to note this is something you can not evaluate without running the shock on a dyno and looking at the forces in the high speed portion of the graph.
As for shock length, you may find that a different swingarm such as the Suter may demand a different shock length because the linkage placement and dimensional constraints otherwise do not allow the required linkage ratio. Remember, you can have the perfect link on paper but will it actually work when built? Are there any clearance issues, will the new arm/link allow full travel of the shock etc etc. I have just designed a new linkage for my trackbike because I am using a different swingarm, my criteria was that the motion ratio should be 2:1 with no more than 3.5% progression over 100mm of travel. Now on paper that took me an hour to get the specifics for linkage geometry correct, in reality when I had the new design 3d printed I had an issue with clearances and have thus had to change the physical design of the link. Finally the newly designed link and swingarm gave me the same geometry as my old swingarm and link.
Ohlins and K-Tech produce Valving libraries for very specific reasons, it saves putting the shock on a dyno to quantify the changes that you want to see and the damper technician only needs to understand what forces they want, choose the valve setup that allows this to happen and then adjust the shimstack accordingally. Revalving a shock on a dyno is a time intensive exercise it can take multiple rebuilds to hit the ideal damping forces and curve you want to achieve, revalving anything without a dyno is just guessing (unless your getting rid of stock and putting in a piston kit/cart kit).
A shock dyno starts a 10K for the most basic of units!!!
In regards fitting, ride height is of no consequence to us, we do not look at seat height. When i design a linkage all that matters is the linkage ratio, the progression of the link and the shock length/swingarm angle. You can have 2 totally different linkage designs that give the same swingarm angle but will give slightly different seat heights, on a Superbike your swingarm angle would be in the region of circa 12 degrees, something like a 600 could have a swingarm angle more in the region of 13 degrees. If you have a bike with 2 different swingarms and when fitted have the same swingarm angle theoretically you should have the same seat height.
Ohlins do not sell a BMW Alpha specific TTX GP Spec shock (i'd have said that was more marketing spin!!!), the TTX GP Shock for the BMW is coded BM568, it is 369mm long +2/-10, with a 54Nmm spring fitted.
The K-Tech DDS shock is 370mm long +2/-4, with a 60Nmm spring fitted.
The BMW geometry is such that the older bike works better with a shorter shock and an adjustment up front to the fork height, this is not because the bike physically sits lower after the change, it is that when you examine the geometry of the whole bike and the interelation between the rake/trail/offset/swingarm angle/tyre size to get good numbers in the Geometry the bike benefits from a change. How it leaves the factory is not the best setting..!
As for the 2019/2020 bike, I have not seen this years team bikes as we are running 2020 non-M spec RR's and I have yet to download the chassis file for my geometry programme.
Just to let you know, the chassis geometry programme user licence is ?1100 per year. Again guessing at geometry changes is like trying to revalve without a dyno, your just guessing!
Once you have a good baseline, then and only then can you truly dial the bike in for the rider, because NO 2 riders end up with the same setting in a race situation.
Also a note on spring rates, linkages are not designed around 1 magic spring rate. The specific target value wheel rate (which I will not disclose) will be achievable by a range of spring rates and those rates will suit most riders. The BMW is undersprung on the shock. The spring rates on the K-Tech and Ohlins shock are sprung to be as per OEM, however K-Tech will supply its dealers a shock sprung for the riders weight when a customer orders a new un. Ohlins do not, I do not know if the dealers offer the option but not being an Ohlins dealer I do not know.
The shock code plugged by Alpha Racing for the link is BM 568 but with the suffix SBK and Ohlins do not list it in their database, Alpha rebuild the shock in house. The shock is 350mm long (20mm shorter) and is the same GP design with a different rate spring, Alpha Racing are an Ohlins centre if i recall?
The length of the shock also has an effect on the progression of the link, understanding linkage design I would say that the stock shock length can not accomodate the more linear rate because of clearance issues. The linkage bolt dimensions change as you adjust the progression of the link which therefore means the link dimensions alter and 20mm is a big change in shock length which has a massive effect on the linkage curve/motion ratio.
Without knowing the specifics of the linkage rate and plotting the trigonometry on my chassis programme it is hard to say what effect the link has with the stock shock length, it is highly likely that is causes major issues with either stroke/travel or clearances between parts.
As to can I make a K-Tech DDS work with this link correctly? Yes I can, very easily ;-) contact
[email protected]
Why do K-Tech not list a shock for the linkage? Very simple, Alpha Racing designed the linkage and to get it to work correctly with the geometry of the race bike it was designed around required a new length shock so they build and sell a new shock- Remember Ohlins would not build the shocks specially for Alpha racing just to fit a new link - thats a bit of clever marketing. Alpha Racing work very closely with some very big teams including Factory BMW and provide UK SBK teams running BMW's race kit, their R&D will come from testing and race data.
Theres alot of stuff in racing and marketing that is never as it seems but marketed in such a way to benefit a company, you may or not be suprised just how much!