The correct way to measure front brake discs

[Aveton Gifford]

So I just went outside to measure my front discs, but I'm not sure if I am missing something, the total came to 295mm

I measured the braking area of the disc first which was 7cm, then doubled it

Then I measured the hub from the edge of the braking area to the edge of the area where the hub bolts go which was 4.5cm and doubled it

Then I measured across the centre of the hub which was 6.5cm

So is this correct??

[RM955I]

It's a good question as I need to measure mine accurately in order to purchase replacements. I have arrived at the conclusion that short of being able to find some kind of identifying stamp, the only other way is to draw around the disc and then measure the diameter. It's too big to get verniers on and the problem with placing a straight edge top and bottom is that unless they are 180 degrees apart and perfectly parallel the reading will be wrong.

It's a long time singe I did trigonometry but there must be an easy way of measuring the circumference to arrive at the diameter.....Diameter = Circumference divided by Pi????

[reggit]

Does this picture help identify your disk size? This is a (somewhat worn) 305mm front disk.

[Georgeandkira]

Hello, It's not trig. It's geometry or even just basic measuring. Use a piece of string which doesn't stretch. Go around the edge of the rotor and cut it flush. Its length ÷ 3.1416 = its diameter. Or take 2 pieces of wood and make a pivot point by drilling a hole through both. Use a bolt and a wing nut so you can hold an angle. Basically make your own crude divider. Or get a piece of thick, single strand wire and bend it to a "C" shape which faithfully holds still on 2 opposite positions on the rotor. You CAN do this. Kira

[T5RatherAmusin]

really?!? you guys dont know how to measure discs

[Aveton Gifford]

Thanks Kira, maths is the only part of my education that I failed on!! but my wife has explained it to me with a piece of Christmas ribbon and a biscuit tin!

Reggit, if yours measure 7.5cm and they are 305's that means that as mine measure 7cm (1cm smaller in total) they must be the smaller 286's I suppose

Looks like I will be taking the wheel off again and using the Christmas ribbon on the brake disc.

It wouldn't be so bad but 295mm couldn't be any more in the middle of 286 and 305!!

Sometimes life makes you feel like a right idiot!!

[coughlanc]

Why not just place a tape meassure over the center line of the disc ?

http://www.google.co.uk/imgres?q=mea...,r:7,s:0,i:103

[Brendan W]

Steel rule across the wheel centre and the Mk 1 eyeball. Practice on cutouts from the cornflakes packet behind the space saver if you have to. How hard can it be to see a 19mm difference?

[Slackey]

If you are really struggling and the discs are off the car just draw around a disc on a piece of paper, cut it out and then fold it in half, then measure the straight edge

[reggit]

Originally Posted by Slackey

If you are really struggling and the discs are off the car just draw around a disc on a piece of paper, cut it out and then fold it in half, then measure the straight edge

If discs are off, you'd just measure the back side as it's easy to get a measure across.