FF: Forks Thornton front shocker

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highbury731

Well Known and Active Forum User
VOC Member
Chris Launders said:
While doing some of the Road testing for the JE steering stem I was testing dampers back to back and in my opinion the Thornton was marginally better than the AVO, but not really enough to warrant the extra cost and of course they are no longer made. When the bearing conversion is done totally different damping is needed so the Thornton would be no good then.
Chris.
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I have an AVO front damper. I also have a ball-bearing lower yoke.
How do I know whether I have the AVO for use with standard bushes or bearings?
Paul
 
T

timetraveller

Well Known and Active Forum User
VOC Member
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.
 
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H

highbury731

Well Known and Active Forum User
VOC Member
timetraveller said:
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.
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Norman, I had a look at my front damper. AVO have sent me a 1447, not a 1448. I have fitted it to my forks, but not put it into service. Suggestions?
Paul
 
rogerphilip

rogerphilip

Active Forum User
Non-VOC Member
timetraveller said:
The advice in the video re speed wobbles is bullshit, no matter the origin of the advice. Having had a proper speed wobble, as opposed to a head shake, then it is not a matter of taking one's hands off the handlebars. The speed and violence of the motion is such that there is no way one could hold on to the handlebars. Mine occurred at one hundred mph on the long straight at Cadwell Park. The amount of kinetic energy of a total mass of, say, six hundred pounds at one hundred mph which is then being converted into oscillations is such that no normal human being, or a friction damper, will be able to control it.
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timetraveller said:
Hi Stu, how fast were you going? I suspect that elementary mechanics comes into this. The faster one is going the more kinetic energy which had to be dissipated through the front tyre leaving curved skid marks as it goes from lock to lock. In my case they went on for several tens of yards. I stayed upright gazing at the demon that seemed to be trapped in the steering system and only fell of when I got to the end of the straight where, unfortunately I landed on my face. No sales pitch here but remember that I can supply a complete hydraulic steering damper kit to fit the JE front end for £125 and that includes every part including the damper. You keep the original damper knob although it no longer serves a purpose.
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rogerphilip

rogerphilip

Active Forum User
Non-VOC Member
Timetraveller
Threads are now blocked to non members such as myself, thus my comment re costs related to the cost of joining the club not being justifiable for me, a non Vincent owner and not the cost of the damper/mod.
Regards rogerphilip
 
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