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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Thornton front shocker
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<blockquote data-quote="timetraveller" data-source="post: 127071" data-attributes="member: 456"><p>On the bottom of the AVO body near to where the lower mounting tube is attached you will find a number stamped into the body. This will be either 1447 or 1448. The 1447s are the ones with the normal damping and the `448s are these with the heavier damping. However, early on when Chris had discovered the lack of damping when the ball race conversion was fitted I started supplying the 1448 to all owners of twins as I assumed that the reduction in friction experienced when the ball race conversion is fitted was the whole story. It took me some time, and I admit it is my fault, before I realised that there is more too it than that. To understand what is going one one has to consider what is sprung and what the damper is trying to control. If we take a twin as weighing 450 lbs and a rider weighing 200 lbs and ignoring fuel etc then there is an all up weight of 650 lbs. However, the front forks, wheel, mudguard and stays, head lamp etc weigh about 100 lbs and are not sprung. In addition the rear end plus any passenger and panniers etc also seem to have very little effect on the total mass being controlled by the front springing. Instead what seems to matter is the engine/gearbox mass, UFM, fuel tank with fuel and the rider. In the above example that would indicate that about 450 lbs are being sprung. If we take the standard set up supplied to mid weight riders then there are two 36 lbs/inch springs which have a preload of 3 inches. That is about 216 lbs. I designed the system to give three inches of further compressed movement which means a total force, when fully compressed, of about 432 lbs.</p><p>Now imagine the bike and rider going down the road and hitting a pothole and further imagine the the front forks, wheel etc are stuck to the road. What is going to happen is that the 450 lb load is going to go downwards and the springs are going to resist that. The damper is going to try to slow down and control the rate at which the springs are compressed. When the pothole has been passed then the restorative force from the springs is going to try to push the engine. rider etc upwards and the damper is there to control the rate at which this happens.</p><p>If we do the same calculation for a 300 lb rider then we start off with two 45 lbs/inch springs, pre loaded by two and a half inches then there is a stationary restorative force of 225 lbs which with a further three inches of compression gives 495 lbs of upwards force.</p><p>So the restorative force has increased from 432 to 495 lbs. It turns out that when that is experienced the standard AVO damper, even adjusted to give maximum damping, is not able to provide enough control to prevent a pogo stick type motion. The modified damper does.</p><p>Now before someone takes all this apart and criticises my approach, I do realise it is naïve. I am sure that the unspring weight of the forks etc has an effect and if we have a suspension expert on the forum then I would be happy for them to show us how to do this properly.</p><p>One final caveat. As mentioned above, after Chris's experiments I supplied the stiffer dampers to a few people who were of lighter weight and they soon found out that the damping was too stiff. I exchanged some of the dampers and a few people sent their dampers back to AVO to have the valving returned to standard. This there are few people who have the 1448 stamped dampers who actually have 1447 internals. If you did this then you should know. So a final question for highbury is do you think that your front suspension is too stiff and if so have you tried adjusting the damping? That is readily done with the damper on the bike.</p></blockquote><p></p>
[QUOTE="timetraveller, post: 127071, member: 456"] On the bottom of the AVO body near to where the lower mounting tube is attached you will find a number stamped into the body. This will be either 1447 or 1448. The 1447s are the ones with the normal damping and the `448s are these with the heavier damping. However, early on when Chris had discovered the lack of damping when the ball race conversion was fitted I started supplying the 1448 to all owners of twins as I assumed that the reduction in friction experienced when the ball race conversion is fitted was the whole story. It took me some time, and I admit it is my fault, before I realised that there is more too it than that. To understand what is going one one has to consider what is sprung and what the damper is trying to control. If we take a twin as weighing 450 lbs and a rider weighing 200 lbs and ignoring fuel etc then there is an all up weight of 650 lbs. However, the front forks, wheel, mudguard and stays, head lamp etc weigh about 100 lbs and are not sprung. In addition the rear end plus any passenger and panniers etc also seem to have very little effect on the total mass being controlled by the front springing. Instead what seems to matter is the engine/gearbox mass, UFM, fuel tank with fuel and the rider. In the above example that would indicate that about 450 lbs are being sprung. If we take the standard set up supplied to mid weight riders then there are two 36 lbs/inch springs which have a preload of 3 inches. That is about 216 lbs. I designed the system to give three inches of further compressed movement which means a total force, when fully compressed, of about 432 lbs. Now imagine the bike and rider going down the road and hitting a pothole and further imagine the the front forks, wheel etc are stuck to the road. What is going to happen is that the 450 lb load is going to go downwards and the springs are going to resist that. The damper is going to try to slow down and control the rate at which the springs are compressed. When the pothole has been passed then the restorative force from the springs is going to try to push the engine. rider etc upwards and the damper is there to control the rate at which this happens. If we do the same calculation for a 300 lb rider then we start off with two 45 lbs/inch springs, pre loaded by two and a half inches then there is a stationary restorative force of 225 lbs which with a further three inches of compression gives 495 lbs of upwards force. So the restorative force has increased from 432 to 495 lbs. It turns out that when that is experienced the standard AVO damper, even adjusted to give maximum damping, is not able to provide enough control to prevent a pogo stick type motion. The modified damper does. Now before someone takes all this apart and criticises my approach, I do realise it is naïve. I am sure that the unspring weight of the forks etc has an effect and if we have a suspension expert on the forum then I would be happy for them to show us how to do this properly. One final caveat. As mentioned above, after Chris's experiments I supplied the stiffer dampers to a few people who were of lighter weight and they soon found out that the damping was too stiff. I exchanged some of the dampers and a few people sent their dampers back to AVO to have the valving returned to standard. This there are few people who have the 1448 stamped dampers who actually have 1447 internals. If you did this then you should know. So a final question for highbury is do you think that your front suspension is too stiff and if so have you tried adjusting the damping? That is readily done with the damper on the bike. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Thornton front shocker
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