Shock oil choice

[bogan]

So long story short, I have a CBR shock which has non-stock oil in it, and is overdamped even when the adjusters are wound right out.
The oil in it is the medium (7wt) from this page as it was all they had in the shop at the time. What weight should I be trying next, and can you blend the oil to get an average weight if I were to go and get the light (3wt) of the same brand?

Now long story long if anyone needs any background, I did a custom linked suspension system for my bros last year so it was a bit hit and miss, but looks like the only thing that missed was the shock oil. I have done the sag checks and they are just in spec. The shock I bought is from a CBR600F4 but required some modification of the top eyelet and I had to remove the reservoir and replace the oil, so I have no idea what the stock fluid weight is or would perform like. The adjustment was a bit on the overdamped side in summer and when I had a different tail on it. But now with the drop in temperature and a 10kg lighter rear, also a harder seat, it feels far too overdamped.

[Robert Taylor]

So long story short, I have a CBR shock which has non-stock oil in it, and is overdamped even when the adjusters are wound right out.
The oil in it is the medium (7wt) from this page as it was all they had in the shop at the time. What weight should I be trying next, and can you blend the oil to get an average weight if I were to go and get the light (3wt) of the same brand?

Now long story long if anyone needs any background, I did a custom linked suspension system for my bros last year so it was a bit hit and miss, but looks like the only thing that missed was the shock oil. I have done the sag checks and they are just in spec. The shock I bought is from a CBR600F4 but required some modification of the top eyelet and I had to remove the reservoir and replace the oil, so I have no idea what the stock fluid weight is or would perform like. The adjustment was a bit on the overdamped side in summer and when I had a different tail on it. But now with the drop in temperature and a 10kg lighter rear, also a harder seat, it feels far too overdamped.

Briefly, theres several things going on here;

1) If youve changed the link system youve likely changed the motion ratio. If for the same rear axle movement you are moving the shock shaft further then you are moving it faster. Damping is proportional to velocity. Think of it in terms of pushing a fire door with a closing damper on it, push it slowly only a little resistance, push it fast and it almost locks up.

2) A lot of these CBR shocks have ridiculously crude valving that is pretty non responsive and the valving would in any event be mosre suitable for the bike it came out of than the Bros. Also the port sizes in the piston are riculously small so they ''choke off'' over abrupt bumps irrespective of how light the valving is

3) The issue is not oil viscosity because of 1 and 2 above. The aim with a shock is to run as thin an oil as posssible but it aint going to significantly fix this issue.

When Im at work Ill have a look at the respective Ohlins spec cards for the CBR600F4 and the Bros. That ( in the absence of any decent info for the oem shocks ) will give an excellent insight into how different the valving and springing etc is.

[bogan]

Briefly, theres several things going on here;

1) If youve changed the link system youve likely changed the motion ratio. If for the same rear axle movement you are moving the shock shaft further then you are moving it faster. Damping is proportional to velocity. Think of it in terms of pushing a fire door with a closing damper on it, push it slowly only a little resistance, push it fast and it almost locks up.

2) A lot of these CBR shocks have ridiculously crude valving that is pretty non responsive and the valving would in any event be mosre suitable for the bike it came out of than the Bros. Also the port sizes in the piston are riculously small so they ''choke off'' over abrupt bumps irrespective of how light the valving is

3) The issue is not oil viscosity because of 1 and 2 above. The aim with a shock is to run as thin an oil as posssible but it aint going to significantly fix this issue.

When Im at work Ill have a look at the respective Ohlins spec cards for the CBR600F4 and the Bros. That ( in the absence of any decent info for the oem shocks ) will give an excellent insight into how different the valving and springing etc is.

Thanks Robert. I tried to get the motion ratio for the link as close as possible to that in the CBR, and the spring settings seem to indicate I got fairly close. The bros shock has a much higher spring rate, which was compensated for in the link design by using more shock travel. So by design and by inspection so far, I think the shock is operating with similar velocities and sag etc in the bros as it did in the CBR. I just think I cocked up and put the wrong oil weight in it; as you say, there was very little info available on the stock shocks when I looked.

While it'd very nice to get a properly valved ohlins unit (I've had a go on my uncle's bros which has a CKT ohlins) I can't afford it on this build. And I'm quite curious as to how much improvement can be gained by putting the correct/best weight fluid in there.

[Robert Taylor]

Thanks Robert. I tried to get the motion ratio for the link as close as possible to that in the CBR, and the spring settings seem to indicate I got fairly close. The bros shock has a much higher spring rate, which was compensated for in the link design by using more shock travel. So by design and by inspection so far, I think the shock is operating with similar velocities and sag etc in the bros as it did in the CBR. I just think I cocked up and put the wrong oil weight in it; as you say, there was very little info available on the stock shocks when I looked.

While it'd very nice to get a properly valved ohlins unit (I've had a go on my uncle's bros which has a CKT ohlins) I can't afford it on this build. And I'm quite curious as to how much improvement can be gained by putting the correct/best weight fluid in there.

Im not trying to sell you an Ohlins as such, Ill just look at the spec cards as a form of reference. Ill come back to you via this thread. A thinner oil will help, but I believe the problem is more invoved than that

[Robert Taylor]

The Bros shock does indeed have a much firmer spring as you correctly say, 18 newtons against 13 or 14 newtons for the CBR. The Bros linkage in fact moves the shock shaft very slowly so it needs a very firm compression stack, also on rebound to control that firm spring.

In the absence of internal valving and piston information for most if not all oem shocks we look at the spec cards for the relevant Ohlins listings. This tells us EVERYTHING about the respective character of the shocks, which are of course matched to the motion ratio, leverage applied, chain / engine pull torque, bike weight and how that weight is dispersed. So, if we are modifying oem shocks ( which we do very often ) we find the Ohlins distributor site very useful indeed

If indeed you have got the motion ratio the same as the F4 what other variables are going on that Ive touched on above? What sag figures are you getting and what is your height and weight in your everyday clothes?

Irrespective the piston and valving design in that shock is crude and ''rigid''. If you spend nothing else on thst bike a 40mm Race Tech piston with a suitably arranged stack will be a lot kinder to your back and rear tyre.

A thinner oil will maybe in your mind yield a ''decent'' result but the potential for it to be a LOT better is HUGE. What was the brand of oil you installed? Not all 7wt ( or any weight ) oils are equal due to the shonky SAE motor oil system used to rate suspension fluids for counter sales. Its wildly inaccurate and it would help somewhat if parts people had some idea of what they were selling.

Relevant to all this lets say you have the correct spring correctly preloaded and sag figures also ''correct. Suspension fluid is correctly ''thin''. But the ride is still harsh. The instinct is then to open out further and further those inaptly named ''fully adjustable'' clickers. What happens then is that there is now so much ''bypass bleed'' the initial slow speed action becomes sloppy and the bike feels a little bit like a rocking horse. But as you then ride over abrupt bumps there is so much bleed that it has lowered the pressure acting upon the valving stacks and they open later, abrupt bump compliance may even be worse. Counteracting / contradicting that the bike rides a little lower in its stroke due to the sloppy bleed and gets to the accelerative part of the link ratio a little earlier, speeding up shock travel. Theres a lot going on and its a matter of getting everything into ''the window''

It makes me laugh when you see an advertisement ( for example ) ''30 clicks of rebound adjustment'' Yeah right, only about 7-8 clicks are actually useful. If its approaching either extremity the internal valving needs reworking.

[bogan]

Yeh the bros 'linkage' is direct to the SA so no surprises there.

Sag figures are 6mm without rider, and 32 with rider (1.82m and 63kg). The preload is clicked right out, so It's probly fair to say it could benefit from a slightly lighter spring, but as the valving is the weak point atm improving that gets higher priority.

Gave a link to the oil brand (Maxima) in the first post and it has a number of specs and viscosity info there, I don't know what much of it means though...

It feels pretty hard through all the bumps, and not getting any rocking motion, also played round with a lot of different clicker position but they both wanted it as low as possible. Initially the back was skipping about over medium abrupt bumps as well.

[Robert Taylor]

Yeh the bros 'linkage' is direct to the SA so no surprises there.

Sag figures are 6mm without rider, and 32 with rider (1.82m and 63kg). The preload is clicked right out, so It's probly fair to say it could benefit from a slightly lighter spring, but as the valving is the weak point atm improving that gets higher priority.

Gave a link to the oil brand (Maxima) in the first post and it has a number of specs and viscosity info there, I don't know what much of it means though...

It feels pretty hard through all the bumps, and not getting any rocking motion, also played round with a lot of different clicker position but they both wanted it as low as possible. Initially the back was skipping about over medium abrupt bumps as well.

The sag figures for road use are actually pretty close to the mark. Id install the thinnest Maxima shock oil available ( its good oil ) and reassess. But in an ideal world it does need a decent piston and valving!

[bogan]

The sag figures for road use are actually pretty close to the mark. Id install the thinnest Maxima shock oil available ( its good oil ) and reassess. But in an ideal world it does need a decent piston and valving!

Cheers Robert, will let you know how it goes. And once I get a job we'll work on getting it ideal

[Pussy]

(1.82m and 63kg).

Mate... it's time you left the salads alone, and start eating more pies!
That's skinny, even by Ethiopian standards....

[bogan]

Mate... it's time you left the salads alone, and start eating more pies!
That's skinny, even by Ethiopian standards....

Pffft, not even, it's still in normal range of the BMI thingumy. I'm just built form lightweight racing parts

[bogan]

I reckon the change has done the trick, it's not plush, but getting good feel and not harsh on little bumps like it was.
Big bumps still quite abrupt, but that'd be something to get them fandangled emulators for I reckon, and it's a lot easier to see the big ones coming up. So this will keep me happy till I look into emulators and possibly springs for front as well.

[ducatilover]

You need a ZZR600 mate.
And boy, do I have a deal for you!

[Brian d marge]

I also can confirm what RT has said the CBR shock is , um err good base line manufacturing which is an anagram for a pile of poo ( didnt the early ones snap the rebound/ compression needle .. pissed and too long ago to remember clearly )

Tony Foale suspension software is a good tool in the right hands

Stephen