CB125F - Front Forks Oil Change

[Chip]

Hi Guys,

The forks on my CB125F seem to bottom out quite easily. I have compared the action of my forks with those on a similar bike in the showroom and they are the same (soft and spongy). I am a sprightly 17 Stone, and there are lots of hills in my area so whilst braking there is potentially a high loading on the forks. I have trawled the internet but there does not seem to be a great deal of service information at the moment for this bike.

Can anyone suggest the best way to stiffen up the front forks, there is no mechanical adjustment that i can see. Also how big a task would I be taking on to replace the oil, I am mechanically competent and have a vast selection of tools at my disposal.

Thanks,

Chip

[Stu]

you need heavier springs in the forks

just replacing the oil is just masking the problem

[Chip]

Just spoke with Hagon Shocks, bike is currently not listed (too new) they recommended measuring springs and spacer when dismantled and they could then try to much with an alternative set of springs.

So no further on really, may just replace the oil and ensure it is at the correct level. Apparently, I am led to believe it has been known for the lower spec bikes to leave the factory with an an incorrect oil level in the forks.

[megawatt]

I had the same problem with a Bandit 600. I added 30ccs of fork oil to each leg. Sorted

Nothing to do with damping, the extra oil reduces the airgap and improves the springing.

[Mr Fro]

Changing the oil is easy - especially if you've got a Haynes or something.

Like Stu says, you probably need stiffer springs. Any decent bike suspension place should be able to sort new springs, oil change and setup. MCT Suspension in Suffolk is very good if you can take the forks off and post them over.

[Chip]

Thanks for the advice guys, spoke to the bike shop this afternoon and have it booked in for a service week after next. Something not quite right. They will check out the forks and advise if we can make a warranty claim with Honda. I wil let you know how I get on.

[Chip]

So, I took the forks off the bike today and drained the oil. Inspected the springs, and fork legs no issues. Flushed the fork legs with some clean oil and allowed to drain off for 10 minutes. Put in Castrol 10W synthetic fork oil to correct level and added extra 10 ml in each leg. Reassembled forks and torqued all bolts. The difference is like chalk and cheese. The front end is reactive but does not dive at the slight touch of the front brake. Handling has improved 10 fold and inspires confidence. Had a good deal with the oil, halfords priced matched opie oils for two 500 ml bottles of synthetic Castrol at £11.95. Changed the engine oil whilst I was at it, bike good to go just need the rain to stop.

[iangaryprice]

Good to hear you have improved the handling.

Just goes to show, before trying to modify a bike make sure it works the way the manufacturer intended.

Its likely that the existing oil had thinned over time and/or some had leaked out.

The extra 10mm would reduce the air space slightly in the forks this would reduce the amount the forks compress during braking,

[Grumpy Old Git]

An alternative that I have seen many times on smaller bikes is to introduce a spacer tube into the top end of the forks to compress (pre-load) the spring slightly - This stops the dramatic 'dive' that some small bikes experience. However, that 'modification' will not stop the forks from 'bottoming' - That is down to the viscosity (thickness) of the oil and the size of the 'air gap'.

Sounds like you have fixed your issue but check regularly for leaks around the seals.

[joned001]

What is the Fork Oil Capacity, please ?

[Guest]

An alternative that I have seen many times on smaller bikes is to introduce a spacer tube into the top end of the forks to compress (pre-load) the spring slightly - This stops the dramatic 'dive' that some small bikes experience. However, that 'modification' will not stop the forks from 'bottoming' - That is down to the viscosity (thickness) of the oil and the size of the 'air gap'.

Sounds like you have fixed your issue but check regularly for leaks around the seals.

Yeh... What GOG said.... a couple of spacers on top of the springs to increase the pre-load would probably have been the best way forward....but we are where we are, just keep an eye on the seals to make sure they don't blow owing to the increased internal pressure created by adding the extra oil.

[fastbob]

Instead of fiddling around with bits of tubing why not buy a bag of suitably sized washers from Tool Station and add or subtract them until you get the desired preload .

[Grumpy Old Git]

Instead of fiddling around with bits of tubing why not buy a bag of suitably sized washers from Tool Station and add or subtract them until you get the desired preload .

Good one - But I had spare tubing of the correct diameter to hand and washers cost money!

[fastbob]

Instead of fiddling around with bits of tubing why not buy a bag of suitably sized washers from Tool Station and add or subtract them until you get the desired preload .

Good one - But I had spare tubing of the correct diameter to hand and washers cost money!

 

But washers allow you to experiment until you achieve the optimum preload . It's unlikely that you will get it right first time .

[Mr Fro]

Instead of fiddling around with bits of tubing why not buy a bag of suitably sized washers from Tool Station and add or subtract them until you get the desired preload .

Good one - But I had spare tubing of the correct diameter to hand and washers cost money!

 

But washers allow you to experiment until you achieve the optimum preload . It's unlikely that you will get it right first time .

That's why you cut it long, try it and then hack bits off until it's right.

[fastbob]

Good one - But I had spare tubing of the correct diameter to hand and washers cost money!

 

But washers allow you to experiment until you achieve the optimum preload . It's unlikely that you will get it right first time .

That's why you cut it long, try it and then hack bits off until it's right.

 

I ain't gonna win this one am I . Good luck , don't forget to stiffen up the back end to match .

[Stu]

Almost 3 year old thread!!!!