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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Modified Steering Stem
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<blockquote data-quote="Martyn Goodwin" data-source="post: 78014" data-attributes="member: 2433"><p><strong>Some thoughts on Vincent Front Suspension. PART 3</strong></p><p></p><p></p><p>Now let’s look at some springs that have been suggested recently. These springs have a lower spring rate but, with an initial free length of 16 .5 inches, when installed in the spring box, they have 2.5 inches of pre load. While we could increase the pre load further with spacers, there is no way of decreasing preload with these springs.</p><p></p><p>[ATTACH=full]15795[/ATTACH] </p><p>Table 6: Linear springs with 2 ½ inches pre-load built in – free length 16 ½ inches</p><p></p><p>Initially I had a set of 16 ½ long, 30 Lb/in springs installed in my bike. My experience was that the front end was topping out with the static sag happening at around 2 to 5% of suspension travel. On the road the suspension was bottoming out with ease. My conclusion is that I need a spring that was much stiffer to prevent bottoming out, but one with less preload so I could realise an acceptable static sag point, around 30% of travel.</p><p></p><p> My on-road testing of springs has shown me that to prevent bottoming out on any but the most horrid bumps I need a set of springs that provides for about 600 Lb of front end impact force at the limit of suspension travel (compression) . If I used a spring with 2 ½ inches of preload – that’s 16 ½ inches free length, then based on the information presented I would need something like a 55 to 60 Lb/in spring – but the problem with that (as can be seen in table 6) is then my static sag will be almost nil.</p><p></p><p>With my personal set up I currently have a Red/White set of David’s springs installed and there is static sag of around 30% which is close I want/need. This equates to an equilibrium force from the pair of springs of 153 Lb. Which is just about the downward force, less the unsprung weight of 50 Lb, the front wheel exerts when I am astride the bike. If I were to change these out for springs 16.5 inches long (thus having 2 1/5 inches of preload) to get the equilibrium point the same ( that’s 30% sag) I would need a pair of 24 Lb/in springs BUT at full compression these 24 Lb/in springs will only be providing for an impact load at the 11 inch mark of 264 Lb and they would bottom out with ease.</p><p></p><p>Using the spring calculator I mentioned earlier I believe the ideal springs for the front of my bike would be a pair of springs each with a spring constant of 90 Lbs/inch and a free length of 14.125”. If I was after a softer overall ride, while maintaining the same static sag, then the front springs would need to be changed to a pair with 75 Lb/inch, each 14.25” long. Even softer ride, still maintaining the 30% static sag point, would be a pair of 60 Lb/in springs, 14.5 inches long.</p><p></p><p>Remember, in all instances to achieve the desired spring free length you can start off with a 14 inch long spring and add a pre-load spacer to get to the required total free length.</p><p></p><p>I hope I have not added confusion to the mix. What I have attempted to do is illustrate that springs and shock absorbers perform totally different functions and that if you know just a few simple things about you bike, yourself and the spring options available it can be possible to select front springs that best suit you and your bike.</p><p></p><p>Once you have sorted your springs, only then can you start to set up your shock absorber, that is IF it has any adjustment capability!</p><p></p><p>All of this assumes you have done all you can to eliminate possible friction points in your front (Girdraulic) suspension.</p></blockquote><p></p>
[QUOTE="Martyn Goodwin, post: 78014, member: 2433"] [B]Some thoughts on Vincent Front Suspension. PART 3[/B] Now let’s look at some springs that have been suggested recently. These springs have a lower spring rate but, with an initial free length of 16 .5 inches, when installed in the spring box, they have 2.5 inches of pre load. While we could increase the pre load further with spacers, there is no way of decreasing preload with these springs. [ATTACH=full]15795[/ATTACH] Table 6: Linear springs with 2 ½ inches pre-load built in – free length 16 ½ inches Initially I had a set of 16 ½ long, 30 Lb/in springs installed in my bike. My experience was that the front end was topping out with the static sag happening at around 2 to 5% of suspension travel. On the road the suspension was bottoming out with ease. My conclusion is that I need a spring that was much stiffer to prevent bottoming out, but one with less preload so I could realise an acceptable static sag point, around 30% of travel. My on-road testing of springs has shown me that to prevent bottoming out on any but the most horrid bumps I need a set of springs that provides for about 600 Lb of front end impact force at the limit of suspension travel (compression) . If I used a spring with 2 ½ inches of preload – that’s 16 ½ inches free length, then based on the information presented I would need something like a 55 to 60 Lb/in spring – but the problem with that (as can be seen in table 6) is then my static sag will be almost nil. With my personal set up I currently have a Red/White set of David’s springs installed and there is static sag of around 30% which is close I want/need. This equates to an equilibrium force from the pair of springs of 153 Lb. Which is just about the downward force, less the unsprung weight of 50 Lb, the front wheel exerts when I am astride the bike. If I were to change these out for springs 16.5 inches long (thus having 2 1/5 inches of preload) to get the equilibrium point the same ( that’s 30% sag) I would need a pair of 24 Lb/in springs BUT at full compression these 24 Lb/in springs will only be providing for an impact load at the 11 inch mark of 264 Lb and they would bottom out with ease. Using the spring calculator I mentioned earlier I believe the ideal springs for the front of my bike would be a pair of springs each with a spring constant of 90 Lbs/inch and a free length of 14.125”. If I was after a softer overall ride, while maintaining the same static sag, then the front springs would need to be changed to a pair with 75 Lb/inch, each 14.25” long. Even softer ride, still maintaining the 30% static sag point, would be a pair of 60 Lb/in springs, 14.5 inches long. Remember, in all instances to achieve the desired spring free length you can start off with a 14 inch long spring and add a pre-load spacer to get to the required total free length. I hope I have not added confusion to the mix. What I have attempted to do is illustrate that springs and shock absorbers perform totally different functions and that if you know just a few simple things about you bike, yourself and the spring options available it can be possible to select front springs that best suit you and your bike. Once you have sorted your springs, only then can you start to set up your shock absorber, that is IF it has any adjustment capability! All of this assumes you have done all you can to eliminate possible friction points in your front (Girdraulic) suspension. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Modified Steering Stem
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