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SOHC.co.uk Forums => CB750 => Topic started by: Lobo on September 27, 2020, 06:45:18 AM

Title: Front Axle query
Post by: Lobo on September 27, 2020, 06:45:18 AM
Hi,
Reassembling the front end of the K2 with a couple of questions.

(1) never particularly noticed in the past, but the axle is about 3mm short of flush with the axle holder. The axle is in the correct orientation (ie hex side by the disc) and looks normal wrt the various clearances. Notably though the 400F’s axle is flush in this respect. Comments? (the forks were removed off the bike)

(2) the fork seals have been replaced; and I note the ‘book says 225ml of oil to replace for the K2..... and other volumes for K3 blah blah. Q. The bike is nearly 50yo.... how can I tell what forks are on it... they may have been changed out in the past?

Ta.
S
Title: Re: Front Axle query
Post by: Bryanj on September 27, 2020, 07:33:58 AM
Try rotating the fork tubes in the yokes and see if the gap varies indicating slightly bent tube, fairly common.
As to oil pick a quantity and make sure both are same thats more important than exact quantity
Title: Re: Front Axle query
Post by: SeanFD on September 27, 2020, 09:22:36 AM
From what I have read, if you have a back-up ring under the circlip, chances are you have early forks. Another external test is whether there is a bolt at the bottom holding the damper in. If no bolt, you have very early forks; I believe the bolt appeared at K0/K1.

The outside appearance of the lowers gives another clue:

[attach=3]

If that doesn’t help, you can tell by the innards.

[attach=1]

It says original because that was what in there when I dismantled them. Later contents:

[attach=2]

When I replaced my springs with IKON progressive springs, this was the advice I was given by IKON rep:

As prior I suggest you remove the springs and fill with about 400cc oil, stroke the fork slowly to bleed, then measure the height of the oil from the top of the tube, with the fork collapsed. Looking for 140mm of space between the top of the fork tube to the top of the oil. Add or remove oil till you get that set evenly, then install the springs and cut spacers as needed to get a total of 25mm of preload on the springs. Measure preload by placing the fork cap on top of the spring and spacer, measure the distance between the top of the fork tube and the sealing surface of the fork cap. Do this measurement with the fork assembly fully extended. 
[/i]

The preload bit may be specific to the springs, so take the above as a suggestion only.

I’d be interested to hear what others think of this, because after replacing the springs and shocks (IKON also) the bike developed a severe head-shake at about 35-35kph if I took one hand off the bars. This quickly settled and at higher speeds it was fine. I have yet, 20,000km later, to solve this problem, so any ideas welcome. I have just rebuilt the forks completely while replacing the seals, so here’s hoping 🤞
Title: Re: Front Axle query
Post by: Lobo on September 27, 2020, 01:10:06 PM
...aah thanks Bryan; that makes a lot of sense; didn’t occur to me. (tho’ the stanchions  are new’ish / DSS pattern & I’d bloody hope true!)

Thanks too Sean; your info does indeed suggest my forks are K2. The forks are now buttoned up with the spec 225ml of W5; I’ll road test it before going the slightly more tech route you outline. (kinda want to see the back of the front end so-to-speak! 😂)

Cheers,
Simon.
Title: Re: Front Axle query
Post by: Bryanj on September 27, 2020, 02:05:51 PM
I have seen bent yokes cause the same problem but that is rarer though as you say new tubes ought to be true!! I have never checked but i would think its possible that axles may vary. Cant be arsed to go through parts books at moment to check
Title: Re: Front Axle query
Post by: Lobo on September 27, 2020, 10:36:36 PM
...the varying axle (length?) theory could hold water as the inner clearance seems minimal... ie I don’t reckon the wheel should be any nearer the fork tube?
Title: Re: Front Axle query
Post by: Lobo on September 28, 2020, 12:31:17 PM
... any ‘750 owners elaborate on the fork tube to speedo hub clearance pls? Does mine look normal...
Ta.
Title: Re: Front Axle query
Post by: Andrew-S on September 28, 2020, 01:05:31 PM
Quote from: Lobo on September 28, 2020, 12:31:17 PM
... any ‘750 owners elaborate on the fork tube to speedo hub clearance pls? Does mine look normal...
Ta.

Here's my 11/70 K1 which is pretty original - hope these help:



Title: Re: Front Axle query
Post by: Bryanj on September 28, 2020, 01:27:06 PM
That gap looks a bit big from memory, i thought the speedo drive more or less touched the fork slider
Title: Re: Front Axle query
Post by: SeanFD on September 28, 2020, 02:30:41 PM
‘72 UK 750 K2.

[attach=1]

Oh, forgot to add; I haven’t tightened the fender bracket bolts, so that slight gap may well disappear when I do.
Title: Re: Front Axle query
Post by: Nurse Julie on September 28, 2020, 02:53:53 PM
Here's our 750 K2. All bolted up.

[attachimg=1]
Title: Re: Front Axle query
Post by: MrDavo on September 28, 2020, 03:11:16 PM
Check out my Z1 resto thread in 'Other Bikes', a suspiciously similar gap between fork and spacer on the front spindle turned out to be a bent bottom yoke, as predicted by Royhall.  8)

That said it was a lot worse than your gap.
Title: Re: Front Axle query
Post by: Trigger on September 28, 2020, 03:12:33 PM
Lobo , it is because you have your mudguard on pushing the forks out  ;)
Title: Re: Front Axle query
Post by: Lobo on September 29, 2020, 12:26:11 AM
Thanks all. Yea Trig, you’re a funny man 😂😂😂!
(PS - I want my original forks back 😂)
Title: Re: Front Axle query
Post by: Lobo on September 29, 2020, 09:15:04 AM
Sorted. With the front of the bike up and axle holders loosened I gained abouts a mm of Bryan’s ‘rotate the stanchions’ suggestion, and a further mm by simply pulling the slider against a spoke. (Minimal forefinger effort).
That leaves the third & last mm as speedo hub to fork slider clearance- can’t imagine it should be any less.
Thanks all.

PS - Andrew! You’re Avator looks spookily like mine! You been raiding my photo files? 😂
Title: Re: Front Axle query
Post by: Skoti on September 29, 2020, 12:33:05 PM
Hello Lobo,

thought you should see this Honda service bulletin, just incase your K2 has been fitted with an earlier K1 wheel at some point.

http://info.sohc.co.uk/download/cb750-service-bulletin-26-23-12-70-front-brake-disc-mounting-bolts/


I was also thinking that if you pull in the fork sliders like you described then tighten up the axle clamps you may increase the friction on the fork operation.

Somebody else may correct me, but it's probably better to compress the forks a few times with the bike off the main stand before fully tightening the axle clamps. 
That would ensure the stanchions and sliders are perfectly aligned.

Is that making sense? 

Good luck

Skoti
Title: Re: Front Axle query
Post by: Lobo on September 29, 2020, 10:25:26 PM
...thanks Skoti, your friction concern did indeed cross my mind. Tbh, the minimal ‘force’ (if that indeed is the correct word) it needed to gain 1mm was here-nor-there I reckon, especially when you go on to consider the same, but massive forces, under braking.

Alas I can’t open your bulletin... perhaps coz I’m on an iPad.

Many thanks,
Simon
Title: Front Axle query
Post by: SteveD CB500K0 on September 30, 2020, 07:02:02 AM
That’s something I’d not noticed before.

iOS is a real bugger sometimes. Apple refuse to open pdf files without using the Acrobat app. PC browsers will open them in a new tab.


Sent from my iPhone using Tapatalk
Title: Re: Front Axle query
Post by: K2-K6 on September 30, 2020, 07:46:53 AM
Quote from: SeanFD on September 27, 2020, 09:22:36 AM

I’d be interested to hear what others think of this, because after replacing the springs and shocks (IKON also) the bike developed a severe head-shake at about 35-35kph if I took one hand off the bars. This quickly settled and at higher speeds it was fine. I have yet, 20,000km later, to solve this problem, so any ideas welcome. I have just rebuilt the forks completely while replacing the seals, so here’s hoping 🤞

Have you verified that the headstock bearings are absolutely smooth with no notching very close to centre?

If you've got a notch that essentially pulls the steering very slightly off centre/dead ahead, the wheel will want to turn the bike slightly with the geometry castor/trail etc trying to neutralise by centering it,  which will produce an oscillation as the two essentially argue it out. As you increase speed the wheel gets significantly more gyro effect and overcomes the above.

It can be almost undetectable as if you lift the front to rotate the forks it doesnt sit on the bottom race,  so hard to feel. More likely on a "turning plate" type of device so you can feel more sensitively with that bottom race loaded with bike's weight.
Title: Re: Front Axle query
Post by: K2-K6 on September 30, 2020, 08:11:26 AM
Also any tire runout,  both front and rear can set off a "shimmy" at low speed that doesn't develop further as wheel speed and  gyro forces rise which then takes  precedence in keeping it straight.
Title: Re: Front Axle query
Post by: Andrew-S on September 30, 2020, 10:13:27 AM
Quote from: Lobo on September 29, 2020, 09:15:04 AM
.......PS - Andrew! You’re Avator looks spookily like mine! You been raiding my photo files? 😂

Hi Simon,
They do look similar, but I guess there's only so many ways you can take a great photo of a CB750 - if I had been on a covert photo raid wouldn't it have looked like this.....  ;D ;D Edit: I think you need to click on the image to get the effect I was after?  ;)  ::)




Title: Re: Front Axle query
Post by: SeanFD on September 30, 2020, 11:04:27 AM
Ah! You were trying to take a covert pic of yourself!  ;D

K2: I have roller bearings in the head. Would the same happen with those?

Thing is, the bike was running perfectly; I’d just completed a 5,000km ride with no issues. Then I changed the shocks and springs, didn’t have a chance to do a test run before heading off on the 2nd leg, and experienced the wobble 200m from home. I went a bit further to see what would happen and once speed increased, the wobble disappeared as quickly as it had appeared. So I went on and lived with it for a further 20,000km.

I took it for a short spin yesterday having replaced the seals, cleaned and rebuilt the forks. The wobble seems to have gone, although previously it was only really bad when the bike was fully loaded. The fact that the tyre pressures were less than 20 psi – the bike has been sitting for a while – didn’t help with the assessment. I’ll be trying again once the rain stops.