tok-tokkie
Race Dog
This is a post I wrote for the American TW200 site.
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Front Forks
I have Cartridge Fork Emulators in my TW. This long report tells the story of how I came to get them and gives some technical explanation of what they are and their advantage.
Other options considered
When I first rode my TW I was pretty surprised at just how harsh the suspension was. My other bike is a BMW F650 Dakar which has gorgeous plush suspension. In Cape Town, which is really just a small provincial town, we are lucky to have a really good bike suspension person, Martin Paetzold. I went to him and asked what he had available. He suggested the forks from a Yamaha YZ85; he did not have them but had sold the owner competition forks so they were available. They were lovely upside down forks:
That is them alongside the TW forks. Somewhat longer and much thicker. I have a cnc machining centre so I could make new triple clamps and the adapter for the brake calliper. However, when i came to have a good look I found the axle hole on the YZ forks is 12mm but the TW axle is 15mm and I judged that there would be insufficient metal remaining if I opened the hole to 15mm. So I abandoned that idea.
Next a used Touratech suspension upgrade for the Dakar became available. The Touratech stuff is actually made by WP. Nice upside down forks complete with the triple clamps and brake calliper mounting plate. So I bought that and fitted it to the Dakar. Now I had the Dakar forks available for the TW.
The upside down forks in the center are another pair that Martin had available. The fork on the right is from the TW and those on the left are the Dakar forks (made by Showa). The Dakar forks have 210 mm travel which is 30% more than the 160mm travel of the TW. I had a good look at it and they could be used (there was enough space for the extra wheel travel) but, in the end, I decided against using them on the TW. I fell off the TW a while back on a good hard gravel road where i must have been gazing at the scenery and hit a lonely pot hole & was seriously concussed. I am scared of that happening again as it took a long time for my brain to recover. The axle on the Dakar is 18mm further forward from the centreline of the fork than it is on the TW. That would reduce the trail from 95mm to 77mm (about 20%). The less trail you have the less stable is the bike & that is just what I did not want so I decided against fitting the Dakar forks to the TW.
The same reduction in trail applies in the case of the Honda forks that Martin had so I decided against them also.
Here is a TW with Yamaha YZ125 or YZ250 front forks. That is what I would really like as the axle is also very close to the centreline of the fork so there would be little loss of trail. That is some German TW enthusiast’s bike whose name I did not save.
This is a close up of the YZ forks showing how close the axle is to the centreline of the fork. The closer it is the greater the trail (all other things being equal).
Martin then offered to make me Cartridge Fork Emulators for the TW. To explain them I need to discuss front fork types.
Damper Rod Forks
Here is an excellent article describing how our TW suspension works – it happens to be a Harley site.
This is the diagram and write up for the front forks from that Harley site.
I want to elaborate on that article. The damping piston is attached to the lower (outer) part of the fork by the long tube sticking up the inside. There are two oil seals; the main one at the top of the lower (outer) part which seals against the outside of the upper (inner) slider and the second seal which is between the damping piston and the inside of the upper slider. The change of volume for the oil in the lower slider is equal to just the thickness of the steel tube of the upper slider – that is all that is moving past the two seals into the lower slider when the fork compresses. There is thus not much oil moving through the compression metering holes. That is one of the great problems with simple damper rod forks – there is relatively little oil flowing through the metering system so it is very difficult to regulate that flow accurately. But that is compounded by the primitive mechanism that is used for that control.
This is a more detailed diagram (from the next link I give) of a Damping Rod Fork as fitted to the TW and Dakar. Pretty primitive forks nowadays.
This diagram from Tony Foale’s book shows, as curve B, the resistance to flow of the oil through the compression orifice. The quicker the fork moves the greater the resistance – but it increases as the square of the speed so the resistance becomes very great for a hard bump. The slope of the curve is determined by the size of the hole (and also the viscosity of the oil). I believe the TW hole is relatively small so it has a steep curve compared to a much flatter curve for the Dakar – the Dakar forks have much more travel and the ride is much plusher compared to the harsh TW ride.
EDIT: What the suspension people are aiming for is curve A. The x axis (bottom scale) is the speed the fork is compressing at; the y axis (side scale) is the resistance (stiffness) of the damping. Curve A hase resistance increasing in proportion to the speed (shown by the straight line indicating the resistance increases exactly in proportion to the speed). What they get with Damper Rod forks is curve B which is below curve A at low speed but then above it at high speed. It is underdamped at low speed and hopelessly overdamped at high speed (it is well below the line A at low speeds and goes miles above it at high speed). Speed here refers to the speed the forks are compressing at – ride over a brick at low speed and the forks have to compress very quickly but curve B is way above curve A so the bike bucks and you have great difficulty controlling it. When the riding becomes challenging the forks are just too stiff. Of course the damping holes can be made bigger which makes curve B slope further to the right so the forks are not so stiff at high speed but that just makes them even worse at low speed than they were. Or the holes can be made smaller so they are better at low speed but then worse at high speed. What Cartridge forks do is give us curve C which is a pretty close approximation of the wanted curve A. (In fact they may even make curve C sag below curve A = regressive damping when the cartridge forks have separate high & low speed damping).
Table 1: Front fork speed when hitting square bumps at various speeds
Bump Motorcycle Speed
Height 20 MPH 40 MPH 60 MPH
1/8" 29 ips 58 ips 87 ips
1/4" 41 ips 82 ips 123 ips
1/2" 58 ips 117 ips 175 ips
1" 83 ips 167 ips 250 ips
2" 121 ips 242 ips 363 ips
4" 283 ips 566 ips 850 ips
(ips= inches per second - an archaic measure of speed)
This is the table from the Harley site I listed earlier. Look at the huge variation in fork speed depending on the bike speed and bump height. That shows the difficulty in making a good Damper Rod Fork – if the hole is to be effective at the lower damper speeds then it will be too stiff (harsh) at the higher speeds, conversely, if the orifice is big enough so that the forks are not harsh during high fork speeds then there is no damping at low speed so the bike wallows up and down like a lazy pogo stick.
---------------------------------------------------------------------------------------------------------
Front Forks
I have Cartridge Fork Emulators in my TW. This long report tells the story of how I came to get them and gives some technical explanation of what they are and their advantage.
Other options considered
When I first rode my TW I was pretty surprised at just how harsh the suspension was. My other bike is a BMW F650 Dakar which has gorgeous plush suspension. In Cape Town, which is really just a small provincial town, we are lucky to have a really good bike suspension person, Martin Paetzold. I went to him and asked what he had available. He suggested the forks from a Yamaha YZ85; he did not have them but had sold the owner competition forks so they were available. They were lovely upside down forks:
That is them alongside the TW forks. Somewhat longer and much thicker. I have a cnc machining centre so I could make new triple clamps and the adapter for the brake calliper. However, when i came to have a good look I found the axle hole on the YZ forks is 12mm but the TW axle is 15mm and I judged that there would be insufficient metal remaining if I opened the hole to 15mm. So I abandoned that idea.
Next a used Touratech suspension upgrade for the Dakar became available. The Touratech stuff is actually made by WP. Nice upside down forks complete with the triple clamps and brake calliper mounting plate. So I bought that and fitted it to the Dakar. Now I had the Dakar forks available for the TW.
The upside down forks in the center are another pair that Martin had available. The fork on the right is from the TW and those on the left are the Dakar forks (made by Showa). The Dakar forks have 210 mm travel which is 30% more than the 160mm travel of the TW. I had a good look at it and they could be used (there was enough space for the extra wheel travel) but, in the end, I decided against using them on the TW. I fell off the TW a while back on a good hard gravel road where i must have been gazing at the scenery and hit a lonely pot hole & was seriously concussed. I am scared of that happening again as it took a long time for my brain to recover. The axle on the Dakar is 18mm further forward from the centreline of the fork than it is on the TW. That would reduce the trail from 95mm to 77mm (about 20%). The less trail you have the less stable is the bike & that is just what I did not want so I decided against fitting the Dakar forks to the TW.
The same reduction in trail applies in the case of the Honda forks that Martin had so I decided against them also.
Here is a TW with Yamaha YZ125 or YZ250 front forks. That is what I would really like as the axle is also very close to the centreline of the fork so there would be little loss of trail. That is some German TW enthusiast’s bike whose name I did not save.
This is a close up of the YZ forks showing how close the axle is to the centreline of the fork. The closer it is the greater the trail (all other things being equal).
Martin then offered to make me Cartridge Fork Emulators for the TW. To explain them I need to discuss front fork types.
Damper Rod Forks
Here is an excellent article describing how our TW suspension works – it happens to be a Harley site.
This is the diagram and write up for the front forks from that Harley site.
I want to elaborate on that article. The damping piston is attached to the lower (outer) part of the fork by the long tube sticking up the inside. There are two oil seals; the main one at the top of the lower (outer) part which seals against the outside of the upper (inner) slider and the second seal which is between the damping piston and the inside of the upper slider. The change of volume for the oil in the lower slider is equal to just the thickness of the steel tube of the upper slider – that is all that is moving past the two seals into the lower slider when the fork compresses. There is thus not much oil moving through the compression metering holes. That is one of the great problems with simple damper rod forks – there is relatively little oil flowing through the metering system so it is very difficult to regulate that flow accurately. But that is compounded by the primitive mechanism that is used for that control.
This is a more detailed diagram (from the next link I give) of a Damping Rod Fork as fitted to the TW and Dakar. Pretty primitive forks nowadays.
This diagram from Tony Foale’s book shows, as curve B, the resistance to flow of the oil through the compression orifice. The quicker the fork moves the greater the resistance – but it increases as the square of the speed so the resistance becomes very great for a hard bump. The slope of the curve is determined by the size of the hole (and also the viscosity of the oil). I believe the TW hole is relatively small so it has a steep curve compared to a much flatter curve for the Dakar – the Dakar forks have much more travel and the ride is much plusher compared to the harsh TW ride.
EDIT: What the suspension people are aiming for is curve A. The x axis (bottom scale) is the speed the fork is compressing at; the y axis (side scale) is the resistance (stiffness) of the damping. Curve A hase resistance increasing in proportion to the speed (shown by the straight line indicating the resistance increases exactly in proportion to the speed). What they get with Damper Rod forks is curve B which is below curve A at low speed but then above it at high speed. It is underdamped at low speed and hopelessly overdamped at high speed (it is well below the line A at low speeds and goes miles above it at high speed). Speed here refers to the speed the forks are compressing at – ride over a brick at low speed and the forks have to compress very quickly but curve B is way above curve A so the bike bucks and you have great difficulty controlling it. When the riding becomes challenging the forks are just too stiff. Of course the damping holes can be made bigger which makes curve B slope further to the right so the forks are not so stiff at high speed but that just makes them even worse at low speed than they were. Or the holes can be made smaller so they are better at low speed but then worse at high speed. What Cartridge forks do is give us curve C which is a pretty close approximation of the wanted curve A. (In fact they may even make curve C sag below curve A = regressive damping when the cartridge forks have separate high & low speed damping).
Table 1: Front fork speed when hitting square bumps at various speeds
Bump Motorcycle Speed
Height 20 MPH 40 MPH 60 MPH
1/8" 29 ips 58 ips 87 ips
1/4" 41 ips 82 ips 123 ips
1/2" 58 ips 117 ips 175 ips
1" 83 ips 167 ips 250 ips
2" 121 ips 242 ips 363 ips
4" 283 ips 566 ips 850 ips
(ips= inches per second - an archaic measure of speed)
This is the table from the Harley site I listed earlier. Look at the huge variation in fork speed depending on the bike speed and bump height. That shows the difficulty in making a good Damper Rod Fork – if the hole is to be effective at the lower damper speeds then it will be too stiff (harsh) at the higher speeds, conversely, if the orifice is big enough so that the forks are not harsh during high fork speeds then there is no damping at low speed so the bike wallows up and down like a lazy pogo stick.
