Perhaps someone can help with a two-part question.
Changing brake discs on our KA3 (Japanese-built)Honda Legend Coupe and after some difficulty getting someone who could obtain the correct size as per what is currently fitted on the car not what is in most catalogue listings for the Legend, I picked them up yesterday. I took an old one in for comparison and everything matched up except the hub centre diameter which is bigger than the old one. The parts shop assistant admitted he hadn't seen quite such a difference before but surmised it would be OK.
Lined the new one up today and sure enough it is just under a centimetre wider all the way round the hub although it sits fine on the hub when screwed in and is a perfect fit round the wheel bolts. The old one has an internal diameter which doesn't quite make a tight fit with the hub. There is surface corrosion on both the outer rim of the hub and on the inner part of the old disc suggesting they are not meant to be a tight fit. My first question is whether this new extra difference in internal diameter matters?
Second question: what is taking the load when applying disc brakes? Is it the wheel bolts on the hub and those two little screws that attach most brake discs I have seen?
PS: The pattern parts are made by Mountain who seem to be a respectable Japanese manufacturer.
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David, I'm not an expert on this but personally I wouldn't fit them unless they located properly on the hub - i.e. without the gap. Apart from anything else, it locates the disc securely so that it is accurately centred. You're right that the bolts & screws take the load, so I don't think that's an issue, but from waht you say I think there is some danger of movement.
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i am also no expert, but would suspect that if a gap existed before, then it will not matter. As Mike says, it's the bolts taking the load, and i would imagine that unless the original was a tight fit on the hub then the replacement will be just a good a job as centreing the disc. Only my opinion though.
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Thanks for those replies but I have probably answered my own question now by looking at the set up again. Although the round disc flange/plate on the hub is smaller and leaving this gap I mentioned there is still a smaller central part of the hub sticking out from it which is a perfect locating fit with the centre of the disc and must be transferring some of that loading when braking to the hub itself.
I was concentrating too much on the obvious external difference and forgetting the internal. Could be a Zen thing?!
Only other explanation for the disc not being a clone is that it has also been manufactured to double up as a part for another car or that Japan has severe copyright laws preventing an exact copy. The pads are, however, an exact replica of the old ones.
Thanks again.
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Simple.......use genuine parts......oh sorry.....they\'re 4 times the price :-(
Honda, they\'re terrible for this.
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Is David\'s car not a grey import?
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I don't know about the Honda setup, but on every vehicle I've changed a brake disc on the central mounting spigot is very important. The close tolerances make it hard work to remove an old rusted disc, and the spigot must be clean and rust free to ensure a succesful new disc fitting. Even with clean surfaces a new disc might be a tight fit. The two small setscrews are only for locating the disc, they don't contribute to any braking effort. If it comes to that, neither do the wheel studs. If everything is fitted and tightened correctly, the friction between the mating surfaces transmits any braking forces between tyre and caliper.
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The braking force is carried to the hub assy. via the bolts. None of the braking force is transferred via the concentric locating ridges since the braking force will always be orthogonal to the contacting surfaces. Any locating ridges are there purely to ensure that the disc is concentric with the hub.
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The bolts should not be carrying any significant load, other than tesnsile, they are to hold the wheel assembly firmly against the disc/hub assembly.
The disc should be an interference fit on the spigot
DO NOT USE
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The disc should be an interference fit on the spigot
No it shouldnt. All the disks on various cars I have changed have never been interference fit. PLus the disk itself gets so much heat in it it needs room to expand.
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Unless rusted in place all the brake discs I have ever removed have been a close, but certainly not interference, fit on the spigot. The purpose of this is to centre the disc during assembly only. Once the bolts are tightened in theory the spigot need not be there! The respondents who say that all braking loads are transferred to the hub by friction between the faces are correct.
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Yes, I have been sitting here thinking about that again, and suppose as long as the wheel is a good fit on the spigot the disc would not have to be.
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David,
Actually I would be worried as these may be a wrong disc. Are you sure they are the same in every respect?
I don't do many Hondas but I thought the Legend had several discs based on a 64.2mm dia hub hole and one for a 70.2mm. I say this because recently I was supplied a pair of Legend discs in error (with large dia hub holes) when I ordered for a Prelude. This caused us to look hard at the book and examine the differences.
As you appear to have started with a smaller size and now upped to the larger you may find the stud holes and overall diameter the same...but the disc thickness may be greater and the inset of the disc from the hub a little less.
M.M
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"the braking force will always be orthogonal to the contacting surfaces...."
Where did you drag that word up from A?! Dictionary for brekkie?
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Forget the hub - surely the braking force has to be transmitted from the disk to the wheel? That means either the studs/nuts in their conical seatings in the wheels, and/or the friction between the wheel and the disk?
Even if all the nuts are loose and the little screws missing, the drive and braking force are still transmitted to the wheels, after a fashion.
There has to be an interference fit somewhere to centralise the disk - the holes for the wheel studs are normally quite a bit larger than the studs.
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Thanks to all and apologies if I initially failed to describe this properly. By concentrating overly on the diameter of the concave part of the new disc being greater than the flange, I overlooked the hub hole still having a tight fit on the spigot/locating protrusion from the flange. That presumably allows braking force to be transferred as well as via the wheel nuts. The flange itself, which had a close but not tight fit on the concave part of the old disc, can now be seen to be acting simply as a mounting plate for the new disc. Applying grease to the mating surfaces shows there is about 1cm extra diameter around the flange compared with maybe 1mm between the edge of the flange and the inner concave of the old disc. All the other disc dimensions are right, down to the Min. and Max. width stampings. The KA3 Legend (Jap-built rather than at Cowley) seems to be the only one using a four-stud 282mm disc rather than the more usual 262mm and five-stud 282mm or 285mm discs on the saloons and later coupes. I had the disc off and measured it which is why I had to turn down the first Japanese 'specialist' who insisted it should take 285mm discs. My second specialist hunted through four catalogues to find an importer which listed the KA3 with the correct size disc. Funnily enough, all listed the correct size pads.
Maybe I should have gone to Honda direct which quoted me 'just' 20% above pattern price for an exhaust a few months back but I am saving up for their £100 sidelight if I don't find one in a breaker soon.
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