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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: klhoskins on October 23, 2015, 11:14:47 AM
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So I've finally got my brick to the point where I can ride it again. I've replaced the fork springs with progressives and replaced the fork oil with what I thought was the correct capacity 310ml.
The front seems to be way too soft and 'bouncy' on the road so I'm thinking either not enough on the preload spacers or not enough oil. That being said after looking at the chart in the link below, I'm not sure which forks my bike has (Showa or BMW). I was hoping someone might have some insight on capacities, proper oil weight and preload for a 200lb rider?
I've got a 1990 K75RT.
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This is why I always speak out against progressive fork springs. They are a "one size fits all" spring. Sure, the progressive nature of the spring accommodates a much wider range of weights than a single rate spring would, but that does not mean that the spring will work well for you. In your case, you could be using a fork oil that is too thin, but based on the intensity of the bounciness it is more likely that your fork springs are simply too weak for the weight of you and the bike.
There are plenty of online calculators (racetech, sonicsprings) that can help you to determine the correct spring rate for your weight so that you don't have to depend on springs made for a broad weight range.
The oil level won't affect the bounciness much; it only determines the air gap in the fork, which will only help you fine tune the suspension's ability to bottom out. If you don't want to spend money on another new set of fork springs (which you probably need), the cheaper option would to just try running thicker viscosity fork oil at the same oil level.
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What was the weight of the oil you put in the forks? Showa forks were stock on models a couple of years later than yours. If the springs were from Progressive Suspension, what is the model number of them? Which link provided you with 310ml as the refill amount after a rebuild? The IBMWR table list 330ml for a rebuild on forks other than Showa or sport.
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You may have S forks(my 91 K75RT does) . You can confirm by checking for a S on the fork caps. If so, 280 +- 10 is the correct amount for stock springs. Don't know how the progressive springs would affect things.