tlartist
Tom Laux
In the process of researching the Sonic spring install I fell down the rabbit hole. Read on....
For those interested in fork oil, here is a good summary: http://www.advrider.com/forums/showthread.php?t=286065. I guess weight is not that accurate of a measure for fork oil, according to the linked post.
The shop manual recommends Pro Honda suspension fluid ss-8. This is 10wt, which as I said does not seem like the most accurate measurement. cSt is more relevant. cSt @ 40 degrees is 39.7, cSt @ 100 degrees is 7.6. Viscosity index I could not find.
I read on another site that the recommendation is that you should stay close to the stock cSt because that is what the forks were designed for. Either way, it is somewhat interesting that when I read the ST site there were recommendations for 7.5 weight. Sonic recommended 10 wt, and I am going to go with this since both Sonic and the service manual recommend this. So, something around the cSt of the recommended Honda fluid - cSt @ 40 degrees of 39.7.
Below is some info from this site: http://www.peterverdonedesigns.com/lowspeed.htm. One main point: "So specifically with regard to suspension, the greater the VI, the more consistent the damping will be over a large temp change."
Also, the cSt and VI vary from oil to oil. There is a chart on the web site above if you want this info.
From the site above:
The oil in a bike's fork or shock not only cools and lubricates the system, but is the heart of all the damping control available for the springs. All phases of the damping involve the viscosity of the oil used in the system. Oil is such a critical suspension tuning decision that is the first priority to get worked out after spring/sag choice. No matter what all of your friends say about what the greatest high speed valves are or wild shim stacks, you should agonize over oil choice.
DO NOT GO BY LABLED OIL WEIGHT! Not only is this a poor way to decide which oil to use, but each manufacturer seems to be on a different scale. For example Maxima RSF 7wt is actually lighter than Spectro 5wt and Silkolene Pro RSF 7.5wt is actually heavier than Showa SS-8 10wt. This is not a judgment about the quality of these oils, just that the 'weight' label leads to a lot of trouble when trying to tune with suspension oils.
Suspension oil is labeled by weight because consumers are used to thinking of oil (ie. motor oil & gear oil) in these terms. The SAE weight system has a very broad and vague viscosity range and does not even cover the viscosity range that most quality motorcycle suspension systems require. Another point of confusion is that motor oils and gear oils are rated for viscosity on the SAE scale at different temperatures, leading to similar viscosity oils having very different ratings. SAE J300 is used to define engine oils, while SAE J306 is used to define drive line (gear) and chassis lubricants.
SAE Motor Oil Grade cSt @100C
0W - 3.8
5W 3.8 - 4.1
10W 4.1 - 5.6
15W -5.6-
##W-20 5.6 - 9.3
##W-30 9.3 - 12.5
##W-40 12.5 - 16.3
##W-50 16.3 - 21.9
##W-60 21.9 - 26.1
Some oils are labeled on the SUS / VI (ie. 85/150) or Saybolt Universal Second / Viscosity Index. Although this is slightly better system than using SAE 'weights', the Saybolt Universal Second is considered antiquated, is vague and is not a valid viscosity reference. ASTM D88-94(1999).
Modern fluid viscosity ratings are on an ISO VG (visocosity grade) (ASTM D-2422-97 (2002) & ISO 3448:1992) system of: cSt@40C / cSt@100C / VI. ?cSt? stands for CentiStokes, an accurate dynamic measure of viscosity (ISO 3104:1994 & ASTM D445-04). Using these numbers, you can tune oil viscosity with a very high degree of precision in a specific heat range. A straight line graph made from these two viscosity numbers at each temperature can give a very close approximation of the oils viscosity at any other usable temperature. A guide to ISO viscosity grades is HERE. A rough conversion table from SUS grades is HERE. A very, very rough conversion table from SAE gear and motor oil grades is HERE.
The Viscosity Index is very important in a suspension oil. ?The viscosity index (V.I.) of an oil is a number that indicates the effect of temperature changes on the viscosity of the oil. A low V.I. signifies a relatively large change of viscosity with changes of temperature. In other words, the oil becomes extremely thin at high temperatures and extremely thick at low temperatures. On the other hand, a high V.I. signifies relatively little change in viscosity over a wide temperature range.? So specifically with regard to suspension, the greater the VI, the more consistent the damping will be over a large temp change.
The Viscosity Index is calculated as such:
VI = 100+(10^(LN((EXP(1.0727+0.6175*LN(cSt@100)+0.9744* (LN(cSt@100)^2)+(-0.3764)*(LN(cSt@100)^3)+0.04824*(LN(cSt@100)^4)))/cSt@40)/LN(cSt@100))-1)/0.00715
On a lovely Sunday morning, the ambient temperature may be 21C (70F). Motorcycle forks will run in the 26C (78F) temp range, rear shocks will run in the 65C (150F) range and rear reservoirs will be around 43C (110F). While motorcycle rear shocks require very high VIs (over 300) to function well over such a huge temperature range, motorcycle forks and bicycles do not. Anything over 100VI will be serviceable for them.
Some suspension oils are synthetic vs. mineral. These are very high quality oils and most general service applications do not require their use. You should, however, consider synthetic oil for performance applications. My Penske rear shock does require a full synthetic oil. Most oils on the market are either full mineral oil or a blend of mineral and synthetic oils. Almost all suspension oils will contain viscosity modifiers, seal conditioners, and anti-foam agents.
I suggest using either Silkolene Pro RSF (PDS)(ester) or Red Line (PDS)(polyol ester) synthetic suspension oils for front and rear suspension systems. They both have very high VI numbers and have enough of a viscosity selection to produce any mix you may need. You should, however, stay with one brand whichever way you go. I have decided to use Red Line exclusively. They range from extremely thin to extremely thick, in five different viscosities
For those interested in fork oil, here is a good summary: http://www.advrider.com/forums/showthread.php?t=286065. I guess weight is not that accurate of a measure for fork oil, according to the linked post.
The shop manual recommends Pro Honda suspension fluid ss-8. This is 10wt, which as I said does not seem like the most accurate measurement. cSt is more relevant. cSt @ 40 degrees is 39.7, cSt @ 100 degrees is 7.6. Viscosity index I could not find.
I read on another site that the recommendation is that you should stay close to the stock cSt because that is what the forks were designed for. Either way, it is somewhat interesting that when I read the ST site there were recommendations for 7.5 weight. Sonic recommended 10 wt, and I am going to go with this since both Sonic and the service manual recommend this. So, something around the cSt of the recommended Honda fluid - cSt @ 40 degrees of 39.7.
Below is some info from this site: http://www.peterverdonedesigns.com/lowspeed.htm. One main point: "So specifically with regard to suspension, the greater the VI, the more consistent the damping will be over a large temp change."
Also, the cSt and VI vary from oil to oil. There is a chart on the web site above if you want this info.
From the site above:
The oil in a bike's fork or shock not only cools and lubricates the system, but is the heart of all the damping control available for the springs. All phases of the damping involve the viscosity of the oil used in the system. Oil is such a critical suspension tuning decision that is the first priority to get worked out after spring/sag choice. No matter what all of your friends say about what the greatest high speed valves are or wild shim stacks, you should agonize over oil choice.
DO NOT GO BY LABLED OIL WEIGHT! Not only is this a poor way to decide which oil to use, but each manufacturer seems to be on a different scale. For example Maxima RSF 7wt is actually lighter than Spectro 5wt and Silkolene Pro RSF 7.5wt is actually heavier than Showa SS-8 10wt. This is not a judgment about the quality of these oils, just that the 'weight' label leads to a lot of trouble when trying to tune with suspension oils.
Suspension oil is labeled by weight because consumers are used to thinking of oil (ie. motor oil & gear oil) in these terms. The SAE weight system has a very broad and vague viscosity range and does not even cover the viscosity range that most quality motorcycle suspension systems require. Another point of confusion is that motor oils and gear oils are rated for viscosity on the SAE scale at different temperatures, leading to similar viscosity oils having very different ratings. SAE J300 is used to define engine oils, while SAE J306 is used to define drive line (gear) and chassis lubricants.
SAE Motor Oil Grade cSt @100C
0W - 3.8
5W 3.8 - 4.1
10W 4.1 - 5.6
15W -5.6-
##W-20 5.6 - 9.3
##W-30 9.3 - 12.5
##W-40 12.5 - 16.3
##W-50 16.3 - 21.9
##W-60 21.9 - 26.1
Some oils are labeled on the SUS / VI (ie. 85/150) or Saybolt Universal Second / Viscosity Index. Although this is slightly better system than using SAE 'weights', the Saybolt Universal Second is considered antiquated, is vague and is not a valid viscosity reference. ASTM D88-94(1999).
Modern fluid viscosity ratings are on an ISO VG (visocosity grade) (ASTM D-2422-97 (2002) & ISO 3448:1992) system of: cSt@40C / cSt@100C / VI. ?cSt? stands for CentiStokes, an accurate dynamic measure of viscosity (ISO 3104:1994 & ASTM D445-04). Using these numbers, you can tune oil viscosity with a very high degree of precision in a specific heat range. A straight line graph made from these two viscosity numbers at each temperature can give a very close approximation of the oils viscosity at any other usable temperature. A guide to ISO viscosity grades is HERE. A rough conversion table from SUS grades is HERE. A very, very rough conversion table from SAE gear and motor oil grades is HERE.
The Viscosity Index is very important in a suspension oil. ?The viscosity index (V.I.) of an oil is a number that indicates the effect of temperature changes on the viscosity of the oil. A low V.I. signifies a relatively large change of viscosity with changes of temperature. In other words, the oil becomes extremely thin at high temperatures and extremely thick at low temperatures. On the other hand, a high V.I. signifies relatively little change in viscosity over a wide temperature range.? So specifically with regard to suspension, the greater the VI, the more consistent the damping will be over a large temp change.
The Viscosity Index is calculated as such:
VI = 100+(10^(LN((EXP(1.0727+0.6175*LN(cSt@100)+0.9744* (LN(cSt@100)^2)+(-0.3764)*(LN(cSt@100)^3)+0.04824*(LN(cSt@100)^4)))/cSt@40)/LN(cSt@100))-1)/0.00715
On a lovely Sunday morning, the ambient temperature may be 21C (70F). Motorcycle forks will run in the 26C (78F) temp range, rear shocks will run in the 65C (150F) range and rear reservoirs will be around 43C (110F). While motorcycle rear shocks require very high VIs (over 300) to function well over such a huge temperature range, motorcycle forks and bicycles do not. Anything over 100VI will be serviceable for them.
Some suspension oils are synthetic vs. mineral. These are very high quality oils and most general service applications do not require their use. You should, however, consider synthetic oil for performance applications. My Penske rear shock does require a full synthetic oil. Most oils on the market are either full mineral oil or a blend of mineral and synthetic oils. Almost all suspension oils will contain viscosity modifiers, seal conditioners, and anti-foam agents.
I suggest using either Silkolene Pro RSF (PDS)(ester) or Red Line (PDS)(polyol ester) synthetic suspension oils for front and rear suspension systems. They both have very high VI numbers and have enough of a viscosity selection to produce any mix you may need. You should, however, stay with one brand whichever way you go. I have decided to use Red Line exclusively. They range from extremely thin to extremely thick, in five different viscosities