Brake disc skimming/grinding and drilling

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TomWaites
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Brake disc skimming/grinding and drilling

Postby TomWaites » Mon Jul 30, 2018 6:35 pm

Greetings all,

Does anybody know of anybody who can skim/grind and/or drill brake discs in the East Midlands area (Derby)? The discs on my 1979 Spada could do with a skim to get rid of the surface rust, and I'd like to get them drilled. Unfortunately I don't have the equipment to do it myself.

Cheers

Tom

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Mike H
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Re: Brake disc skimming/grinding and drilling

Postby Mike H » Mon Jul 30, 2018 6:40 pm

Using the brakes will get the rust off. Then when you leave the bike can get rusty again. Sometimes overnight!

Personally I wouldn't mess with them.
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."

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cyclobutch
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Re: Brake disc skimming/grinding and drilling

Postby cyclobutch » Tue Jul 31, 2018 1:03 pm

I would expect most engineering shops to be able to do this work. Whilst Mike is right that routine use should keep the dics clean, those on my V50 are now horribly ridged and could really do with some attention. Not so sure about drilling - personally I prefer the look of a plain disc, but they must do it for a reason. I'd expect that to cost more than a skim, and if they get the pattern wrong that might introduce vibration. Maybe?
1980 V50 II........................1981 T3.......................1973 Eldo Loop
- 'The Black Pearl'.............- 'Blue'...............................- 'Jug'

various others.

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Chris950s
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Re: Brake disc skimming/grinding and drilling

Postby Chris950s » Tue Jul 31, 2018 2:36 pm

I had mine skimmed because they were quite heavily pitted and even in use they tended to induce a shudder to the steering when braking. I got Star Engineering, in Chelmsford to do the work. As Butch said any engineering company should be able to do it, go for one locally that gets a good recommendation from your local branch members/bike club/friends etc.
Drilling will reduce unsprung weight but will not improve braking performance, personally I would not bother with it.

Best of luck
Chris and Karen - Essex
2011 Stelvio NTX, 1974 950S (750S replica still in bits), 1966 Triumph T100SS 50 years young last year!

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Mike H
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Re: Brake disc skimming/grinding and drilling

Postby Mike H » Tue Jul 31, 2018 3:37 pm

Unless I'm mistaken the holes are to clear water off the friction surface, I.e. somewhere for the water to go when the pads close up, same sort of idea as a tyre tread.
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."

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Chris950s
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Re: Brake disc skimming/grinding and drilling

Postby Chris950s » Tue Jul 31, 2018 4:05 pm

Mike H wrote:Unless I'm mistaken the holes are to clear water off the friction surface, I.e. somewhere for the water to go when the pads close up, same sort of idea as a tyre tread.

According to more knowledgeable people on this subject, water is only third on the list after heat and gas https://auto.howstuffworks.com/auto-parts/brakes/brake-parts/brake-rotors1.htm Quite frankly for normal road use it is of no benefit at all. However if you are racing and constantly braking sharply dispersing the heat maybe of importance to you. Clearing water however seems to me to be a non starter, where does the water go? Into a hole that is blocked the other side of the disc by the pad the other side? Tread on road tyres force the water away from the contact patch to the sides of the tyre, how do holes in discs do that? I would think that the holes in discs would just collect brake dust and other crud. I can see how slotted holes would work with water clearance but round ones would just fill up and be of no further use.
Chris and Karen - Essex
2011 Stelvio NTX, 1974 950S (750S replica still in bits), 1966 Triumph T100SS 50 years young last year!

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Mike H
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Re: Brake disc skimming/grinding and drilling

Postby Mike H » Thu Aug 02, 2018 3:20 pm

To be honest I'm only quoting what I've read (probably in a bike magazine from long time ago) they could've been wrong too of course.
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."

David_M
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Re: Brake disc skimming/grinding and drilling

Postby David_M » Thu Aug 02, 2018 3:51 pm

Would the water swept into the holes not get flung back out when the hole emerges from between the pads?
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Chris950s
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Re: Brake disc skimming/grinding and drilling

Postby Chris950s » Thu Aug 02, 2018 5:34 pm

Good call, and what of the water not near a hole?
Chris and Karen - Essex
2011 Stelvio NTX, 1974 950S (750S replica still in bits), 1966 Triumph T100SS 50 years young last year!

David_M
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Re: Brake disc skimming/grinding and drilling

Postby David_M » Thu Aug 02, 2018 7:16 pm

Wasn't trying to present an argument in favour of drilled discs, just a thought about what may happen. I agree that the slotted ones would seem to make more sense practically. I suspect that drilled discs were a knee jerk reaction to the many road tests on early disc braked machines which condemned the poor performance in the wet. Maybe more to do with the appearance of having done something than an actual improvement. (Cynical, me, never!)
David M
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Chris950s
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Re: Brake disc skimming/grinding and drilling

Postby Chris950s » Thu Aug 02, 2018 8:02 pm

No argument mate, only discussion. I remember my mate Batty not only drilled his discs but also his fork brace, front caliper mounts and the flange around the rear diff on the RH side, he was thinking of weight only in those days!
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Chris and Karen - Essex
2011 Stelvio NTX, 1974 950S (750S replica still in bits), 1966 Triumph T100SS 50 years young last year!

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Mike H
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Re: Brake disc skimming/grinding and drilling

Postby Mike H » Thu Aug 02, 2018 8:14 pm

Back in the late '70's I think it was Motorcycle Mechanics did an article describing comparison tests of various discs from different sources, to test their braking effort when wet. They made a test rig comprising of a huge iron flywheel they found left over from the Victorian steam age, put the disc on the other end of the shaft, span it up to speed (always the same), sprayed the disc with water (again consistently same rate) applied the brake (again, same pressure each time) then measured how long it took for the flywheel to stop. Can't remember if they said anything about drilled discs, quite likely no appreciable difference. The tests were prompted by the issue of Japanese bikes of the period that wouldn't stop in the rain. They did test Brembo discs as fitted to Guzzis etc., which performed poorly as a matter of interest. That bit I remember.
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."

David_M
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Re: Brake disc skimming/grinding and drilling

Postby David_M » Thu Aug 02, 2018 8:38 pm

Sorry, I meant argument as in point of discussion, not as in dispute.
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Re: Brake disc skimming/grinding and drilling

Postby Oldrat » Thu Aug 02, 2018 9:34 pm

Cast iron discs rust, as said sometimes over night so regular use is the best remedy, and the material provids one of the best brake rotor surfaces much better than stainless steel.

Lipped edges through wear can be easily removed with a hand angle grinder if you are a confident user and do it carefully. But make sure that what remains does not go below minimum recommended thickness (in your manual)

Drilled discs dissipate heat and to a fashion water, just look at modern brakes all drilled, it als reduces unsprung weight which is one of the holy grails of good handling. The holes also reduce the braking surface area slightly reducing efficiency, so it all a trade off and the choice is yours.


Don’t forget to report back sonothers can benefit from your experience
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Chris950s
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Re: Brake disc skimming/grinding and drilling

Postby Chris950s » Fri Aug 03, 2018 8:42 am

David_M wrote:Sorry, I meant argument as in point of discussion, not as in dispute.

No problem David, it is often difficult to get the tone and emphasis of chat on here without the spoken word, something that we all come across! :D
Chris and Karen - Essex
2011 Stelvio NTX, 1974 950S (750S replica still in bits), 1966 Triumph T100SS 50 years young last year!

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cyclobutch
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Re: Brake disc skimming/grinding and drilling

Postby cyclobutch » Fri Aug 03, 2018 8:44 am

I'm running the original stainless disc and caliper on my H1 Kawa. That's a '76 and undrilled. Seems pretty good on modern pads though I don't take it out in the rain from choice (running NOS tyres from '85).
1980 V50 II........................1981 T3.......................1973 Eldo Loop
- 'The Black Pearl'.............- 'Blue'...............................- 'Jug'

various others.


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