GPz750 Fork Oil Recommendations

[Grumpy Grandad]

I'm planning on changing the fork seals as I've noticed a small leak in one of them.

Good News is they weren't changed when the bike was recommissioned last year, so they're OLD!

I was pissed off because I'd thought they were New Seals that had failed after only 7 months.

I'm not impressed with the 39 year old suspension but such is life. I have heard that putting a heavier 15W oil can make an improvement, anybody had any experience of that?

The A3 model is fitted with Anti-Dive but I'm sure that was more a gimmick than a practical solution. I understand a common mod is to bypass it, anyone tried the bypass kits?

Cheers.
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Life may begin at 30, but it doesn't get interesting until about 75 mph.

[MarJay]

I have a friend who used to have an A1 back in the day, he said that the anti dive, when it worked, was really damn good. He thinks the A models handled better than the later ones with more modern tyre sizes and suspension.
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[stinkwheel]

I run halfords multigrade oil in most of mine. Less fade if you've been working them hard.

However, is damping the issue or is 40 year old springs that were probably on the soft side when they were new anyway the problem?

The oil viscosity will only affect the damping rate, which is a function of the valving and hole sizes in the damper. That's probably not changed much over the years.

So my suggestion would be to plug your details into the spring calculator on https://racetech.com/product-search/ . It'll give you a suggested linear spring rate, spacer size, oil viscosity and air gap. Do that. It's a US site so you may need to look at different years for your model.

For example, if I plug my details in there for your bike, it recommends a spring rate double that of standard and 20wt oil. Then set your static sag by adjusting spacers.

Having done this on a couple of bikes, I'll say the only downside is it'll make you realise you now also need a new shock. Having done that, you'll realise you need to upgrade the brakes.

There's no point fucking with any of the other settings on your suspension if the spring rate is wrong. Then sag, then other stuff.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Grumpy Grandad]

[stinkwheel]

Well, you can at least check the springs are still within spec. Your workshop manual will have a minimum free length listed. I can almost garauntee they are under it, just by dint of being compressed in a fork with a bike sat on them for nearly 40 years.

Bushes are easy to fit, the hard bit is sometimes getting the forks apart, this necessitates slide-hammering them apart to remove the bottom bush. It generally either goes very easy, a few knocks and they're apart or they start to override and you're on the struggle bus.

My other top-tip is, if at all possible, use OEM bushes and seals and check the bush end gap is correct. Aftermarket seals are a false economy.

If you don't have a sel driver tool, it's a good investment.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Grumpy Grandad]

[Robby]

I can highly recommend putting a set of cartridge emulators in traditional damper rod forks. In fact I would do that first, on standard springs and 10W oil, and work from there.

Heavier oil and stiffer springs both restrict your suspension movement. This is good when your suspension is moving too much - and bottoming out a lot, or diving too much for comfort under braking. The downside is that if you go too far you end up with suspension that only moves under the biggest loads, and moves slowly.

On my Guzzi, which is quite new but had awful suspension from new, I have the following setup:
Hagon springs progressive springs. I did previously have a set of racetech springs, but they were too hard. They only offered one spring rate, Hagon do a few.
10W oil. It's the right weight to allow good movement on my bike.
YSS cartridge emulators. They make the biggest difference.

I now (finally) have forks that use their full range of motion, are comfortable, and don't bottom out.

Make one change at a time. Each aspect affects everything else.

[Grumpy Grandad]

[yen_powell]

[Grumpy Grandad]

[Robby]

[Grumpy Grandad]