Year-round shock oil for cold weather
Posted: Tue Dec 18, 2018 8:08 pm
Before I ordered my 2.5" Kings from DamageWagon, I heard stories about how King shocks ride like a dump truck in the cold. Given that I live somewhere cold, I was concerned about this. I did some research. I ordered my Kings dry. I chose my own shock oil and filled/charged them myself. I've driven on them. They're awesome. Here are my findings.
Bottom Line Up Front: Maxima Racing Shock Fluid 7wt is a winner in these shocks.
First, a little technical schooling on hydraulic oils. I'm a mechanical engineer, but this is not my area of expertise, I had to do some reading.
The viscosity of a hydraulic oil changes with temperature (hence the reported cold weather dump truck ride). A number called the Viscosity Index (VI) is the measure of an oil's temperature stability. It's calculated based on the kinematic viscosities of the oil at 100*C and 40*C (roughly 210*F and 100*F), compared to a reference oil with a VI of 100. If a hydraulic oil data sheet doesn't give you the VI, you can calculate it if the viscosities are given for 100*C and 40*C. When the "Viscosity Index" was invented, 100 was a good VI. With modern advancements in additives, there are oils with VIs in the 200s and 300s (that's just what I've come across, VIs may go even higher). For shock oil, the higher the VI, the more consistent the ride will be across a range of temperatures.
The pour point is the temperature at which the oil loses its ability to flow. Obviously, we want oil in our shocks, not gelled goopy semi-solid. If the ambient temperature is lower than the pour point of your shock oil, you're gonna have a bad time. For shock oil, the pour point needs to be lower than the lowest temperature you expect to drive the truck in, and the lower the better.
So, knowing what I was looking for, I set off on a quest to find out what exactly is in King shocks. Every other brand of shock oil I could find published at least SOME technical data. Somewhere on their website you could find a full blown datasheet, or a pamphlet or flier with a few numbers on it, or a Safety Data Sheet with a few properties. King publishes NOTHING. That was kind of a red flag to me. Either their shock oil is soooo special that its properties are a trade secret, or their shock oil's properties don't stack up against the competition. Given what I had heard about the cold weather ride, I assumed it was the latter. I did a ton of internet searching and sent a few emails, and eventually I found what I was looking for. Given how secretive King is about it, I'm not going to say what they're using for shock oil. I will say it's a quality oil suitable for use in shocks, especially racing shocks that can be tuned for a fairly consistent operating temperature. If your truck is a daily driver instead of a race truck, and you live somewhere cold, King shock oil is probably not the best oil for you.
As far as alternatives go, there are a LOT of options out there. I focused my research efforts on a few alternatives that are easy to get in useful quantities, and affordable at truck shock volumes. Maxima Racing Shock Fluid 7wt and Amsoil Shock Therapy #5 are what you want in your Kings for cold weather. The Maxima has a higher VI, the Amsoil has a lower pour point. Maxima can be bought on Amazon, Amsoil is available through their nation-wide distribution chain, both are about $16 per quart.
How do Maxima RSF 7wt and Amsoil Shock Therapy #5 compare to King?
(I also plotted Maxima 3wt exploring the possibility of a seasonal shock fluid change, which I now don't think will be necessary.)
The Maxima has an extremely high VI and a very flat viscosity-temperature curve. The Amsoil, not as flat as the Maxima, is still miles better than the King oil curve. The viscosity is plotted on a log scale. At the cold end, the viscosity difference between King and the other fluids is huge. Hence, the dump truck ride.
King
VI: 100
Pour point: -30*C (-22*F)
Maxima RSF 7wt (highest VI)
VI: 338
Pour point: -42*C (-44*F)
Amsoil Shock Therapy #5 (lowest pour point)
VI: 209
Pour point: -53*C (-63*F)
I ordered my 2.5" Thuren Kings valved at stage 2, wanting them to still be driveable if they firmed up in the cold. I filled with Maxima RSF 7 wt. The coldest it has been so far is 7*F (-14*C), and I can't feel any noticeable difference when the shocks are cold. I'm sure the ride is a little firmer because the fluid is colder and is more viscous (and every rubber bushing in the truck is harder), but I can't go from 0*F to 70*F instantly to compare, so all I have to go on is seat feel. I'm going to keep an eye out for lower temperatures throughout the winter, but I don't expect to find anything cold enough to change my current impression and I'm contemplating whether I want to try stage 3 or stage 4 next.
Bottom Line Up Front: Maxima Racing Shock Fluid 7wt is a winner in these shocks.
First, a little technical schooling on hydraulic oils. I'm a mechanical engineer, but this is not my area of expertise, I had to do some reading.
The viscosity of a hydraulic oil changes with temperature (hence the reported cold weather dump truck ride). A number called the Viscosity Index (VI) is the measure of an oil's temperature stability. It's calculated based on the kinematic viscosities of the oil at 100*C and 40*C (roughly 210*F and 100*F), compared to a reference oil with a VI of 100. If a hydraulic oil data sheet doesn't give you the VI, you can calculate it if the viscosities are given for 100*C and 40*C. When the "Viscosity Index" was invented, 100 was a good VI. With modern advancements in additives, there are oils with VIs in the 200s and 300s (that's just what I've come across, VIs may go even higher). For shock oil, the higher the VI, the more consistent the ride will be across a range of temperatures.
The pour point is the temperature at which the oil loses its ability to flow. Obviously, we want oil in our shocks, not gelled goopy semi-solid. If the ambient temperature is lower than the pour point of your shock oil, you're gonna have a bad time. For shock oil, the pour point needs to be lower than the lowest temperature you expect to drive the truck in, and the lower the better.
So, knowing what I was looking for, I set off on a quest to find out what exactly is in King shocks. Every other brand of shock oil I could find published at least SOME technical data. Somewhere on their website you could find a full blown datasheet, or a pamphlet or flier with a few numbers on it, or a Safety Data Sheet with a few properties. King publishes NOTHING. That was kind of a red flag to me. Either their shock oil is soooo special that its properties are a trade secret, or their shock oil's properties don't stack up against the competition. Given what I had heard about the cold weather ride, I assumed it was the latter. I did a ton of internet searching and sent a few emails, and eventually I found what I was looking for. Given how secretive King is about it, I'm not going to say what they're using for shock oil. I will say it's a quality oil suitable for use in shocks, especially racing shocks that can be tuned for a fairly consistent operating temperature. If your truck is a daily driver instead of a race truck, and you live somewhere cold, King shock oil is probably not the best oil for you.
As far as alternatives go, there are a LOT of options out there. I focused my research efforts on a few alternatives that are easy to get in useful quantities, and affordable at truck shock volumes. Maxima Racing Shock Fluid 7wt and Amsoil Shock Therapy #5 are what you want in your Kings for cold weather. The Maxima has a higher VI, the Amsoil has a lower pour point. Maxima can be bought on Amazon, Amsoil is available through their nation-wide distribution chain, both are about $16 per quart.
How do Maxima RSF 7wt and Amsoil Shock Therapy #5 compare to King?
(I also plotted Maxima 3wt exploring the possibility of a seasonal shock fluid change, which I now don't think will be necessary.)
The Maxima has an extremely high VI and a very flat viscosity-temperature curve. The Amsoil, not as flat as the Maxima, is still miles better than the King oil curve. The viscosity is plotted on a log scale. At the cold end, the viscosity difference between King and the other fluids is huge. Hence, the dump truck ride.
King
VI: 100
Pour point: -30*C (-22*F)
Maxima RSF 7wt (highest VI)
VI: 338
Pour point: -42*C (-44*F)
Amsoil Shock Therapy #5 (lowest pour point)
VI: 209
Pour point: -53*C (-63*F)
I ordered my 2.5" Thuren Kings valved at stage 2, wanting them to still be driveable if they firmed up in the cold. I filled with Maxima RSF 7 wt. The coldest it has been so far is 7*F (-14*C), and I can't feel any noticeable difference when the shocks are cold. I'm sure the ride is a little firmer because the fluid is colder and is more viscous (and every rubber bushing in the truck is harder), but I can't go from 0*F to 70*F instantly to compare, so all I have to go on is seat feel. I'm going to keep an eye out for lower temperatures throughout the winter, but I don't expect to find anything cold enough to change my current impression and I'm contemplating whether I want to try stage 3 or stage 4 next.