Yamaha MX backing plate choice.

[firko]

Pictured is a ratty old MX250 wheel that I'm going to rebuild for my Cheney. I have a number of questions regarding the wheel that need some expert replies.
1: What model Yamaha is the pictured backing plate from?
2: I would like to use the backing plate in conjunction with the MX hub instead of the original floating hub, purely because it looks less busy and doesn't require the brake stay. I merely have to weld a male plate to the inside of the swingarm to accept the female slot on the backing plate....much neater. Now the question..is there any advantages or more importantly, disadvantages in using this setup over the original floating brake?
3:Does anybody have a complete MX250/360/SC500 back wheel with straight and uncracked mudcatcher rim? A rim by itself would do as well.
The pictured wheel was my spare dirt track wheel from the 'DT1 from Hell' (as you can see by the Bridgestone wet weather slick) and unfortunately it's not as nice as I'd remembered after being buried in the garden shed for 15 years. I've ordered new spokes from Thailand but now need a nicer rim fairly urgently.

[mx250]

IIRC That hub first appeared on the YZ D 250/400 ('77). I didn't know they inter changed  - I would double check the internal distances.

To my knowledge full floating backing plate feed braking torque into the frame and reduced wheel hop under braking. Why they changed back then I would have no idea .

[firko]

- I would double check the internal distances.

Graeme, I compared both backing plates and they're both pretty much the same internally, same back spacing, the same brake shoes..or at least my new EBC MX250 shoes are a perfect fit. I can't see any reason it can't be used.

[Nathan S]

I reckon Graeme's right: That brake first appeared on the '77 models.

Yamaha occasionally did weird things with brake plate offsets, particularly in this era. I've found several brake/hub combinations where the brake rubs on the hub, or sits out far too far. Obviously, these are not impossible problems, but you should be aware of the possibility...

For short travel bikes, the full-floating rear brake thing is a wank, IMHO. The fact that Yamaha went to a non-floating brake for the YZ250A (over the floating brake on the MX-A) says a lot. That they didn't return to floating rear brakes until their first "proper" long travel bike (the YZ250/400F), also says a lot.

[GMC]

If it's a D model backing plate then it also matches...
Part # = 1W4-25321-00-98

Part Description = PLATE,BRAKE SHOE (rear)

Model Count = 7

TT500F 1979 500 TT500 Off-Road

TT500G 1980 500 TT500 Off-Road

TT500H 1981 500 TT500 Off-Road

YZ250D 1977 250 YZ250 Competition Motocross

YZ250E 1978 250 YZ250 Competition Motocross

YZ400D 1977 400 YZ400 Competition Motocross

YZ400E 1978 400 YZ400 Competition Motocross


But the hub is used in many more models suggesting intermatching can be achieved

Part # = 483-25311-13-98

Part Description = HUB,REAR

Model Count = 22

IT250F 1979 250 IT250 Dual-Purpose

IT250G 1980 250 IT250 Dual-Purpose

IT400C 1976 400 IT400 Dual-Purpose

IT400F 1979 400 IT400 Dual-Purpose

IT425G 1980 425 IT425 Dual-Purpose

MX250B 1975 250 MX250 Off-Road

MX400B 1975 400 MX400 Off-Road

TT500C 1976 500 TT500 Off-Road

TT500D 1977 500 TT500 Off-Road

TT500F 1979 500 TT500 Off-Road

TT500G 1980 500 TT500 Off-Road

TT500H 1981 500 TT500 Off-Road

YZ250A 1974 250 YZ250 Competition Motocross

YZ250C 1976 250 YZ250 Competition Motocross

YZ250D 1977 250 YZ250 Competition Motocross

YZ250E 1978 250 YZ250 Competition Motocross

YZ250F 1979 250 YZ250 Competition Motocross

YZ360A 1974 360 YZ360 Competition Motocross

YZ400C 1976 400 YZ400 Competition Motocross

YZ400D 1977 400 YZ400 Competition Motocross

YZ400E 1978 400 YZ400 Competition Motocross

YZ400F 1979 400 YZ400 Competition Motocross

[mx250]

The fact that Yamaha went to a non-floating brake for the YZ250A (over the floating brake on the MX-A) says a lot.

Oh dear!!!




On the YZ's I think the non-floating was only for '77 '78 - when they established themselves as Yammahops . Was it the non-floating brake or poor suspension geometry?

I haven't tried it Firko but I would double check the distances along the axle so that there's no binding when axle is pinched up. As Nathan said with Yamaha what looks like a match up between models there is often little details that preclude an easy match up. I'm only saying "double check".

I don't think there is a world of difference in practical terms between floating and non-floating backing plates. In theory there is a difference. I think the biggest issue with the Yammy rear brake is it's too powerful and over sensitive. Easy dealt with - reduce the surface area of lining .

[firko]

You beat me to it Graeme....I thought the YZA had a floating brake but I'd never challenge Nathan on old Yamaha stuff without checking.....which both you and I obviously have. Turns out he's right, the hub arm is connected to the swingarm and not to the frame as on the DT1 and MX-A.

I've verniered everything up on both backing plates and for the life of me can't see a problem. I haven't fitted the wheel to a swingarm and nipped it up yet but I have an inkling after thirty years of doing weird shit like this that I'm hopefully on to a winner. I'm just aiming to avoid using the ugly floater brake stay and over engineered and (in my opinion) ugly original backing plate. I'm trying to keep the lines of the bike as clean as possible without sacrificing performance. This bike is intended be a serious pre '70 class racer, not just a Classic Dirt show pony so I want it to work. I think Geoffs list proves I'm on the right path.

[GMC]

The floating brake is all about the non rotation of the backing plate through wheel travel.

On the non floater the backing plate rotates with the arc of the swingarm and so slightly rotates.
The problem is the brake rod becomes loose and tight through suspension movement. If you hold your foot in one place on the pedal than braking pressure raises and lowers causing chatter.
The more wheel travel the worse the rotation of the backing plate and the worse the chatter.

The floating backing plate moves in parallel keeping an even pressure on the brake pedal.
Look closely and you will see most torque arms are mounted in parallel to the brake rod.
A cable operated brake can also avoid chatter as do hydraulics, that’s why you don’t see floating rear brakes on moderns.

[firko]

The more wheel travel the worse the rotation of the backing plate and the worse the chatter

Thanks Geoff, that answers the question so that even a dubbo like me can understand. I don't think the 4" of Cheney travel would create enough movement to cause much chatter but I'll look into it a bit more by looking at some example bikes in the pre '75, seeing what the different philosophies were at the different factories.

I personally doubt it makes much difference with 4" though.

[Nathan S]

FWIW, the MX250A had shocking brake chatter at the Nationals, despite having the floating brake. The non-floating YZ-X had none (that I noticed).

There's more to brake chatter than floating vs fixed brakes!

[Graeme M]

My 05 YZ125 chatters like crazy under brakes (well, at least it makes a Helluva lot of noise that is...). I've played with the clickers etc and can't seem to make a difference to it. I posted the question on a modern forum and no-one seemed to know what I was talking about. Brake chatter? Whassat? So yes, Nathan, I think there are suspension forces involved as well.

[vandy010]

I posted the question on a modern forum and no-one seemed to know what I was talking about. Brake chatter? Whassat?

Graeme, they probably didn't understand what it was because most of the younger guys have stuffed linkage bearings/etc on thier bikes anyway.
bearings? Whassat?
Firko,
the small rubber grommet on that backing plate of yours could even lend it to being from a DT250/400 monoshock back wheel.

[GMC]

Yes there are many things that can cause chatter.
The floating hub helps reduce chatter caused by a moving brake pedal.

My first bike with rear disc was my 87 KX 125 which suffered from a lot of chatter.
I thought it might be from the wheel rotating slightly with the suspension so I built a floating disc for it.
At first it behaved the same, then it got a lot better.

The torque arm broke and I put it back to standard and it was still better than when I first rode it.

Eventually I had to concede that I simply got used to having a powerful brake.

[mx250]

The torque arm broke and I put it back to standard and it was still better than when I first rode it.

. The same thing happened to me with one of my 'experiments' . Can't remember the details now but I remember it stopped 'forward progress' in some what of a hurry .

Eventually I had to concede that I simply got used to having a powerful brake.

I think that is the real answer. We use to bend the brake rods on the Yammies to put a bit of springiness into the rod and reduce the immediate braking effect. I haven't done it but these days I would like to try to reduce the surface area by removing some of the braking material - probably by drilling. Ah, to experiment .

[firko]

Besides all of this chatter chatter.....has anyone got a straight mudcatcher rim to suit the MX hub?