Too much oil in the forks - what will happen?

Oil type and viscosity aside; what happens if you run the bike with too much oil in the forks? I´ve seen recommendations to go up to 180cc instead of the standard 150cc. The reason for my asking this is that I have had extensions made for the sliders, so the upper bushing is moved up 50 mm, about 2". And the reason for that is that I have put a fork brace on there. So I´m worried that there wont be any oil splashing its way up to the bush if I dont increase the oil level?
\Tommy

180ml was the "hot tip" on a recent Andover Norton Tech Tip video. I know on my modern air cooled bonneville the oil volume was stated as being important/critical because it sets the correct air gap above which plays a role as bump stop.

fiatfan said:

Oil type and viscosity aside; what happens if you run the bike with too much oil in the forks? I´ve seen recommendations to go up to 180cc instead of the standard 150cc. The reason for my asking this is that I have had extensions made for the sliders, so the upper bushing is moved up 50 mm, about 2". And the reason for that is that I have put a fork brace on there. So I´m worried that there wont be any oil splashing its way up to the bush if I dont increase the oil level?
\Tommy

Click to expand...

Assemble a leg without the extension and put in 180ml of oil.

Then measure the air gap at the top of the fork.

Next, strip, drain and assemble with your extensions and fill as required to achieve the same air gap.

Measure the oil as it goes in so that you know for future.

Measuring the air gap is the most reliable and accurate way (there’s always ‘some’ residual oil left inside, but how much?). Many competition forks don’t give a cc/ml figure, only an air gap.

There are some good vids on YouTube about how to measure the air gap.

When you brake on a neutral handling bike, the forks compress and thec steeringhead becomes steeper and you have less trail, so the bike becomes more stable when you are braking. If you hold the front brake on when you get into a corner too hot, you will teand to run wide faster, because the forks compress.
If you have too much oil in the forks you might shift the handling spectrum. The bike might become less stable when you are using the front brake.
The resultant force which acts when you steer a motorcycle works at the trail by addition of vectors. The biggest vector is the one which is in line with your front axle. That can chuck you down the road. A spinning front wheel contains a lot of energy and it can all act through the handle bars. Most guys do not change the trail on their bikes's steering because what you get can be a gamble.

And blown oil seals if you get it wrong with too much oil.

From what I've read, there's 2 things going on.

One, is that you have to cover the top of the damper tube with fluid by a certain distance so the damper doesn't suck air from it's top bushing when the slider compresses. So there's a minimum of oil that you must have to keep your damper submerged in oil so it doesn't suck any air

the other is, as Tornado said, the spring effect that the air gap has when the slider moves up the fork leg and compresses the air above the fluid

The method I've seen used to find the minimum amount of fluid needed was to assemble the lower parts of the forks, but not the fork cap. Remove the spring from the damper cartridge, since the spring keeps you from being able to pump out the air trapped in the damper, then fill with fluid as Nigel said, keeping notice of how much fluid you are adding while pumping the damper rod. Once the damper rod stiffens, the damper is full of fluid. Then you add fluid until the top of the cartridge is submerged and the damper rod is stiff and at the top of the damper. Then add 10ml more to be safe. That's your minimum amount of fluid necessary.

After that, the more fluid you add beyond the minimum will increase the pressure in the declining volume of air as the forks compress... and of course that pressure pushes on the fluid and the seals...

It is interesting learning about the old post war Velocette front fork design...they had always run girder front on production bikes until just following the war, when they switched to Oleomatic forks...no springs at all, just oil and air as the damping/suspension setup...these came from an aircraft landing gear company. Only had them for two years before the supplier went out of business...Velocette them came up with their own oil/spring units which wre quite typical for the time. Reports are the Oelo's had longer term issues as the seals at the time were not up to the task of keeping the air in....there was an air valve, just like on a tire, fitted on the pressure tubes to allow roadside topping up. Bikes also had onboard storage for a hand air pump, alongside the tank.

ashman said:

And blown oil seals if you get it wrong with too much oil.

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Yeah , you could risk blowing out the fork seals on a big pot hole .
In my part of the world you have 24 hrs. to make a case against the municipality for that .

Tornado said:

Only had them for two years before the supplier went out of business.

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Dowty was the company


Went bust but many years later so that had nothing to do with the air forks being dropped.

kommando said:

Dowty was the company

Dowty Motorcycle Oleo Shock Absorbers

Dowty was happy to supply the experimental motorcycle-sized Oleo shock absorbers in 1936 for Veloce and the following year went ...

www.dowtyheritage.org.uk

Went bust but many years later so that had nothing to do with the air forks being dropped.

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Yes you are right. Checking my Velocette history books, I got my facts wrong, 'twas the girder forks from the Webb company that ended production in '47, necessitating telescopic forks on production bikes. No reasons were given for dropping the Dowty Oleomatics by 50-51, for Velo in house designed spring/oil forks, but I'd guess it was to cut costs.

180cc will cover the Commando damper tube, if you look a the Domi publications they suggest 150cc and this will cover a Domi damper tube which is nearly two inches shorter. You would then need to add a serious amount of additional oil above this 180cc to get into trouble as this would then start to reduce the air volume in the stanchion tube where the spring is located. Might have to get the sliced fork slider out again and mock a fork leg up.

ashman said:

And blown oil seals if you get it wrong with too much oil.

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Yeah, that did cross my mind....

Fast Eddie said:

Assemble a leg without the extension and put in 180ml of oil.

Then measure the air gap at the top of the fork.

Next, strip, drain and assemble with your extensions and fill as required to achieve the same air gap.

Measure the oil as it goes in so that you know for future.

Measuring the air gap is the most reliable and accurate way (there’s always ‘some’ residual oil left inside, but how much?). Many competition forks don’t give a cc/ml figure, only an air gap.

There are some good vids on YouTube about how to measure the air gap.

Click to expand...

That I like! Sounds very logical, thanks!

But only measure once the damper bodies are full or the height will be higher than once the oil gets to all the right places.

Fast Eddie said:

Assemble a leg without the extension and put in 180ml of oil.

Then measure the air gap at the top of the fork.

Next, strip, drain and assemble with your extensions and fill as required to achieve the same air gap.

Measure the oil as it goes in so that you know for future.

Measuring the air gap is the most reliable and accurate way (there’s always ‘some’ residual oil left inside, but how much?). Many competition forks don’t give a cc/ml figure, only an air gap.

There are some good vids on YouTube about how to measure the air gap.

Click to expand...

Not required, but handy if you do this job routinely.

I've changed my damper fluid 10 times in the last 2 months for an ongoing project..... That "pro" oil level tool (which I have but don't use) is a way to measure any brand of motorcycle's fluid height by having each model's published air gap height as a reference. So,... If I am a mechanic servicing motorcycles, I look up the bike I am servicing and use the published air gap measurement as my guide to get the fluid volume correct for each different model of motorcycle that I am servicing, without having to do the procedure I described previously. The air gap method also accounts for any fluid left inside your damper when you drain the forks to change the fluid too, so I see why a service tech person would use that method given the variety of motorcycles he might service and his desire to avoid removing the spring off the damper and doing a fluid volume inspection on every model he services.....

the only problem is the air gap method is that you can't pump the air out of the damper easily without fastening the fork cap nut if you don't take the spring off,.... So how do you measure the air gap without pumping the damper first?? Do you add the fluid, fasten the fork cap to pump the forks... 5 times? 10 times?? to purge the air from the damper tube, then remove the fork cap again to measure the air gap? Then add or subtract fluid with your Pro tool and reinstall the fork cap?? Seems like a lot of work...

I like my method better, but I am not servicing many different models of motorcycles and don't need a tool which can work with multiple manufacturer's specifications. I'm not even working with commando dampers, so I have to do a bit more to find my fluid volume, but once I knew my volume, I just use a syringe to inject that amount. I don't use the air gap tool... I just don't think think the air gap is that critical. I think it's more critical if you don't have enough fluid and your damper isn't completely submerged.

Is that slightly smaller volume of air really that detrimental to performance? anyone?

I had a Yamahaha XS 1100 .
The air gap / pressure was so very important to front suspension .
Can't comment on a Norton much , I'm going to tear down the front end on mine with Acadian in 2 days with all new parts , using 180 c.c. per leg of 20 W. This should bring up any air pressure gap higher . No way to measure that .

Torontonian said:

I had a Yamahaha XS 1100 .
The air gap / pressure was so very important to front suspension .
Can't comment on a Norton much , I'm going to tear down the front end on mine with Acadian in 2 days with all new parts , using 180 c.c. per leg of 20 W. This should bring up any air pressure gap higher . No way to measure that

Click to expand...

It's not like air pressure in forks was such an effective mechanism that it caught on and it's widely used.... It was an idea that was tried and may have some applications, but it wasn't a game changer in motorcycle forks...

The stock rebuild is straight forward. Take apart, clean everything replace old with new and reassemble. Use the best fork oil you can find, judging it by it's high viscosity index. It ensures that your fluid viscosity will change as little as possible as the oil heats up due to normal fork action. It might not be as critical with a primitive design like the commando damper because the fluid isn't being forced through valve openings like modern dampers. You'll notice the new seals will be sticky in comparison to the ones that you removed. Definitely put some lubricant on the seal to lower the friction.