Author Topic: Fork Spring / oil  (Read 1474 times)

Offline Paulreade

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Fork Spring / oil
« on: June 23, 2018, 02:24:43 PM »
Hi guys, working through a process of elimination to remove a hop or bob out of my k6 front forks. I will attempt to ensure wheels are true and steering head tight in due course. I will then probably try a thicker oil first before replacing springs.
Q: if I remove the springs - is there a way of checking if they actually need replacing or is it just worth doing due to the age of the bike ?
Q: David silver do not stock any longer - I have located progressive springs that may be available via Hagon - is this the best outlet ?
Q: I believe I have 10w oil in there at present, might it be worth draining and trying a slightly thicker oil before a spring change to say 15w or even 20w. I'm only around 11-12 stone so I don't want to stiffen it too much as I believe some of the thicker oils are needed for a heavier load ?
Any advice welcome
Regards
Paul
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

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Re: Fork Spring / oil
« Reply #1 on: June 23, 2018, 05:01:00 PM »
Making the assumption that you're not talking about wheel balance which would be consistent and regular as speed increased?
Then, my interpretation of "bobbing" is too much damping in the fork.  This effectively gives a hydraulic locking effect when asked to move at a certain linear rate.

The feeling is that the front of the bike is bounced upward a small amount rather than the suspension moving to accommodate the road irregularities.

To try and see a different effect,  I'd put in 5w oil to make sure the fork will move quickly enough and prevent it trying to move the vehicle mass upwards.  In other words, getting the fork to respond within the range you are operating it with your total all up weight.

I'd not initially consider the fork springs unless there is a very clear measurable sag in static suspension setting. 

At what pressure are you running the front tire? This older and larger setup don't normally need excessive pressure as they are more supported than modern 17 inch rims with small sidewalls and lighter carcass construction. The tire is an undamped spring which will easily introduce bounce into the overall setup if out of pressure range.
Your low body weight would allow you to use lower than spec,  perhaps 2 psi drop to make a difference.  Quick and easy to try.

Offline hairygit

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Re: Fork Spring / oil
« Reply #2 on: June 23, 2018, 05:06:23 PM »
Alternatively, Honda recommended "Premium quality A.T.F" (Automatic Transmission Fluid) which is fairly thin, but has always worked fine for me, plus ATF contains an addative to swell the oil seals and reduce risk of leakage.
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Offline Green1

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Re: Fork Spring / oil
« Reply #3 on: June 23, 2018, 05:08:41 PM »
You check the springs by measuring there length. I can't remember it of the top of my head but I should imagine there fine unless the bikes done a lot of work.
My gold bike has around 23000 on the clock and there like new. When I got my green bike it had around 80000 miles and they were on there limit. I replaced them with Hyperpro progressive springs and it made a massive difference. The bike was far more stable at speed or mid corner.
I think they supplied 10w with the springs and I recently changed it to ATF fluid and it feels exactly the same.
I only weigh 10 stone if I make a pig of myself and have 28psi in the front tyre.

Well described Nigel I get that happen occasionally but only on rippled roads at certain speeds so I just go faster.
Mick
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Offline Underdog1

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Re: Fork Spring / oil
« Reply #4 on: June 23, 2018, 08:12:54 PM »
Off the top of my head you want the spring length to be over 18.5 inches. Not sure where I’ve got that from though!

Offline Paulreade

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Re: Fork Spring / oil
« Reply #5 on: June 24, 2018, 11:17:29 AM »
Interesting feedback - thanks. I will try a thinner oil first as suggested, seems to make sense.
Thanks again for your input.
Regards
Paul
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

Offline spanners

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Re: Fork Spring / oil
« Reply #6 on: June 25, 2018, 01:20:16 AM »
Hi,  yes, the book says 19"  I have mine out at present 1972 750 K2  and they measure just 1/4 inch under that, which is pretty close (I'm talking inches not mm  because that's straght from the manual) either they have not lost their spring much in all that time or have been replaced at some point.  It's the seal type p/n / depth of seal (seal stiction debate) I ended up giving the most thought to on my older K2

My 1975 CB550 had progressive springs as standard honda fit.  That seemed a good idea for the 750,  but it was suggested by hondaman on the usa website that progressive springs are not such a good idea for the CB750  so I decided to stick with normal non progressive on that.

Type of oil you end up with for riding style I guess depends a lot also on how worn they are internally and is a suck it and see thing.  I think honda went to ATF because it extended seal life, but to me it is not viscoss enough maybe for older forks with a bit of wear.  Also I hear people say that 10w is the same vicossity as 10 w30 at 20C  but to me 10w seems pretty obviously a lot thinner than 10w 30  just shaking it. 
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Offline Paulreade

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Re: Fork Spring / oil
« Reply #7 on: June 25, 2018, 09:51:18 PM »
I think for the price of the oil, it will be worth a bit of experimentation to see what oil has what effect. I'll report back after I have tried a couple. Spring length I will check when they are out but great to know what they should be.
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

Offline Paulreade

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Re: Fork Spring / oil
« Reply #8 on: August 06, 2018, 04:44:37 PM »
Just looking at various 5w fork oil types. I seem yo have a choice of classic blends from millers ( specified for vintage / classic motorcycles) to the more newer blends of fully synth and semi synth.

Does it matter if you use modern fully synth types - ie will it affect seals etc or leak past them ?.
Any advice welcome.
Thanks
Paul
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

Offline Nurse Julie

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Re: Fork Spring / oil
« Reply #9 on: August 06, 2018, 05:01:14 PM »
We use this in all our old Hondas [ Guests cannot view attachments ]
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Offline Paulreade

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Re: Fork Spring / oil
« Reply #10 on: August 07, 2018, 01:42:00 PM »
Thanks Julie, I'll look into it.
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

Offline SumpMagnet

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Re: Fork Spring / oil
« Reply #11 on: August 07, 2018, 02:25:06 PM »
I was warned against running 'progressive' springs on anything not meant to have them as the whole suspension system was designed around a constant spring rate. Progressive springs just have a soft start, which gets used up when you sit on it, but droops too easily. Your damping is constant...so effectively it's overdamped....then briefly OK...then underdamped as the spring gets firmer.

Getting what you have working as well as it can is usually the best approach
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Offline Paulreade

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Re: Fork Spring / oil
« Reply #12 on: August 07, 2018, 05:45:43 PM »
Point noted. I'll check Spring length when I get to it before considering replacement.
Current bikes:
Suzuki GSX 1250 FA
Triumph t100 bonneville
Honda cb750 k6

 

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