11 replies to this topic

[TimmyG]

Hi there,
As promised in a previous discussion on the forum with BMCecosse I’ve done some basic tests to prove whether or not different types of damper can affect ride height, as there are those that say they can and those that say they can’t.
First things first, which dampers are which?

• Basic twin-tube design oil-filled dampers as fitted to all standard dry cone suspension mini’s (except maybe sportspack, i'm not sure whether koni's are just oil or oil/gas). Examples of which are;- Sachs/Boge, KYB oil, Unipart etc. These can be eliminated from the test as they are non-pressurised. They do not extend on their own and therefore cannot impart any force by themselves which in turn means they cannot raise the ride height.
• Adjustable oil-filled twin-tube;- examples include AVO, GAZ, and i think SPAX dampers. (if they don’t extend on their own they are not ‘gas pressurised’) though may be marketed as such by saying ‘gas filled’ which is not at all the same. (Air is a gas.) As above re cannot raise ride height.
• Gas-pressurised twin tube;- examples include KYB gas (not gas-a-just) GMAX, and i think Koni sport (haven’t seen these). These are low pressure gas charged and will extend on their own but with low force. Theoretically will raise ride height a minute amount.

· High Pressure gas Monotube;- KYB gas-a-just, Bilstein, De-Carbon. These use nitrogen (or other inert gas) at a pressure about ten times that of the low pressure type, and this causes the damper to extend with considerable force. Theoretically will raise ride height by a measurable amount.
The test i did was merely to get some ballpark figures on the force with which the dampers extend, and therefore their likely effect on ride height. When i get the chance i will add to this with actual ride height figures if anyone is interested.
The dampers used for the low pressure gas twin tube were 200ish mile GMAX, and the High pressure monotube type were KYB gas adjust. Front’s had covered about 5k miles, rear’s brand new.
Using a pillar drill and a weighing scales i held the dampers at just in from fully extended, half stroke, and just shy of full stroke and took readings from the scales at each point once stabilised.
Results were predictable but here they are in approximate round numbers, taken from doing all four dampers of each type. (they matched anyway).
· KYB gas-a-just;- at full nearly full extension = 15kg force.
At half stroke = 20kg force.
At nearly full stroke = 25kg force.
· GMAX;- in the same order, 1.5kg
3kg
5 kg.

So at half stoke which is about where the damper will be operating, a pair of this type KYB will lift the rear of your mini to the tune of lifting 40ish kilos out of the boot! Thats the equivalent of FOUR batteries!
At the front where the wheel to damper movement ratio is about 2:1 the effect will be half of that but the suspension will still be reacting like a still considerable 20kgs has been removed from under the bonnet. So yes, monotube KYB’s do have a spring rate to a small degree (about 3.5lbs/inch) but have a large pre-load. (the force doesn’t increase by a large amount through it’s travel but it has high initial force).
With the low pressure dampers, the effect is too small to be of any consequence.

I hope this info is of help so someone anyway! :)


Tim.

[tiger99]

That is very interesting information. But I think that the effect on ride height of even the highest pressure gas filled dampers is not huge and although it might be quite dramatic on a coil sprung car, the highly variable spring rate of the Mini cones should reduce the dependence of ride height on damper pressure. Measurement of the force/displacement curve of new and partly worn cones, of the several different types available, would be another very useful bit of information.

It is amazing how many people, not just Mini owners, think that they need to change dampers to fix problems with the ride height, especially when they are not using pressurised dampers. That error even creeps into certain magazines. Lack of basic education unfortunately, the basic damped second order system (spring, mass, damper) or its electrical equivalent (capacitor, inductor, resistor) should be taught as part of basic science in all schools.

[TimmyG]

I'm glad somebody found it interesting! Agreed on your comments, the overall effect is still going to be quite small due to the rapidly rising spring rate mini has with rubber cones, but on the rear the damper is working directly with wheel travel whereas the spring is 'diluted' 5 to 1 at the wheel. I estimate putting 40kg's in the boot would probably close the wheel to arch gap by 1/4 to 1/2 an inch. Which is why some people have noticed that their cars ride height has changed after replacing normal dampers with the high pressure type, only to be shot down by some on this forum, the tech section, where facts are supposed to overthrow fiction, myth and opinion. As you say these myths and opinions become legend and even find their way into the press which is why i don't just believe everything i read. Makes me mad when misinformation is presented in magazines, mind you, even Keith Calver has said previously that dampers cant affect ride height, so you can't blame anyone for believing it!

[A-Cell]

Good piece of observation and myth busting facts. And the mini with rubber cones was not designed or optimised for gas pressurised dampers. The original spec was for Telescopic orifice control double acting. Gas pressurised dampers spoil the secondary ride of the standard car, making it jerky with very small inputs.

[Guess-Works.com]

Urm.... right I've had time to inwardly digest the above and give a constructive reply, after sitting down for a while, drinking a cup of coffee and counting to ten...

I was not aware of the original thread...

but..

You experiment is missing one important fact...

The corner of a mini weighs considerably more than 25kg's

For a pressurised damper to lift the side of the car it must first take the weight of the car off the spring, this aint gonna happen !

To complete your experiment, take your scales, put them under a jack and then lift the body of the car, just enough to move it, and then look at the scales..

[TimmyG]

For a pressurised damper to lift the side of the car it must first take the weight of the car off the spring, this aint gonna happen !

But this is exactly what is happening. If the damper is mounted at one end to the wheel, and the other to the body, and it has an extension force of 20 kgs, that 20 kgs has just bypassed the spring straight to the wheel. I'e the wheel is still carrying the full weight but the spring isn't.

The jack and scales was my next task..... :)

Edited by TimmyG, 26 January 2013 - 12:05 PM.

[Allrounder]

Or how about removing the dampeners from behind the rear wheels, measure the ride height, then to apply upwards force to the lower part of the dampener to see if the height is altered?

[Ethel]

The pressure in the damper only has the cross sectional area of the pushrod to act on, perhaps a 20th of a square inch. For comparison a Mini coilover spring will be rated at about 200lbs/in. Your shockers would have to be pressurised similar to a firing assault rifle to see any appreciable extra height.

[TimmyG]

Its not the spring rate thats going to make the difference though, the rate works out at a measly 3ish pound per inch, as you say a tiny amount and negligable, but it's the pre-load that will raise the ride height. The dampers have a pre-load of 33 pounds each at full extension, which at the rear of a mini which is light, is going to lift it a little, not a huge amount granted, but a little, maybe 1/4 to 1/2 inch? If we add 65lbs of weight into the boot the suspension will compress? Therefore adding 65lbs of preload has to lift it by a similar amount. More preload means less sag.

The high-pressure gas damper is basically acting like a very very long but low rate spring thats been compressed right up, so it is pre-loaded to a high degree but another few inches further compression doesn't add much to that.

[Dusky]

Very intresting , just ordered gas a just shocks. Ill measure various points moe (spax chocks) and later with the a just shocks.

[nicklouse]

Don't waste your time.

The pressure is there to stop/reduce cavitation and foaming of the damping oil.

[Spider]

The effect of any gas pressure in shocks on a Mini's ride height is negligible / academic and good luck actually measuring it.

 

The effect of any non - gas pressure in shocks on a Mini's ride height is zero.