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Drilled Disc Hole Sizes

Discussion in 'The Garage' started by 5twins, Apr 29, 2021.

  1. 5twins

    5twins XS650 Guru Top Contributor

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    I made mention of this in some other threads and now here's the full explanation. I had seen or read somewhere on the web several years ago that so as not to lose surface area, it was best you kept the hole size no bigger than the disc is thick. To prove that to myself, I needed to do a bunch of math calculations. I found a nice, simple to use calculator for cylinder surface area on-line .....

    https://www.calculatorsoup.com/calculators/geometry-solids/cylinder.php

    All I had to do was plug a couple numbers in (hole radius, cylinder height) and it gave me all the area values I needed (area of the holes, area of the sides of the holes). With these, I was able to determine the ideal hole size for the disc thickness I was drilling. So, it turns out that using holes bigger than the thickness of the disc won't lose surface area as opposed to no holes until you get very big, like around 3/8" or 10mm. But, you will create the most additional surface area if you use holes the same size as the disc thickness. So, I drew up a chart, highlighting the ideal hole sizes in red. Go above or below those sizes and the amount of additional surface area you generate begins to drop off .....

    [​IMG]

    Drilling your disc has several advantages. Probably the biggest and best known is improved wet weather braking. On a solid disc, in wet weather the pads will hydroplane on the disc when you 1st apply the brake, until the water is forced out. You will actually feel and notice this when you first squeeze the brake lever. You'll have very little, if any, braking force momentarily until the water is dispersed. Drill holes and you give the water someplace to go. You will also save some weight but the other big advantage is creating more surface area for better cooling. So, you want to create as much as possible.
     
  2. member28833

    member28833 Race the wind Top Contributor

    5twins, curious if you either feel or hear the holes passing through the pads? I ask because having only owned one motorcycle with a drilled disc, a BMW F650, that feel and sound of the disc was very noticeable?
    And, next question, what is your opinion of slotted discs as like the late XS1100's were equipped with ?
     
  3. 5twins

    5twins XS650 Guru Top Contributor

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    I know what you're saying about sound, some drilled or slotted rotors do make noise, but I haven't noticed it on these 650 discs. I don't "feel" the holes either. But I discovered they do improve the braking a little even in dry weather. My '83 came with the stock solid rotor. I swapped on a drilled disc and it braked slightly better.

    I suppose slotted rotors are fine too although I've never tried one. I think the important thing is to have some sort of opening through the swept area of the disc, either holes or slots, for water to get pushed through. I have a stack of extra solid rotors I've acquired over the years so drilling them allows me to put them to use still. My latest came on this $25 parts bike .....

    [​IMG]
     
  4. SomervilleXS650

    SomervilleXS650 XS650 Addict XS650.com Supporter

    Nice work @5twins this is a great reference. Suggestion would be to expand the table to include the number of holes used in the calculations? Is it fixed pattern, or did you assume smaller diameters would have more holes?
     
  5. 5twins

    5twins XS650 Guru Top Contributor

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    The chart is giving data on individual holes so the number of holes used or the pattern isn't relevant. The total number of holes used will only come into play if you're trying to figure out the total amount of surface area you've gained. For instance, I use a pattern of 15 "swirls" of 8 holes each. That's 120 holes total. At 7mm diameter through a 7mm thick disc would give me 120 x 77 = 9240mm², or about 14.32 square inches of surface area gained.

    But, now that I've figured all this stuff out, I have a bit of a dilemma. Several years back I decided to use 3/16" or 5mm holes for drilling all my discs, and I made up a permanent metal, reusable pattern utilizing that hole size. I chose this smaller hole size because they're easier to drill than bigger holes and, not knowing what I do now, thought that smaller size would be just fine even in a 7mm thick disc .....

    [​IMG]

    [​IMG]

    [​IMG]

    Now I know better, that 7mm holes in a 7mm thick disc are better, but my permanent pattern places the outer 3/16" or 5mm hole in each "swirl" as close to the edge of the disc as I'm comfortable with. I wouldn't want to make it bigger which would put it closer to the disc edge. The innermost hole is also a little off the pad swept area of the disc so I don't see the point of enlarging that one either .....

    [​IMG]

    So, the best I can do using this pattern is to leave the 1st and last hole in each "swirl" small and just enlarge the 6 middle ones to around 7mm. This nets me a surface area gain of 14 square inches, so not much less than all 7mm holes would .....

    [​IMG]
     
    Last edited: Apr 29, 2021
  6. SomervilleXS650

    SomervilleXS650 XS650 Addict XS650.com Supporter

    Ahh. Yeah, I should have looked at the units and realized. I thought you were doing the cumulative analysis.

    I suppose the interesting question would be to factor in the number of holes you can fit on a given rotor and seeing if the optimum is still the same. I realize that it's probably a moot exercise since in reality people are using fixed patterns and no one is maximizing the number of holes.

    Thanks for the table....I know you've always made that point about the hole diameter not exceeding the rotor thickness, but it's cool to see the figures illustrated.
     
    timbeck likes this.
  7. Jim

    Jim Beyond the edge is the unknown. Here be Dragons XS650.com Supporter Top Contributor

    Good info on disc cooling 5twins. More info on weight loss if I may....
    2T is your break even point. 2 times the disc thickness (T= thickness). As 5twins points out 1T gives you the best increase in cooling area. You can see from his chart that a 5mm hole in a 5mm disk loses 39mm in disk area but gains 78mm in total area... it doubles. For maximum weight loss, 2T or a 10mm hole loses 157mm in disc area and gains 157mm in total area.... it's a wash. So you didn't gain anything in cooling area, but you did gain in weight reduction. When you start to go bigger than 2T, then you're reducing weight at the expense of cooling area
    So, like everything else, it's a tradeoff. 1T gives the best gain in cooling.... 2T gives the best gain in weight loss without losing cooling area. Holes larger than 2T and your cooling will start to suffer. If weight loss is your reason for drilling, remember the 2T rule.
     
  8. 5twins

    5twins XS650 Guru Top Contributor

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    Drilling doesn't reduce the weight that much on these porky stock discs, lol. I weighed some and was surprised, even somewhat disappointed. A 7mm thick undrilled disc weighed about 6.8 pounds. Drilling it with 120 3/16" holes only reduced the weight to 6.6 pounds, not even a 1/4 pound savings. I'll have to weigh one with mostly 7mm holes but I doubt you'll see much more savings, maybe another 1/4 pound if you're lucky. A bigger savings is had by switching to a 5mm thick disc. Undrilled, it only weighs about 5.25 pounds. I didn't have a drilled one to weigh but the savings would be even less than on a 7mm thick disc. So, when drilling one of these, I don't think your main goal should be weight savings but rather more surface area gained for cooling.
     
    Signal and SomervilleXS650 like this.
  9. JJames

    JJames XS650 Enthusiast

    Fascinating. Wasn't there a beatles song about this?
     
    SomervilleXS650 and Jim like this.
  10. XSLeo

    XSLeo XS650 Guru Top Contributor

    On the holes, The heat disapation is valid as well as the water desersion.
    The holes or slots have other benifits as well.
    As you use your brakes the pads get hot. As the pads heat up they outgas, This gas can build up between the pads and rotors, lifting the pads off the rotor. The holes let this gas escape. As the pads rub on the rotors, bit of szteel get peeled of the rotor and embed in the pads. If the edges of the holes are sharp they shave the pads a bit to remove the embedded steel. This keeps the brakes working bewtter.
    These ca slightly shorten pad life but pads don't cost that much.
    Leo
     
    SomervilleXS650 and GLJ like this.

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