Fork suspension query

[SirFallalot]

Hey all,

If any of you could confirm/give any advice on my suspension it would be great.

I soon realized that my 94 CB400SF had way soft forks, bottoming out when exiting kerbs or hitting some potholes (I figure, form the CLANK) and diving quite a bit when braking, It bounces a lot too. I was planning to buy these £96 Hagon shocks, but decided it was worth to take current sag measurements and get an opinion.

I measured from the fork brace or whatever and the edge of the outer tube in centimeters

17.3 Fully extended

12.3 static after compressing
14.1 static after lifting
13.2 average for 4.1cm sag

12.1 rider after compressing
11.5 rider after lifting
11.8 average for 5.5cm sag

I am not sure how much travel I got, I weight 75kg and this was with no gear so maybe 5-10 extra kilos, and sometimes I pillion with my 50kg gf for long rides. Not sure how much it is with her as she was the one taking the measurements.

Someone told me to add heavier oil but that will just make it less bouncy and still bottomed out right? Also wondering what weight oil I should put back in.

Thanks in advance

[-]

Just go up in oil weight and leave less air gap. You can also mix weights, so a 50/50 mix of 10wt and 15wt will give 12wt.
I stiffened up my TZR race bike forks by going from 10wt to 15wt and reducing the air gap by 40mm.

The springs may also be out of tolerance now. Take them out and measure them. The manual should give original spec.
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TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[Riejufixing]

First, are you sure that they are actually assembled properly with the right amount of fork oil in, of the right viscosity?

[SirFallalot]

Hm, no I am not sure if they are currently to spec or assembled properly.
I guess measuring them and the air gap might be an initial step. I only have a russian translated manual though, it has the spring size, but not the sag specs.
I need to undo the handlebar as it's in the way, I guess that's one point for clip-ons heh.

The oil/air gap won't decrease sag though right? Only new springs? I've read around but found anything from 2.5 to 4cm rider sag, mine are over that, so I was wondering if the stiffer/new springs would be a good idea too.

[-]

If your forks are bottoming out, thicker oil, less air gap and spring being up to spec/compressed/spacered out more or higher rate, will all help.

The cheapest and easiest way to start is to check the current air gap/oil level and condition. Thicker oil of at least the correct amount is very cheap and easy to do.
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TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[SirFallalot]

Thanks, I'll see to that before buying new shocks

[stinkwheel]

If your forks are bottoming out. You need stronger springs. End of. Any of the other things you can fiddle with are just fucking with how quickly they move up and down.

Go here:

https://www.racetech.com/ProductSearch/12/Honda/CB400SF%20Super%20Four/1994

Click on the "Calculate spring rates." hyperlink. This will give you a spring rate, oil viscosity and air gap. Do all that. Set static sage to 25-30mm depending on how you like to ride.

Even at 85kg, you'd want linear springs 50% stiffer than standard (and the standard ones will be knackered because they are 25 years old.

There are UK suppliers of linear springs much cheaper than racetec.

Now there is the "how much do you want to spend on a 25 year old motorcycle? option.

Your bike has damper rods. They are shit.

You could fit cartridge emulators (try Brooks suspension, or go full gold-valve).

Alternatively, you could see if you can find either a full front end or a set of cartridges that fit your bike (might be cheaper). You will find the difference remarkable. Doing a lot of setting up with 25 year old damper rods in is polishing a turd. Fitting springs strong enough to keep you from bottoming out is not.

I just upgraded the (already cartridge but basic cartridge) suspension on my VFR750 using linear springs and a set of fettled firestorm cartridges and it's like a different bike.


...Looks like a CBR600F3 fork stanchion is the same diameter. There are other considerations too if you wanted to swap the whole front end. If you are really lucky, you might be able to bolt the whole cartridge in, some Hondas are a bit like lego in that respect. I suppose a CB400 owners forum would be the best bet to ask about them. If you can fit those, then you can buy a pair of rusty, pitted forks off ebay for much cheapness. The cartridges are probably ok on account of being in oil.

But yeah. New springs.
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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.

[SirFallalot]

Thank you stinkwheel, by far the best advice.

[SirFallalot]

Well I never got round to this and forgot about it but after scraping the exhaust again and getting a shitscare bottoming out the forks I feel I best get on with it. (Also I'd like to crash my bike by learning to wheelie)

The CBR600F3 fork swap seems very good but I'd be afraid start the project as I have limited tools, can't decommission the bike for more than a weekend and can't find any within budget compared to springs.

I don't think I measured the springs when I had them out, but the manual says 338.2mm, and 130mm travel (which I measured and is accurate).

I could only find Hagon springs on Wemoto, but they are way big at 550mm and progressive with 5.8 to 10.5 kg/cm. Racetech's calculator recommends 0.7-0.75 kg/mm for me, so even if they fit in there not sure if appropriate.

Does anyone know where I could get some springs from for ~£100?
I found https://motorcycleshockabsorbers.co.uk/, on which you can pick from K-Tech/Racetech/FC/Bitubo and send a form with your bike, weight, pillion weight etc etc, anyone have experience with them or have other recommendations?

Thanks in advance.
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Lexmoto Valiant 125; 94 CB400; 96 CB750F2; 81 CB750 (restoring, lol not gonna happen); 2001 ZX9R(It's about to go :c); 2012 R1200R; 2015 R1200RS

[JackButler]

Your suspension was crap when it left the factory, but it's safe to say it wasn't this bad. It just got worse over time & mileage.

I can't say that I'd chuck much money into rebuilding a 94 CB400, but it does sound like you do need to rebuild those forks.

New springs (those Hagons are a good idea), new bushes, new seals & new oil. I'd steer away from anyone who claims to be able to make them drastically better than when they left the factory, they were crap when they left that factory & I don't recall anyone having much success with them back then.
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I clean my guns with the tears of the snivelling, left wing scum.

[redeem ouzzer]

Stick a CBR600 or blade front end on. Probably cheaper than getting the original forks reworked so they aren’t shit.
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Be a REAL MAN!

[SirFallalot]

[stinkwheel]

Aftermarket springs are not necessarily the same length as the originals. They are generally a lot longer and use much shorter spacers.

Try giving Brook Suspension a call/email.
____________________
“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.

[SirFallalot]

[stinkwheel]

[SirFallalot]

Never mind, I do remember seeing that, looked at the manual and it's there. I tried using long screwdrivers and a coat hanger to get the spring out. my bad.
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Lexmoto Valiant 125; 94 CB400; 96 CB750F2; 81 CB750 (restoring, lol not gonna happen); 2001 ZX9R(It's about to go :c); 2012 R1200R; 2015 R1200RS

[wintelf94]

Interesting and amazing contributions. Very educative.
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Beyond Limit