XRAY - Model racing cars

hehe..OK here's a quote from the best r/c handling guide online with the proper "science" if you like of shock holes/oil thickness etc..

If there's anything a racing enthusiast needs to know about fluid dynamics, it's that there are two basic ways for a fluid to flow; laminar and turbulent. A flow is said to be laminar if the particles move parallel to each other, creating flow lines that never intersect. Laminar flow occurs when the velocity is low, the fluid has a high viscosity, and the surface is smooth and well-rounded. A flow is said to be turbulent if the particles move randomly, creating eddies. Situations where the velocity is high, the fluid is thin and the surface is rough favor turbulence. In case of turbulence, a lot more energy is required(or wasted, depends how you look at it) because there is a lot more friction between the particles. Also, for a laminar flow the pressure (resistance, in case of a damper) is proportional to the velocity of the fluid whereas in case of turbulence, it's proportional to the velocity squared. There is no strict distinction between the two types; there's a big gray area in between.
To predict whether or not a flow is turbulent, the Reynolds number is used. It's defined as Re = D * V /n . D is the diameter, V is the velocity of the fluid, and n is its viscosity. If Re is smaller than 2000, the flow is most likely to be laminar, if it's in between 2000 and 4000 it's something in between, and if it's greater than 4000, the flow is most likely turbulent.

Now consider a typical R/C damper unit: you have oil of a certain viscosity passing through orifices of a certain diameter at a certain speed. Some oil flows around the outside of the piston, this is almost always laminar, since the gap between the piston and the housing is so narrow, so it creates a lot of drag. For the oil flowing through the holes in the piston however, it's hard to predict. When the shaft speed is very low it will be laminar, and when it's high it will be turbulent. Exactly when the transition will happen is hard to predict, but easy to feel: because the resistance of the shock is proportional to the shaft speed when the flow is still laminar, and proportional to the shaft speed squared the very next moment, when the flow has turned turbulent, it feels like a kind of hydraulic lock has occurred because the difference in resistance is usually quite substantial. The transition is sometimes also described as 'pack'; it feels as if the shock 'packs up'.

This effect can both be useful and unwanted: it can prevent your car from slapping the ground when landing from a jump, but it can also make your car bounce very badly over sharp ruts or bumps taken at high speed. So it's pretty important to get this adjustment right.

The way to achieve this is to select the right piston and shock oil: both the combination of a piston with small holes and a low viscosity oil and the combination of a piston with large holes and a high viscosity oil will yield the same static damping; it will feel the same when you bump your car by hand. It will also make the car handle the same in low-speed transitions, such as smooth cornering and low-frequency bumps. But the real difference is in the high-speed damping: the first combination will pack up very rapidly because of the low viscosity fluid and the increased fluid velocity. (the same amount of oil has to pass through smaller holes in the same amount of time, so its speed must be higher) The second combination will have a relatively high resistance to turbulence, because of the very thick fluid which flows at a much lower speed. Hence, turbulence will occur at much higher shaft speeds, or it may not occur at all.

So selecting the right piston and oil depends largely on the track layout. Killer jumps or chassis-wrecking bumps require pistons with small holes to prevent the chassis from slapping the ground and usually making the car very unstable. On the other hand, if the track has lots of bumps or is very rutted, any packing up of the shocks would make the car bounce and thus very unstable. In that case try pistons with larger holes.

there you go.... basically small holes/thin oil make the oil flow faster creating turbulence which gives the "pack" effect. A further thing to take into consideration is air bubbles trapped in the oil and passing through the holes cause a lot of friction and heat and degrade your oil performance as it heats up.This is why you bleed your shocks to remove the air.however any air left is "held " longer in suspension by thicker oil so the heating effect is increased.Thinner oil allows the bubbles to rise faster so theoretically they will be at the top of the shock and not passing through the piston..Nothing beats testing to find what works for you though!!

very informative!! thanks!

hello all, i must admit i am finding this thread very informative and interesting, i have experimenting with tapered pistons for a couple of weeks now, and am looking forward to trying some of these ideas at the srs in kent next week, i was running some home made tapered 6 hole 1.3mm with some 50 wt front and 55 wt rear shock oil which worked well at herts during the qualifiers but the rear lost a lot of damping about 8 minutes into the main the front was looser but still just about managable. after reading this i think the oil must have thinned considerably after that time, and i am now looking at getting some 10 hole, either 1.1mm or 1.0mm. in the meantime do you think its worth going a lot thicker oil maybe up to 70 or 80 wt front and up 5 on the rears until i can get the proper pistons that is. i am trying to work it all out, but many heads are better than one, especially mine.

It's no wonder we get confused with what's going on with the oils when we have false statements on all shock oils "temperature stable" yeah right.

The quick fix for you is to keep with the same oils during qualifying and go up 10w for the finals till you get the pistons you want. In hot weather start higher anyway.

What weight shock oils is everyone most commonly using with the new 10x1.1 pistons

Im running 40-45 front 45-50 rear, this is for a medium bumpy dirt track