Mechanical grip

Old 01-30-2004, 01:10 AM
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Default Mechanical grip

what does the term mechanical grip mean? also, ive read that you should change to a higher rate spring for a harder susupension instead of adding preload. but when i add preload the shock does physically feel harder, so i dont understand. like if the ride height doesnt get any higher after adding a certain amount of preload will i make the shock harder if i add more preload after that point where the rideheight stopped increasing?(provided if i dont want to run any droop) i know this is abit confusing to read but i really dont understand.
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Old 01-30-2004, 03:33 AM
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Mechanical grip, is grip gained via the suspension. You always want to run some droop, even if it's just a touch. So with that said, if you use preload to make the suspension harder you will have no droop since the suspension has bottomed out. Always change to stiffer springs instead of adding preload.
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Old 01-30-2004, 04:11 AM
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Mechanical grip refers to the cornering force generated by the suspension and tires. The other type of cornering force is aerodynamic grip, and it is generated by the body and what we in RC call a wing (although it is really a spoiler).

Your question of pre-load is an excellent one. If you were to convert spring load and pre-load into wheel rate (the combined effect seen at the contact patch), yes, the initial movement would be the same. Where it differs is after you have "broken" the pre-load. You see if you take a 20 lbs/in spring and apply 0.250" of pre-load (after you hit the droop stop), you now have to apply 5 lbs to break pre-load. So even if you were to have a 25 lbs/in spring, that 5 lbs of load on the spring would generate 0.200" of spring movement, but your 20 lbs/in spring hasn't moved!
The big difference is after, another 5lbs of load would break pre-load and you would have 0.250" movement, where your 25 lbs/in spring will have moved 0.400" total. You see that quickly the 20 lbs/in spring will generate more movement for equal load than the 25 lbs/in spring.

This is not to say that it is bad! Front suspension on Formula cars almost all work this way, except on bumpy tracks. In the rear, however, droop is almost always required.

Hope this helps!
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Old 01-30-2004, 04:44 AM
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I'll say that one of my biggest mistake during my initial setup learning fase, was to not use any droop at all.
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Old 01-30-2004, 06:39 AM
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I think you have preload a little confused. The spring you are using dictates the stiffness not the adjusting collar. all that does is set ride height. With a suspension spring its not actually preload as it should not be bottoming out when the car is sitting idle. Preload would mean that the spring is holding the arm tight against the droop screw or limiter and that would be wrong. If you have no droop you have no suspension movement and therefore no suspension it has to move to do its job. Its the same spring no matter how much you twist on the collar. Only use the adjusters to set ride height. The stiffness of the spring dictates roll in the corner and shock oil dictates speed at wich the roll takes place. Its a hard thing to visualize how a RC car is acting in a corner you have to see it not feel it like in a real car.
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Old 01-30-2004, 07:05 AM
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Thanks for the different replies i think i get hte idea, but i still dont quite understand pilmet's explanation, if pilmet, or anyone else would be kind enough to make it simpler itd be appreciated!

Thanks again
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Old 01-30-2004, 08:59 AM
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Explanation in the simplest terms I can imagine...

A spring is a certain length when it is unloaded.

When you have 4 springs supporting the weight of the car, each will compress a certain amount, say 1mm each as an example.

Now, to adjust the ride height, we will use the spring collars.

Lets set the ride height to 5mm (still with droop).

The springs will still be compressed 1 mm, but the collars will have been moved.

Now imagine we remove all the droop, so 5mm is the maximum ride height the car can have.

Cranking down the collar another 1mm will not affect the ride height, but the spring will now be compressed another 1mm, making it "pre-loaded".

Newtonian Physics states that every action must have an equal and opposite reaction.

Say the spring is rated 250g force for every 1mm movement.

This 1mm preload on all 4 springs means the springs are pushing "up" on the car a total of 1000g MORE than they would without the pre-load.

So, if you were to push "down" on the car with your hand, you would need to use an EXTRA 1000g of force before the car even started to move (hence it "feels" stiffer, even though the spring is in fact the same rate as before).

Once you've applied that extra 1000g force, the suspension will compress the same amount for any extra given force as it would if the springs were not pre-loaded.

That is, the spring is still the same rate, you just meed more force to actually start compressing it in the first place.

Only a harder spring will make the suspension rate stiffer.

Obviously, this pre-load has a substantial effect on the cars pitch/roll and bump handling - it needs bigger cornering forces, and bigger bump forces, to start moving the suspension.

Pre-load is not desirable unless you REALLY know what you're doing with the car's handling (and F1 engineers REALLY know what they're doing - they're also working with much bigger forces, particularly aerodynamic ones, than we do in RC)

Does this make sense?

Last edited by sosidge; 01-30-2004 at 09:08 AM.
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Old 01-30-2004, 09:02 AM
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oops!
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