Skimming discs

rjen · Post by **rjen** » Mon Feb 26, 2007 1:26 am

My TRX has done 53000kms. I've not had it long and it's always had the following problem: The front brake doesn't work evenly. i.e. for the same lever force the brake bites more on certain points of the discs than others.... indicating that the discs may be slightly warped. You get this a lot on cars and to solve it you simply remove the discs and get them skimmed. Is this also possible on a bike (as long as you don't go below the min disc thickness) or should you just buy new discs?
The problem isn't bad. You can only feel it at really low speeds. It certainly doesn't judder or anything like that when braking at higher speeds.
I'm about to fit a set of blue spots so thought I'd like to get it all sorted out while I'm down there.
If it is possible to skim the discs do you need to actually remove the discs from the wheel? Can anybody suggest a place in Sydney where they will skim bike discs?
Cheers!

Greg · Post by **Greg** » Mon Feb 26, 2007 2:33 am

To be honest Rjen I'm not sure if there's enough meat on the disc to skim it safely. Car discs are much much thicker (I worked in a disc foundry in Poole for a while...), and even them it's mostly not worth the effort and cost of skimming compared to renewing.

You mentioned that the brakes have more bite at some points than others...? I wonder if the problem is something other than you think as IMVHO think you'd be hard pressed to tell (in angular rotational terms) where the pads actually hit the disc... Have you had a dial gauge on the rotors? Can you 'eye them up' sufficiently to visually check for warpage? Generally speaking a warped disc will manifest itself as a pulsing in the lever as the disc bangs the pads back into the caliper each time it goes round. Usually also it's only more obvious at higher wheel speeds, although it depends entirely on the extent of the warpage in the first place.

If you can get the front wheel off the ground you could either listen/look for binding at certain points, or maybe try a chalk line around the face of the disc, spin the wheel and gently apply the brake to see if there's a high/low point somewhere (the pad will rub the chalk of the high spots).

Other than that it could be a fluid issue. When was the fluid last changed or the system bled? A useful trick with (some) spongy brakes is to zip-tie the brake lever back to the handlebar and leave it overnight. This can sometimes allow trapped air to rise to the top of the line where it's sucked back up into the reservoir when the lever's released... Just a thought, it might help...

Greg

rjen · Post by **rjen** » Mon Feb 26, 2007 5:47 am

Very imformative. Thanks Greg! Disc warpage was only a theory of mine as it seemed like the most likely reason from the problems I'm seeing. There is deffo no pulsing back thru' the lever. You can only really tell it's happening when you are moving very slowly and braking lightly. It's subtle... but being a perfectionist I like my bike to be working just right.

I've got a set of blue spots coming with a matching m/c and new lines. I think I'll bolt that lot on and see if I've still got the problem. At least that way I'll know the problem doesn't lie with pads, calipers,lines,m/c or fluid!

Quan-Time · Post by **Quan-Time** » Mon Feb 26, 2007 7:31 am

as he said...

summary, check fluid / pads 1st (prefer full flush of fluid) and check, dont machine disks, just get new ones..

From experience they are a bastard to cut.. Cos they chatter (floating disks)

Solid disks like rear / harley's etc.. fine to do.. but floating is just WAY too annoying.

Greg · Post by **Greg** » Mon Feb 26, 2007 1:09 pm

Only moving slowly?? lol.... there's a line there somewhere!

It's prolly not relevant, but I changed to blue spots recently and had heard that they really work best with an R1 master cylinder... I got one, installed it and spent the next 3 hours bleeding the system to try and get rid of sponginess. I'd changed brake lines too so assumed it was a bleeding problem, but still couldnt get rid of the sponge - thought fukkit then put the old master cylinder back on and bingo, no sponginess!! So the Ebay MC was fukked ... hmmm!!

How long have you had the bike? Not asking a DAQ but are you used to its little querks? I took friggen ages to use the rear without locking the wheel - it needed a feather light touch compared to stamp my ST1100 needed..!

rjen · Post by **rjen** » Tue Feb 27, 2007 1:04 am

Greg wrote: How long have you had the bike? Not asking a DAQ but are you used to its little querks? I took friggen ages to use the rear without locking the wheel - it needed a feather light touch compared to stamp my ST1100 needed..!

DAQ?
It must be about 3 months now tho' I've only ridden about 800kms on it. I've pretty much always had sports type bikes so was used to only using very light pressure on the rear brake.
Quirks? Well it took me a while to get the proper clutch control needed at very low speeds/pulling away due to the tall 1st gear. I hate slipping it but it seems the only way to ride it smoothly below 10km/h or while pulling a U-turn. The only other is that when it runs of petrol on 'ON' and you switch it to 'PRI' you have to crank the engine for ages to get it to start again! Maybe this is normal for big bikes with electric start? I've only ever had kick start 2-stokes up until now!
That's it really.
I love everything else about it....especially the noise!
-rjen.

BTW last time I had a bike (early 90's) I remember buying dot 5 silicone based brake fluid. It was awesome stuff! With a braided brake line I remember my brakes being *SOLID*. I've been looking for dot 5 recently at my local bike shops and can't find it. What's going on? Can you still get the stuff?

burty · Post by **burty** » Tue Feb 27, 2007 9:52 am

I think some might debate whether you should use DOT5 silicone based brake fluid. It doesn't disperse water throughout the system like DOT3/4/5.1, which makes the system more prone to internal corrosion, and localised boiling. It's not always compatible with the seals, it's not compatible with any Glycol based fluid and it's actually more compressible.

The best approach must be to replace your DOT 3/4/5.1 fluid regularly.

Greg · Post by **Greg** » Tue Feb 27, 2007 10:42 am

DAQ ... dumb ass question ...

...

I was just wondering if you were taking time to get used to the brake. Some bikes have awesome stoppers and some just need a lot more 'pull' to get them to do their job ... Same goes for this feel thing, I've often heard of bikes having 'wooden' brakes (here's where you have visions of some old geezer lovingly crafting a set of rotors and matching pads from a nice piece of tight-grained rosewood - maybe QuanTime's next project !!

). I've never had any experience of being able to 'feel' brakes... maybe all of my bikes have been wooden, who knows, but they've all stopped! I had a ride on a new GSXR 600 K6 last autumn - was very very surprised at how flat the bike felt (not used to having only 3nm of torque see!) but amazed at the front brake - it almost endo'd me off with a single finger on the lever!! Is that having 'feel' ?? not sure about that!

burty · Post by **burty** » Tue Feb 27, 2007 11:34 am

I would describe the brakes on my TRX as having a wooden feel.

I'm running Bluespots, braided lines and an R1 master cylinder. The master cylinder made a big difference in feel over the TRX master cylinder. Feel is excellent, and they are far more effective than the TRX standard calipersm but it does feel like the pad/disc interface isn't making the best of it.

Personally I think that the wooden-ness comes from the pads I have in place. I'm using the Carbon-lorraine A3 pads, but have been meaning to get something a bit softer for a while.

Post by **Max** » Tue Feb 27, 2007 11:58 am

I "wood" describe my (front) brakes as having a hot-wired-to-my (pea sized)-brain feel. I'm running standard issue import (domestic for me) "Yamaha Brembo" calipers with the "stock" pads, braided lines (my own with a single line running down to the right pot and a cross-over to the left) and... (insert beating dead horse joke here) a genuine brembo forged racing radial master. It all came to life with addition of the master cylinder. Never, and I mean NEVER have I ridden anything with better feel, control or feedback.

Now there are others round these parts that claim it's all about blue (or black) spots, this pad or that (don't recommend Maxi-pads...), "R6", "R1" or some other wanabe production pump. For me, best piece of cash (besides my flat slides) that went on the bike was that pump.

And everyone I've let ride it gets off with their tongue on the deck saying "WTF!"...

slow codger · Post by **slow codger** » Tue Feb 27, 2007 8:43 pm

Agree with Max, my import Brembo's work for me.

By the way, I always find when skimming discs that if you launch low into the water just over low wave height that it will bounce well into double figures, but small stones are better!

Dave E

Greg · Post by **Greg** » Tue Feb 27, 2007 9:10 pm

I think I see my next purchase on the edge of the blue horizon...

Triton · Post by **Triton** » Wed Feb 28, 2007 8:16 am

Jeez Codger, wot you bin smokin' N!gger?

Kirk · Post by **Kirk** » Wed Feb 28, 2007 11:58 am

I agree with Codger, and after a big storm is the best time, cos the sand washes away and leaves heaps of good size 'discs' exposed on the beach...
Oops, is this off the topic? sorry...

Post by **Max** » Wed Feb 28, 2007 12:03 pm

Getting back on topic...

After one of those big storms, maybe you could find a couple the right size to mount on the TRX! You could just reach your legs down there and give them a rub. Flintstones!