Fork flex

[Swissrider]

I have fitted a 220 disk to my 2018 Kenevo which has Rockshox lyrics. When braking on a steep hill, it feels as if something is loose, like the headset ( which I’ve checked and it’s fine). I can see the bottom of the forks flexing backwards and forwards, as if the bushings are worn. I think the movement might be caused by uneven braking on the disk, but I bedded in properly and it feels smooth. If I turn the bike upside down I can’t really produce any play in the fork legs by pulling on them. The performance isn’t really affected, but it feels vaguely unpleasant. Any ideas what is going on and what I can do about i?

 

[R120]

SRAM 220mm Rotor

I just converted my front rotor on my 2019 S-Works from 200mm to 220mm. It took some research to get the conversion right so I wanted to share what I did. 220mm Rotor: SRAM CenterLine 220mm 6-bolt Disc Rotor with Rounded Edge 40mm Adapter: Avid/SRAM Disc 40mm Post-Mount Adaptor, Fits 200mm...

www.emtbforums.com

 

[steve_sordy]

Too much braking torque for the fork.

 

[Gary]

you've checked the headet and fork bushings and seem happy it's neither of those. and I guess when you tried flexing the legs you're also happy it's not a CSU issue.
so...
Next try holding the front brake on with the wheel lifted (or bike upside down) and try to rotate the wheel. - if it rotates a few degrees forwards and back with the brake on it's likely to be one of these issues:
Loose rotor (6 bolt or centrelock) - re-tighten
Loose caliper/mount - re-tighten
Loose maxle - re-tighten
slightly undersized pads moving in the caliper - fit new pads
You haven't mentioned what brake or rotor you are using but some rotors can cause vibration with certain caliper/pad combinations - seeing as it's only started since you switched rotor it could be that the rotor you've fitted is prone to this with the brake you're using or it could be that your old/worn pads not being bedded in/aligned to the new rotor's braking track is the issue

all forks do flex forwards and back in use BTW. it's perfectly normal. just a little disconcerting when you first realise it.

 

[Eddy Current]

Unbolt the fork, then with help lift the frame with the wheel on the air pressing the crown of the fork against the headset tube from below. The bolt again starting first from the spider and then the stem. Sometimes is just that

 

[Dirk74]

Which 220mm rotor do you have? Magura MDR-P or maybe another rotor with two piece design?

Loose boost spacers

Does anyone else have the above on the standard roval hubs on the Levo? Mine rattle around between the hub and fork legs and it drives me bonkers!

www.emtbforums.com

 

[Swissrider]

It’s a SRAM 220 rotor with Code R brakes. I’ve done all the checks as suggested above and nothing is loose. If I apply the front brake with the bike upside and try and move the wheel, I can see some flex in the forks. My theory is that there is some uneven deposits from the brake pads on the disk which means the breaking force changes the disk goes around. I’ve cleaned

 

[Swissrider]

The disk and I’ll see if this makes any difference - maybe I should try some fine grade sandpaper on the disk. I’m using sintered pads which are supposed to be for EMTBs and they certainly are wearing well but maybe I should try some trickstuff pads which I’ve used in the past and they seem good, except they wear out quite quickly

 

[Dirk74]

It is also possible that the pistons in the brake caliper do not extend evenly.

Get the wheel and the pads out. Push the pistons back (use a plastic tire lever).
On a 4 pistons caliper block one pair with a small blead block (the SRAM pro bleeding kit includes a small one)

Gently! push the brake leaver and look if the pistons come out equally on both sides. Attention, do not push either one piston out of the caliper! Otherwise you would need to rebuild and bleed the caliper.

Clean the outside of the pistons with Isopropanol (use a ctton era stick) and then put a small droplet of brake fluid (DOT 5.1 for SRAM Brakes) on it, to reduce friction. Push them back (again use a plastic tire lever). Repeat pushing them out (gently) and back until they come out easily and equally.

Set the small blead block on the already service pistons and repeat the same with the other pair.

When done (with the 4 pistons all pushed back) clean everything carefully with Isopropanol to remove the brake fluid as it would ruin the pads (and other stuff).

Put the pads and the wheel back.

Readjust the alignment of the caliper with the rotor if necessary.



There are very good videos from GMBN Tech on Youtube for it.
Here is one of them (can’t just quickly locate the other one about it)

 

[Swissrider]

Thanks for these clear instructions. I have done this before - quite a fiddly job and some pistons seem to have a mind of their own no matter how well you clean them. I did clean the disk and it seems to be better. I noticed the fork flex an a very long descent (2500m over 30k) with some steep sections over loose stones and gravel and I wonder is this also contributes to uneven braking which would make the forks flex. I guess Specialized choose the Lyric forks for the Kenevo because they are stiff enough to cope with a heavy Ebike, although I’ve always thought that when it comes to braking the difference between the lightest and heaviest bikes (5kilos) is trivial compared to riders, some of which weigh 40 kilos more than me!

 

[jerry]

Have the same issue, after fitting a larger disk + adapter. Guess the forces this conversion puts on the fork are a tad different. Check the following: pulse your front brake while riding. If, at each pulse, your wheel has a tendency to (ever so slighly) visibly be pulled to the left, it's just fork flex.