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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Batan on July 26, 2024, 01:07:34 AM
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I am getting conflicting info, does an 87 K75S have progressive springs in its fork?
If not, what's a good upgrade? Danke.
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you asked same question on FB, yes?
https://www.realoem.com/bmw/enUS/showparts?id=0572-USA-08-1988-K569-BMW-K_75_S_0563,0572_&diagId=31_0369
no progressive springs, but two springs of different rates which have similar effect.
however, over the many years since the bike was built, chances are someone has changed them. you won't know what is on your bike until you take the forks apart and look.
"Progressive" is also a brand name for a company that makes progressive springs for these bikes.
I have a set of Progressive springs from my 94 K75s I will be listing for sale soon.
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As I recall, my K75RT had progressive rate springs. The bike had an unacceptable amount of brake dive and the front end bobbed up and down with throttle changes at low speed. Despite the squishy front end the ride was harsh over pavement joints.
I replaced the factory springs with Race Tech single rate springs and their Gold Seal cartridge emulators. The improvement in handling and ride was significant. Cost $300+, but was definitely worth it.
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K75S's had different forks than RT's. early S's had two springs per fork, of different rates. later S's with Showa forks had one single rate spring.
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All K75 models came with Showas as of 8/91.
Forks overview:
(http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/forks/BMW.Forks.Overview.Diameter.jpg)
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you asked same question on FB, yes?
https://www.realoem.com/bmw/enUS/showparts?id=0572-USA-08-1988-K569-BMW-K_75_S_0563,0572_&diagId=31_0369
no progressive springs, but two springs of different rates which have similar effect.
however, over the many years since the bike was built, chances are someone has changed them. you won't know what is on your bike until you take the forks apart and look.
"Progressive" is also a brand name for a company that makes progressive springs for these bikes.
I have a set of Progressive springs from my 94 K75s I will be listing for sale soon.
I have, yes. But I'm getting a conflicting info.
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K75S's had different forks than RT's. early S's had two springs per fork, of different rates. later S's with Showa forks had one single rate spring.
So technically speaking the early S had progressive springs and the later one did not???
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no progressive springs, but two springs of different rates which have similar effect.
So technically speaking the early S had progressive springs and the later one did not???
No.
A progressive spring is a single spring that is built with variable rates of compression along its length. There are two types. One is more commonly known as the progressive spring and the other is known as a dual rate spring. In either you will notice how the coil space changes along its length—more gradually in the progressive example.
Progressive
(https://www.motobrick.com/gallery/4/1601-270724103416.png)
Dual rate progressive
(https://www.motobrick.com/gallery/4/1601-270724103458.png)
In the early K75S with Sport fork there were two main compression springs and two secondary compression in both legs. One leg had different valving to control rebound. I don't know whether one of the two main springs had differing compression rate from the other because I haven't disassembled one but if member K1300S indicates they do, that's enough to convince me they do.
Sport fork
(https://www.motobrick.com/gallery/4/1601-270724104113.png)
In the later Showa fork, there was one main compression spring in each fork leg. I bought a '95 K75 that had approximately 25K miles on the clock. When I dismantled the fork tubes to inspect their components, what I saw is illustrated below. The main spring looked like the spring above labeled Progressive. I don't know if the original owner had installed this but he seemed savvy regarding motorcycle performance.
(https://www.motobrick.com/gallery/4/1601-270724104604.png)
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In the early K75S with Sport fork there were two main compression springs and two secondary compression in both legs. One leg had different valving to control rebound. I don't know whether one of the two main springs had differing compression rate from the other because I haven't disassembled one but if member K1300S indicates they do, that's enough to convince me they do.
Thank you. I am familiar with progressive springs but less so with dual-rate ones.
So, I guess the answer so far is "probably" regarding the progressive spring in the Sachs fork K75S. And if those two springs were not of different rates, it begs the question why bother with two - one spring is cheaper to make than two.
More important question is what is a worthy upgrade to it that doesn't cost a an arm and a leg?
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So, I guess the answer so far is "probably" regarding the progressive spring in the Sachs fork K75S.
:laughing4-giggles:
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If you are looking at an upgrade, the Race Tech springs and emulators I mentioned really changed the character of my bike for the better. You might want to look into them.