The Joys of Antidive...

What is antidive, and what's it for?

"Suzuki's anti-dive system, developed from the feedback of racing technology, is now equipped on many of Suzuki's 1982 models. The new system is attached to the outer tube of the front fork. The brake line of the front brakes master cylinder leading to the caliper is connected by a hose to the antidive device. When the master cylinder's hydraulic line functions to brake the front wheel, it simultaneously operates the anti-dive device's plunger, which regulates and limits the flow of oil in the front fork. This reduces the compression of the front fork, which also reduces the extension of the rear shock absorber. Hence, the device serves to counteract the change in the motorcycle's attitude during braking." (from the Suzuki GSX 750/1000/1100 Supplementary Service Manual, Sept. 1981, p.6)

That makes antidive sound like the most wonderful thing since the invention of the motorcycle! But while antidive does indeed limit front-end diving under relatively heavy braking, the fact is that it has a couple of key disadvantages. For starters, you inevitably will lose a degree of braking bite at the calipers, because some of the brake fluid pressure is initially diverted to operate the antidive plungers.

The other downside is that having antidive often means that bleeding the brake system is a proper pain in the butt. Not to mention the waste of time and brake fluid in the quest to evict that elusive air bubble. And that's after following the correct bleeding procedures as per the manual, I might add. Which is: 1. left antidive, then 2. left caliper, then 3. right antidive, and finally 4. right caliper. But the sad reality is, you can do this time after time, and still have that spongy brake lever and sub-standard braking.

So while it might have been a good idea at the time, the fact is that there is no substitute for stiffer springs, better damping and properly-tuned suspension (and, looking at the photo above, stainless fasteners...). And for proof of that, look around you: are any current manufacturers using anti-dive? Let's just say that the complete absence is instructive.

In the meantime, unless you've swapped out the front end of your Katana with some later-model running gear, you have got antidive with its foibles. So, what to do?

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Bleeding those brakes

Here's a brake-bleeding procedure I have used a number of times with excellent results:
  1. Squeeze the brake lever rapidly about 10-15 times, until it's nice and hard. (It's hardened-up because those air bubbles that are the cause of the sponginess, are now compressed to microscopic size.)
  2. Whilst maintaining a strong squeeze on the lever, tie it back to the handgrip, as tight as it will go. You can use rope, wire, cord, a leather strap... whatever takes your fancy.
  3. Leave the brake fluid under pressure for a couple of hours. What happens is that the wretched elusive air bubbles, the cause of the spongy brake lever, are dissolved under pressure into the surrounding brake fluid. So far, so good...
  4. Take the top off your brake fluid reservoir. Make sure it's topped-up nicely with fresh brake fluid.
  5. Now it's time to bleed that air-bearing fluid from the system. So, whilst maintaining your grip on the brake lever, untie it. We do not want to slacken the pressure off the lever at any stage, otherwise those wretched microscopic bubbles will re-form, see, and start to float back up the brake lines, away from the brake-bleeding nipples...
  6. So, while maintaining that squeeze on the brake lever, slowly crack open each brake-bleeding nipple with your 8mm (or whatever) ring spanner, bleed out the fluid, clamp up the nipple, and then...
  7. ... release the lever, but make sure that you immediately squeeze the brake lever tight again. Let me say it again: we do not want to slacken the pressure off the lever at any stage, and give those wretched air bubbles a chance re-form!
  8. Do this a number of times for each nipple, left and right calipers, left and right antidive units.
  9. Whenever the master cylinder reservoir is getting low on brake fluid, and you don't have a mate handy to top it up for you as you go, you will have to tie the lever back to the bar to maintain that all-important pressure while you top things up.
  10. Once all the brake-bleeding nipples have had their turn, all going well you should now have a nice, rock-hard brake system without a trace of sponginess.

Of course, that's if everything goes well. There may be other reasons for spongy brakes, besides air bubbles in the system. Like, how old are your brake hoses? What, they're still the original ones, nigh-on 30 years old?! Get rid of them; they are unsafe and not worth entrusting your life to. Instead, install a set of braided lines and you'll be amazed by the improved feel and braking power.

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Removing the antidive units

The antidive units on the old Katana had been showing signs of their age when they started leaking fork oil. For a couple of years it had been only the slowest of weeps and nothing to worry about. But by May 2010, the units started doing a proper job of leaking the stuff everywhere. Which is the about last substance you need oozing out near your brake discs and calipers, I'm sure you'll agree.

So I tried fixing them: replaced the two O-rings on each side where the unit bolts to the front fork. But no joy there; oil was still appearing. Then I noticed the oil was coming from the tiny 'breather hole' on the rear of each unit — aha! But consulting the microfiche parts diagram, I found to my dismay that none of the internal O-rings or plunger seals were available as spare parts. Only the whole lower half of the antidive unit was available as a part. Enquiries revealed the crippling price of AU$232.60 each. Well there's no way I'm paying that amount of dosh.

After repeated fruitless episodes of removing and dismantling the units, trying to coax the tired old plunger seals into retaining their oil, I finally gave up. Sadly, the time had come to remove the antidive units after nearly 3 decades of service, and consign them to my little museum of motorcycle bits.

The procedure:
  1. First, unbolt the short antidive brakelines. Now use the shorter banjo bolt at the antidive unit, to join the brake line to your brake caliper. That was the easy bit. (Obviously you will need to bleed the air out of your brake system, clean up spilt brake fluid, etc.)
  2. Now unbolt and remove the antidive units. Put trays under each fork because now the fork oil is going to come flowing out of each fork. Removing the small drain screw on each fork will help at this point. (If you really want to speed up the draining of the fork oil, remove the caps at the top of the forks. But only do one at a time, otherwise you'll lose all fork spring pressure and your bike will end up diving forwards toward the floor... a potential disaster.)
  3. Now the fun part: fabrication! Yes folks, it's time to make up a pair of square blanking plates. I used some 1/4" aluminium I had lying around. Place your old antidive unit on the aluminium plate, and trace around the mating surface with a scriber. Now get your hacksaw, files, disc grinder — whatever you want to you use — and cut out and finalise the shape of the blanking plates. Drill the two mounting holes for each blanking plate, using a 6.5mm drill (I'd be lost without my drill press; I couldn't drill a straight hole without it! If you don't have one, go get one; you won't regret it).
  4. You must cut a channel in the rear of the blanking plate where it will join the fork, because otherwise the fork oil won't be able to travel through, and you'll lock your front forks solid. And probably burst the fork seals the first bump you hit! (Now that would be messy.) I cut a channel by using a 6mm drill and drilling a row of overlapping holes, as in the pic below. Using the depth gauge on the drill press ensured that I didn't go too far.

  5. Well, barring a bit of smoothing off the edges with a small file, that's the blanking plates made up.
  6. Clean away any oil residue on the forks, smudge a tiny amount of silicone goop on the rear of your nice new blanking plates, and bolt them to the front of the forks using the original bolts.
  7. Put fresh fork oil in the forks. I like to use 20W as the fork internals get a bit worn after 30 years, so thicker oil gets the damping back up to scratch.
  8. You're done!

Of course, now that the antidive units aren't there to anti-the-dive any longer, you might like to invest in a firmer set of fork springs. Aftermarket springs are readily available; the IKON crew in Albury, Victoria stock 'progressive' springs. I don't know what their springs are like, but they seem to have a good reputation on the forums. If ever I get a set I'll let you know what I think.

Heavier-duty fork springs

With the antidive gone, the front-end dive of the heavy old Katana whenever I hauled the anchors on, was something I just couldn't get used to. So (June 2010) I set about looking at the front suspension more closely.

First I measured the 'static sag' of the suspension — ie. the amount it compresses from fully-extended, when you take the bike off the stand and sit with your natural weight in the seat. (The suspension's sag is easy to measure if you put a cable-tie around the fork tube, and just measure the amount it slides upwards.) The correct amount of static sag for a road-going motorcycle is probably somewhere around 20-30mm. On the old Katana it was 40mm, which sounded a bit too much to me. Especially considering that the 4-position preload caps (standard fitment on the Katana) were already done up to their max. setting...

So I made up a pair of 16mm thick aluminium spacers (27mm diameter to fit nicely inside the fork tube), and tried that out. Predictably, the static sag was now 40 - 16 = 24mm. I took it for a ride and YES! a great improvement. These things can be a bit subjective, I know — and so much about suspension settings does get down to individual preferences and riding-styles — but I was sure I was on the right track.

Now at this stage, I could have just left things at that, I suppose. But there is a problem with putting in whacking-big spacers — and that is that by using them, you have shortened the suspension travel and increased the risk of the springs reaching the point where they lock up. That is not a good point to arrive at — because at that point you don't have any suspension happening, which is asking for trouble (your suspension is what helps your tyres keep their grip and do their best job, see). So, checking out the specs for the suspension stroke of the Katana in the manual (150mm), and comparing that to the actual compression of the bike under full braking, I realised that spring-lock probably wasn't far away. Eeeek.

Folks, let me underscore this: it's the suspension we're talking about here... and that's what keeps your tyres on the road... which is about all that lies between pleasure and pain (or worse). NB: if anyone thinks suspension is primarily for comfort, then let me tell you now: you're on the wrong website!

So I gave the chaps at IKON Suspension in Albury, Victoria a call. We talked through what I felt I needed (and why). Geoff got me to measure (i) the free-length of the existing fork springs (480mm), (ii) count the number of coils (each spring did 59 turns or coils), and (iii) measure the thickness of the wire (it was 4.5mm). He crunched a few numbers, and then was able to say that they really had nothing off the shelf that would do what I wanted. The answer would be to get a pair of specially-made springs, which it turns out wouldn't cost that much more than the more standard stuff they already had available.

Well the new springs arrived from IKON last week (early August 2010). Visibly they are indeed heavier than the springs that were in the bike, being made from 5mm wire instead of 4.5mm. But otherwise, a similar number of turns, and the same length. So I popped them in, and putting the preload adjusters on setting #3, I got the 20mm of static sag I wanted. So that's good: there's room to adjust things firmer or softer, if I want to. Then it was off for a quick strop around the suburb for a 10-minute ride — and I'm happy to report that, err, I'm happy! No longer is there that excessive dive under braking, and all-in-all the whole plot feels much tidier and better controlled.

Which is what it's all about, eh.

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