Inverted struts
13 January 2002, 09:40 PM
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Inverted dampers provide extra strength by having the thicker part of damper at top (high stress position). This means the thinner part (piston) is contained within an extra casing which is externally threaded to hold the adjustable spring platforms.
I used to run a road/rally 205 and managed to break a front damper (piston at top) under hard braking. The damper snapped just under the top mount. I then went out and bought some Bilstein inverted dampers/casing fom Peugeot Sport...not more worries
There might be some other reasons though.....
I used to run a road/rally 205 and managed to break a front damper (piston at top) under hard braking. The damper snapped just under the top mount. I then went out and bought some Bilstein inverted dampers/casing fom Peugeot Sport...not more worries
There might be some other reasons though.....
14 January 2002, 02:14 PM
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Also the main weight of the unit is bolted down and the lighter bit moves, giving a lighter unsprung weight, which helps speed of response of the suspension = better handling
15 January 2002, 11:12 AM
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Inverted dampers are weaker, but there is a saving in unsprung weight. That's the only reason for using them.
I haven't seen a WRC car using inverted units, but strength is the main priority for these cars.
I haven't seen a WRC car using inverted units, but strength is the main priority for these cars.
15 January 2002, 06:51 PM
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I am very possibly (read: most likely) wrong about the WRC bit.
Would the saving in unsprung weight compensate for the increased weight of the brembo brake setup?
Andy
Would the saving in unsprung weight compensate for the increased weight of the brembo brake setup?
Andy
15 January 2002, 07:51 PM
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On bikes the advantage is in unsprung weight (and fashion). Important on a bike as the sprung/unsprung ratio is much closer, but on a ton of car...?
Richard.
Richard.
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17 January 2002, 09:57 AM
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The bennefit on bikes is both strength, (as pointed out by Peter) and unsprung weight.
Don't forget, on a bike, the forks are also a "stressed member" whereas in a car those loads are carried by the wisbone Etc. The extra strength on a bike arises from the extra rigidity of being able to brace (In the yolks) the thicker outer tubes of the dampers making the front end more rigid.
I can't see why inverted dampers on a car should or would be weaker... they should be exactly the same?
Mike.
Don't forget, on a bike, the forks are also a "stressed member" whereas in a car those loads are carried by the wisbone Etc. The extra strength on a bike arises from the extra rigidity of being able to brace (In the yolks) the thicker outer tubes of the dampers making the front end more rigid.
I can't see why inverted dampers on a car should or would be weaker... they should be exactly the same?
Mike.
17 January 2002, 08:44 PM
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Mike
The strength issue is down to hubs ability to resist rotational forces, be there fore/aft (as generated in braking/accelerating) or lateral (resulting from sideways loading).
In a McPherson strut set up, like ours, the hub is held in place by the damper. The damper tube is normally very strong and the sideways load is transmitted onto the rod or tube about 300mm away from the centre. The rod or tube is always narrower than the case and will therefore always be weaker.
If you imagine the sideways loads imposed by a tarmac WRC car landing sideways, on slicks, or worse still a gravel WRC car (with a high ride height and long dampers) drifting across a hard forest stage road which has ruts in it, I know how I would want my dampers configured.
Generally motorbikes do not incur sideways loading because the rider banks into the corner keeping the mass over the vectored force. If they do receive a sideways loading, I think the result is called a "high side"
As the load is almost always vertical down the line of the damper, this allows them to use very light inverted units with very thin piston rods.
The strength issue is down to hubs ability to resist rotational forces, be there fore/aft (as generated in braking/accelerating) or lateral (resulting from sideways loading).
In a McPherson strut set up, like ours, the hub is held in place by the damper. The damper tube is normally very strong and the sideways load is transmitted onto the rod or tube about 300mm away from the centre. The rod or tube is always narrower than the case and will therefore always be weaker.
If you imagine the sideways loads imposed by a tarmac WRC car landing sideways, on slicks, or worse still a gravel WRC car (with a high ride height and long dampers) drifting across a hard forest stage road which has ruts in it, I know how I would want my dampers configured.
Generally motorbikes do not incur sideways loading because the rider banks into the corner keeping the mass over the vectored force. If they do receive a sideways loading, I think the result is called a "high side"
As the load is almost always vertical down the line of the damper, this allows them to use very light inverted units with very thin piston rods.
18 January 2002, 11:54 AM
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Pete,
the reason you use inverted shocks on a car is to have a more responsive shock. A standard shock has a rod diameter of appr. 22 mm (where the bearings are situated), an inverted one has a tube of appr. 45 to 50 mm, so the bearing pressures are much less. This is the reason why inverted shocks have less friction (practical experience from the behaviour of the bearings, before anyone starts a discussion!) and so a better respons.
Forks for motorcycles are also used in motocross bikes. The sideways loading (bending) when landing after a jump is enormous. This is why you have seen an increase in tube diameter to appr. 50 mm in for the forks for motocross bike. This diameter is not absolutely necessary for strength (you could also increase the wall thickness), but to reduce the bearing pressure on the bearings and so to have a more repronsive fork.
P.S. I have headed the engineering department of WP Suspension and have so gained experience on shocks and forks.
the reason you use inverted shocks on a car is to have a more responsive shock. A standard shock has a rod diameter of appr. 22 mm (where the bearings are situated), an inverted one has a tube of appr. 45 to 50 mm, so the bearing pressures are much less. This is the reason why inverted shocks have less friction (practical experience from the behaviour of the bearings, before anyone starts a discussion!) and so a better respons.
Forks for motorcycles are also used in motocross bikes. The sideways loading (bending) when landing after a jump is enormous. This is why you have seen an increase in tube diameter to appr. 50 mm in for the forks for motocross bike. This diameter is not absolutely necessary for strength (you could also increase the wall thickness), but to reduce the bearing pressure on the bearings and so to have a more repronsive fork.
P.S. I have headed the engineering department of WP Suspension and have so gained experience on shocks and forks.
18 January 2002, 12:05 PM
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Something I forgot.
The forks of motor cycles can be conventional or upside down. Conventional means the thinner part is on top and clamped, with the upside down fork the thicker part is on top and clamped. Due to the spacing of the clamps the bearings of the upside down fork can be spaced further apart. Thus reducing the bearing pressure and friction, with the result a more responsive fork. Don't think these friction forces are small. If you reduce the friction you need to make a completely different damping set up for the fork. And (coulomb) friction is also a kind of damping (just look at the very old type of friction "shocks") but difficult to control and tune. You really want to control your damping with your shim package (among others).
The forks of motor cycles can be conventional or upside down. Conventional means the thinner part is on top and clamped, with the upside down fork the thicker part is on top and clamped. Due to the spacing of the clamps the bearings of the upside down fork can be spaced further apart. Thus reducing the bearing pressure and friction, with the result a more responsive fork. Don't think these friction forces are small. If you reduce the friction you need to make a completely different damping set up for the fork. And (coulomb) friction is also a kind of damping (just look at the very old type of friction "shocks") but difficult to control and tune. You really want to control your damping with your shim package (among others).
28 January 2002, 06:00 PM
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I think we are talking at cross purposes here.
You have rod dampers and mono tube dampers. Mono tube are much stronger, much more expensive to make and much harder to make adjustable. An example of a mono tube is a Bilstein or DMS.
A classic inverted damper is like the sort you would see on a bike or a Caterham. Rod going down to the wheel, damper body attached to bike/car.
An inverted monotube is the same and this is what you see on motorbike front forks. If the rider keeps the his centre of gravity and that of the bike in the line of the force acting on the tyre (he leans into the bend), the vertical loads they can take are enormous. If the rider slides and hits a kerb or something, they get bent and he gets tipped off.
The damper shown in the picture is somewhere between the two. It may have the mechanism at the top, but it has a substantial casing/spring seat support at the bottom, which surrounds the piston. This unit may be very strong, but I would expect it to be quite heavy too.
Ideal for gravel rallying, but the age old advantage of inverting the damper to reduce unsprung weight doesn't work here.
You have rod dampers and mono tube dampers. Mono tube are much stronger, much more expensive to make and much harder to make adjustable. An example of a mono tube is a Bilstein or DMS.
A classic inverted damper is like the sort you would see on a bike or a Caterham. Rod going down to the wheel, damper body attached to bike/car.
An inverted monotube is the same and this is what you see on motorbike front forks. If the rider keeps the his centre of gravity and that of the bike in the line of the force acting on the tyre (he leans into the bend), the vertical loads they can take are enormous. If the rider slides and hits a kerb or something, they get bent and he gets tipped off.
The damper shown in the picture is somewhere between the two. It may have the mechanism at the top, but it has a substantial casing/spring seat support at the bottom, which surrounds the piston. This unit may be very strong, but I would expect it to be quite heavy too.
Ideal for gravel rallying, but the age old advantage of inverting the damper to reduce unsprung weight doesn't work here.
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