So let's say, for Lemons or other cheapo race car only suspension tuning, you refilled a stock type shock with thicker oil. How thick is too thick? 5w-30? 90w? grease? What happens to a car that has dampers that are too stiff?
No street use, no rough roads, nothing aftermarket.
I would guess 90W. Too stiff means it doesn't take bumps very well. Wonder how it would affect bump and rebound..
iceracer wrote: I would guess 90W. Too stiff means it doesn't take bumps very well. Wonder how it would affect bump and rebound..
That was my first though, too. "too stiff just means it will lose traction over bumps, and this is a race car, so who cares" - but the engineer in me said "Okay, brian, so the shocks are now infinitely stiff, now what?" and obviously there has to be some detriment. If there wasn't, no shocks would be adjustable, right?
Motorcycle fork oil is usually 5w, 7.5w, 10w, etc. Changing from 5w to 10w is considered a huge change from what I've seen when this is discussed. There are other measures of how the stuff flows though and weight isn't a great way to get that information.
There's a ton to learn if you really want to play with this stuff. And 90w will most likely blow the seals out of the shock in a few miles. It will also feel like a huge change in spring rate. Assuming this is an oil filled shock without some sort of gas charge like nitrogen, a guy can play with effective spring rate of the shock by playing with the amount of air at the top of the shock.
Honestly I'd just get used shocks in good condition if you are really limited on budget and play with actual springs to affect ride and handling instead of trying to get into this. And stay safe when playing with shocks on a bench.
Tuna, what kind of car are you playing with?
Brotus7 wrote: Tuna, what kind of car are you playing with?
Does it matter?
Currently the team is working on a 70's LTD and the main car is a 66 Amazon and there is also an old Simca that might get some love.
tuna55 wrote:Brotus7 wrote: Tuna, what kind of car are you playing with?Does it matter? Currently the team is working on a 70's LTD and the main car is a 66 Amazon and there is also an old Simca that might get some love.
of course not. just put the LTD springs in the simca with some knee-action dampers from an MGA and you'll be fine.
now that i got that out of my system, here's what i think i know, and how i think, about dampers:
dampers slow down the motion of the body in the jounce direction, and they slow down the motion of the suspension in the rebound direction. so, too stiff in jounce will transmit more force upward into the body, resulting in more body travel and making the ride feel harsh. too stiff in rebound will not let the wheel follow the downside of a bump (or a dip), causing the tire to lose grip with the surface.
http://www.teamswift.net/viewtopic.php?f=10&t=25593
This is really the only thread I've seen on the subject, but it seems like a pretty decent 'labor only' type of suspension mod. I'll definitely be doing it when its time to build a lemons car. Being that you're in pretty uncharted territory, I'd say be prepared to do some guess and check. Start thin, and go thicker till you find a happy medium. Good luck, be safe.
GREG
I changed the oil in some VW GTI shocks years ago. I used motorcycle shock oil, can't remember what weight, but it stiffened them up a lot. Then, I hit a big dip at the bottom of a hill and blew out both front struts.
Tuna we did this on the Civic for the last lemons race. Drilled the bottom of the struts, welded in bungs and filled them with ATF. Worked like a charm for about 6 hours. By the end of the race the front end was bouncing so bad we ended up breaking the transmission mount and almost dropping the engine out of the car.
Toyman01 wrote: Tuna we did this on the Civic for the last lemons race. Drilled the bottom of the struts, welded in bungs and filled them with ATF. Worked like a charm for about 6 hours. By the end of the race the front end was bouncing so bad we ended up breaking the transmission mount and almost dropping the engine out of the car.
It scares me the way my brain works. I read this as "We were able to take free shocks and make them into performance shocks that last for the equivalent of 6 autocross seasons" (or 100 challenges 
)
I've never done anything like this to car shocks, but I have rebuilt motorcycle forks, and I would say don't use anything heavier than 5 wt. Motorcycle forks are much larger than shocks or struts and have fairly large orifaces for the oil to pass through, I'm sure the orifaces in a car shock are smaller and made to be used with a thinner oil.
Some guys use 20wt on motorcycle forks. What is the stock thickness? Any idea?
93EXCivic wrote: Some guys use 20wt on motorcycle forks. What is the stock thickness? Any idea?
The bikes that I was working on were mostly dirtbikes from the late 80s to mid 90s with lots of tuning for compression and rebound. Simpler forks probably use the 20wt.
HappyAndy wrote:93EXCivic wrote: Some guys use 20wt on motorcycle forks. What is the stock thickness? Any idea?The bikes that I was working on were mostly dirtbikes from the late 80s to mid 90s with lots of tuning for compression and rebound. Simpler forks probably use the 20wt.
Yup these were old Yamaha enduros. And the stock thickness was for the stock shocks just to clear that up.
Couldn't you drain the stock oil out and measure the weight?
Now that I'm thinking about it, This might be a good question to take over to a moto-cross forum, those guys are always modifing and tuning shocks and forks.
tuna55 wrote:Brotus7 wrote: Tuna, what kind of car are you playing with?Does it matter? Currently the team is working on a 70's LTD and the main car is a 66 Amazon and there is also an old Simca that might get some love.
It only matters from the point of view that I thought most cars used gas shocks/struts, so I was curious what used oil and if this was something I should look into for our LeMon.
I'm the smart engineer who drilled out the rear shocks on our LTD and welded in a bung (heheh...he said, "bung") and refilled them with 90W. After talking and reading this thread, I'm reconsidering and think I will refill them with some 30W oil I have. They seem really, really stiff with the 90W.
ATF seems like a bad idea to me, as most ATFs (especially Ford type F) have friction additives to make it less oilly and more grabby. This is so your A/T will work. I think a straight mineral oil would be the best- no additives, just a straight weight.
I did order new shocks for up front, when they come I'll compare them subjectively against my homebrew rears.
volvoclearinghouse wrote: I'm the smart engineer who drilled out the rear shocks on our LTD and welded in a bung (heheh...he said, "bung") and refilled them with 90W. After talking and reading this thread, I'm reconsidering and think I will refill them with some 30W oil I have. They seem really, really stiff with the 90W. ATF seems like a bad idea to me, as most ATFs (especially Ford type F) have friction additives to make it less oilly and more grabby. This is so your A/T will work. I think a straight mineral oil would be the best- no additives, just a straight weight. I did order new shocks for up front, when they come I'll compare them subjectively against my homebrew rears.
ATF is in fact often used on high end/specialty dampers. I've used Dextron III on a number of apps.
RE: gas filled shocks. The gas isn't used to "stiffen" damping so much as reduce fade and cavitation (twin tubes) or ensure the damper doesn't lag/replenishes properly (monotube). A twin tube w/o gas pressure is no big deal generally, but a monotube simply won't work without sufficient gas pressure.
"Shim restackor" is a neat program to play with, to determine the effect of shims on the damping curve, and there is a lot of good information on the impact of damping on vehicle performance.
Generally, lots of low speed damping will help with body motion control (roll/pitch). Thickening the oil will do this, but will ALSO greatly increase high speed damping (and internal damper pressure), which could result in blown seals or other mechanical failures.
njansenv wrote:volvoclearinghouse wrote: I'm the smart engineer who drilled out the rear shocks on our LTD and welded in a bung (heheh...he said, "bung") and refilled them with 90W. After talking and reading this thread, I'm reconsidering and think I will refill them with some 30W oil I have. They seem really, really stiff with the 90W. ATF seems like a bad idea to me, as most ATFs (especially Ford type F) have friction additives to make it less oilly and more grabby. This is so your A/T will work. I think a straight mineral oil would be the best- no additives, just a straight weight. I did order new shocks for up front, when they come I'll compare them subjectively against my homebrew rears.ATF is in fact often used on high end/specialty dampers. I've used Dextron III on a number of apps. RE: gas filled shocks. The gas isn't used to "stiffen" damping so much as reduce fade and cavitation (twin tubes) or ensure the damper doesn't lag/replenishes properly (monotube). A twin tube w/o gas pressure is no big deal generally, but a monotube simply won't work without sufficient gas pressure. "Shim restackor" is a neat program to play with, to determine the effect of shims on the damping curve, and there is a lot of good information on the impact of damping on vehicle performance. Generally, lots of low speed damping will help with body motion control (roll/pitch). Thickening the oil will do this, but will ALSO greatly increase high speed damping (and internal damper pressure), which could result in blown seals or other mechanical failures.
I know nothing about dampers. Pretty much literally nothing. I could read a dyno plot and, properly directed, select a close match given ample data and instruction, but I know nothing at all beyond that.
Are most cars shocks "mono"? Are most cars shocks "gas" or "oil" filled? What can you do on the cheap to avoid raising the high speed damping too much?
The only oil you should use is motorcycle fork oil! Any other kind of oil will not work and will most likely blow out the seals. This trick has been done for over 30 years. I did it to a set of stock struts on a Capri. A friend did it first using some kind of motor oil and blew out the seals. I used 10W motorcycle fork oil and had a set of struts that had higher damperning and didn't blow out the seals!
The ports on a shock piston are very small. Trying to force 90W oil through them just will not work!
Bilsteins are almost exclusively Mono-tubes Koni's are usually twin-tubes but they do mono-tubes as well. "Most" cheap OE dampers (Monroe and the like) are twin tubes.
All car shocks are both gas (air) and oil filled. The gas is there to deal with rod displacement. IE: if there was no gas the (relatively incompressible) oil wouldn't allow the rod to enter the shock.
On the cheap? I'd look for some worn out Bilsteins and rebuild them myself. Parts are cheap and available, and there's good documentation on the interweb. On the very cheap, there is nothing that says you can't experiment with the thicker oil..understanding the limitations. Cutting a shock open is interesting, to say the least: at the base level, they're pretty simple devices. The "best" way to increase the low speed damping is increase the spring preload on the damper piston while still allowing the valving to open fully (IE: blowoff).
I agree, motorcycle oil would be the best, and if you want to play keep purchases of different weights in the same brand so you can be reasonably sure that one actually flows more or less than the next weight in a way that you expect. 30w motor oil will be crazy stiff on high speed damping.
Toyman01 wrote: Tuna we did this on the Civic for the last lemons race. Drilled the bottom of the struts, welded in bungs and filled them with ATF. Worked like a charm for about 6 hours. By the end of the race the front end was bouncing so bad we ended up breaking the transmission mount and almost dropping the engine out of the car.
The ATF is roughly a 15 wt oil. That's very roughly. Most car dampers use thin stuff like 5 or 7.5 weight, from what I have seen. The problem Toyman refers to was caused by a damper that was already leaking before it was molested.
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