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Author Topic: unusual question from the MOT today!  (Read 815 times)
chippyzip
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« on: June 08, 2018, 17:28:45 »

Hoi!

OK, just had my second MOT- it passed with flying colours last year, but this year, the tester said" I'm going to have to fail you this year, because of a loose front brake disc.

I went over to the scooter to have a closer look..

He said " one of the discs just has a little bit of movement, not much as if the bolts holding the disc on are either loose or badly worn"
I took off one of the bolts..
he said "there's no wear there, and because it's got less than 9000 miles, it must have come out of the factory like that" with that, he exclaimed that the other disc also had slight movement! he also had a closer look at the hole where the bolt goes into, and it was not completely round, but slightly elongated
He scratched his head- "it must be designed like that, but god knows why" and he gave the scooter a pass in the end..
I said " i'll find out on the forum as to why and if the disc is supposed to have slight movement, as I've never noticed this before, and I've never noticed any problems with braking!"

So, the question is this- is the front brake disc supposed to have slight movement, and if so why?? but thank god it passed as I'm on an advanced motorbike course tomorrow, and off to Ireland on Wednesday, for World Vespa Day in Belfast!

Graham 
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« Reply #1 on: June 08, 2018, 21:44:53 »

It could be that the holes are supposed to be round but the constant movement caused by loose bolts has worn them elongated.

Bob
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« Reply #2 on: June 09, 2018, 08:02:29 »

I think the screw has a top hat bush plus a wavy washer between bush and disk. So when its tight it clamps the bush and the disc is semi-floating by design.
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Dave Milnes
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« Reply #3 on: June 09, 2018, 11:29:33 »

No they are very slightly oval, or on the X9 they are. When you place the disc on the lugs and loosely fit a bolt in each hole you can slightly turn the disc, only 1mm or so and I always centre it in the then nip the bolts up in a cross sequence 1-3-5-2-4 then go round a second time. I use Screwfix flattened cheese head SS socket screws not the OEM ones and apply  dab of coppaslip and have never had one come loose or be particularly easy to undo again. The slight chamfer from the shank to the head of the screw should sit the hole central. They don't move once nipped up.
I have also never seen an MOT tester check the discs! Is this an addition to the regs this year?
It is possible the X10 is designed for slight movement to simulate a floating disc but it's a poor way to engineer it.
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chippyzip
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« Reply #4 on: June 09, 2018, 17:04:49 »

It passed with flying colours last year for it's MOT, but this year, he was going to fail it because of the slight movement in the discs. I noticed than when I buggered up the ABS sensor on the rear, there was no movement at all in the disc, it was a solid as a rock, and I don't want to buy new front discs just in case the holes are elongated at well! I did some quick reading up, and you can get "floating discs" designed to stop warping when the brake is applied- https://www.biketorqueracing.co.uk/pages/about/btr-tech-station/fixed-or-floating-brake-discs.htm

The only way to be sure that the front disc is in fact a floating disc is:
A, ask Piaggio, as they must make bloody sure!
B. Check another X10 and see if the disc is fixed or floating
But as I've the only X10 in existence in Cambridgeshire/Northamptonshire, I can't possibly know!

Graham  ::
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« Reply #5 on: June 09, 2018, 19:30:17 »


I have also never seen an MOT tester check the discs! Is this an addition to the regs this year?
 

Yep, and they are checking the thickness of pads too
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« Reply #6 on: June 10, 2018, 07:59:31 »

To get slightly technical this is all about over-constraint. If all six bolt were a tight fit in the disk and the disk also had a tight tolerance on the hub centre it would be possible that when fully tightened it would be under strain and more likely to warp slightly, especially when it got hot.  A better design is to run a tight tolerance on the hub centre and allow some float on the bolt holes but without any danger of the bolts becoming loose. Unfortunately I don't have a really good picture from my old X10 but I can see that the top hat bush under the bolt head is a large diameter ( "brim" of the top hat)so I think it fits this concept. If you search ebay for a replacement disk you will find pictures showing just how oval the mounting holes are.
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Mike H
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« Reply #7 on: June 10, 2018, 08:54:38 »

The calipers are single sided pistons, sliding type, so discs don't need to be in any way floating type.
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« Reply #8 on: June 10, 2018, 09:03:47 »

They don't but some bikes simulate a floating disc for aesthetic reasons although this method on an X10 isn't asthetic.
You could put a very thin fibre washer under two of the screws to grip the disc. It would be enough that a person gripping the disc couldn't move it.

I don't understand why they check brake disc thickness either, as brakes will be the same efficiency right down to the metal unless it is just to warn you that they need changing soon and so they can issue more tickets with advisories.
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