billysbones Posted January 23, 2011 Report Share Posted January 23, 2011 I have a project on the go. VFR swingarm into early CBR frame. Project thread here The VFR shock was mounted through linkages to the swingarm, and I had knocked up a bit of a conversion, but was never really happy with it, as it meant that I couldn't use the original shock mounting on the frame. I also would like to use the shock I have from the CBR, as it's nearly new, and black, which I like, If I do use the original frame mounting, and can weld a bespoke bracket on the swingarm, then 1) I think it will look much better and 2) well, I can't really think of a 2. Pics below. I know the linkages provide a more gradual compression of the spring. Would mounting it directly to the arm have a massive effect. I have posted some pics below, and could post some screenshots of autocad drawings, which would give dimensions of pivot points etc. This would be about the height that the swingarm mounting would need to be (maybe a touch higher. That was the thickest tape I had to prop it up with!) Any help with this would be greatly apreciated. I can get ally bits machined and welded through work, but I need to know if the theory holds water, and I don't know enough about the behaviour of shocks!! Thanks in advance. Link to comment Share on other sites More sharing options...
billysbones Posted January 29, 2011 Author Report Share Posted January 29, 2011 Took a few measurements tonight and knocked up a couple of quick and dirty autocad drawings with the basic angles at the correct dimensions: showing how it is under it's own weight: Showing how it would be with the spring compressed (probably more than would happen in practice) Any ideas? How much travel would you normally expect to see on the average back wheel? Link to comment Share on other sites More sharing options...
lorenzo Posted January 30, 2011 Report Share Posted January 30, 2011 I know this is a really vague answer, but I think you're starting from a too flat a position on the swingarm. I've read somewhere in the long and distant past that give or take a couple of degrees more or less all bike have a swingarm angle of about 12deg when they're sat under their own weight. From your drawing it looks like yours is at about 2deg. You know the distance from the swingarm pivot to the top shock mount, you know the distance from pivot to the bottom shock mount and you know the length of your current shock. From this you can calculate the angle on the pivot corner of your triangle. Add about 10 degrees to this angle and then work out the new shock length from that. See, you know when your GCSE maths teacher told you to pay attention, they weren't kidding... Link to comment Share on other sites More sharing options...
Superdunc Posted January 30, 2011 Report Share Posted January 30, 2011 You will end up with a falling rate as the spring compresses, very bad. The motion ratio between the wheel and the damper looks wrong also. The damper needs to be much closer to vertical. Link to comment Share on other sites More sharing options...
billysbones Posted January 30, 2011 Author Report Share Posted January 30, 2011 I know this is a really vague answer, but I think you're starting from a too flat a position on the swingarm. I've read somewhere in the long and distant past that give or take a couple of degrees more or less all bike have a swingarm angle of about 12deg when they're sat under their own weight. From your drawing it looks like yours is at about 2deg. You know the distance from the swingarm pivot to the top shock mount, you know the distance from pivot to the bottom shock mount and you know the length of your current shock. From this you can calculate the angle on the pivot corner of your triangle. Add about 10 degrees to this angle and then work out the new shock length from that. See, you know when your GCSE maths teacher told you to pay attention, they weren't kidding... Mmm. Maybe my sketch of the swingarm angle isn't totally accurate. I will go back out and check that. It does seem a bit shallow, now you mention it. What about the theory of directly mounting the shock? I can make the bottom mount to suit, as I'm not restricted by already having a botton mount to use. Whatever I do, i will have to make the botton shock mount and get it welded to the swingarm. Edit: I know it is possible, as I have seen bikes with a directly mounted shock almost horizontal. What I dont know is wether the shock is suitable, or wether directly mounted shocks are set up differently. Thanks for comments so far. Food for thought. Keep 'em coming! Link to comment Share on other sites More sharing options...
lorenzo Posted January 30, 2011 Report Share Posted January 30, 2011 Having swapped a directly mounted shock on the bros for one off a blade (which would usually have a rising rate linkage attached to it) I know that the spring rate needs to be a good bit higher, as does damping rate. There's nothing that focusses your atention to your suspension mods more than the back end of your bike bottoming out when you hit a bump exiting a corner and your back tyre hits the underside of your mudguard! Link to comment Share on other sites More sharing options...
suspensionsmith Posted January 30, 2011 Report Share Posted January 30, 2011 What you are doing looks sound enough But you need to work out how the leverage ratio will be different The standard CBR600 is around 2.7:1 Workout how different your new setup is.(Swingarm length divided by distance of shock mount too swinger pivot) By the looks you will be near 3.8:1,if so the shock will be way too soft and too much wheel travel. If so you need to get the shock mount closer to the tyre and the shock more verticle. I would look at putting the upper shock mount up where you had it before and brace the area with some rectangular section tube in place of that flat section you have now along with abit of gusseting of the two upper round subframe tubes Link to comment Share on other sites More sharing options...
suspensionsmith Posted January 30, 2011 Report Share Posted January 30, 2011 I had another read of your project thread and see the shock you are using is for the VFR not the CBR so the VFR shock is setup to suit around 3:1 leverage ratio but also a heavier bike than the CBR. It will be close if you can get the shock mount nearer to the tyre (lower leverage ratio) and the shock more verticle Again rearrange the top shock mount support structure ,strengthen it considerably. Link to comment Share on other sites More sharing options...
billysbones Posted January 30, 2011 Author Report Share Posted January 30, 2011 In light of Lorenzo's post about the angle of the swingarm, I took some more measurement and the swingarm looks more like this: The angle of the line between the swingarm pivot and the wheel centre is 10.5 degrees, so a lot closer to what is ideal than I first thought. Suspensionsmith: Thanks for the information. I will have another look at the top shock mounting, and see if there are any more options for the top end. Link to comment Share on other sites More sharing options...
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