18" rear wheel in ETZ251?

[Puffs]

Hi, great forum this is!

I'm fixing up a '89 (G-reg) ETZ251. It's done 30k miles and I've used it for 7 years as a commuter, but the last 12 years it hasn't been used. Before I started I took it for a spin, and suddenly remembered why I did so many miles on it.

ETZ251 '89 org, LH.jpg (43.13 KiB) Viewed 127 times

ETZ251 '89 org, RH.jpg (41.88 KiB) Viewed 127 times

When MZ moved from the ETZ250 to the ETZ251 they reduced the rear from 18" to 16", but I would prefer it slightly higher at the rear. I'll re-build the rear wheel (corroded spokes), and I'm considering to change it to an 18" rim. I realise it would then not pass an MOT, but there is no MOT for motorcycles in Belgium (where I now live, Ardennes).

Does anyone have experience or ideas to share on this?

[Blurredman]

I'm not sure, but it might require a longer swingarm to accomodate the larger size wheel, and because of that, the wheel would also be further back meaning the rear mud-guard might need to be changed or angled in a way so that the wheel does not hit the mudguard. This will also require a longer chain too, of course. 2 links more. So.. A bit of work required I think.
The rear hub system is the same though.

Can I ask, why would doing this modification fail the MOT?

[Puffs]

Thanks for your input!
The idea came up after I took the wheel apart, so I did not measure for it. It took me an hour to take off 3 spokes, and then I got fed up and cut the rest, so now I can't really measure it properly anymore...
But it looks like a 400x18" tyre has an outside diameter of a little less than 26", and in my current chain adjustment (roughly in the middle of the range) I have some 13" between the centre of the axle and the transverse bar in the swing arm. So that might just fit, and of course I could add a link or 2, or use a smaller 350x18" tyre. But the ETZ250 and the ETZ251 have a different swing arm, the ETZ250 one probably being longer; so it's bound to remain an imperfect fit with limited chain adjustment possibilities. It may also require different shocks, and maybe more than 2 links (if the wheel moves backwards 1", you need to extend both the top and bottom part of the chain by that amount). And yes, then there is the mudguard. But, see the pics, at the rear there is quite some clearance, but if it all fits remains to be seen. And the centre stand might be too short too; could be fixed with some blocks below it. I also expect the handling to change slightly, with the steering becoming slightly more direct.

Though the bike is a bit small for me I don't think I'll do it; it seems a lot of hassle, and expensive too. Best keep it simple. I just wondered if someone had tried it and had experience with it.

The type approval, for use on public roads, contains the wheel & tyre sizes, so if you change those the bike is no longer approved. But I'm not sure if they actually check that in an UK MOT. Here in Belgium they do, for cars, but as said bikes don't need an annual test.

[DAVID THOMPSON]

one of my bikes has a 16 inch rear tire .. here in the USA the selection of 16 inch tires is very poor and the 18 inch fronts are all to wide
makes it a pain at new tire time
dave

[Blurredman]

UK mot's only really check the safety of a motorcycle, and don't really do red tape like "this rear wheel should be x size for this one year". If you had a later 251, it would have the much larger/longer plastic mudguard. I think you might be able to get away with the centre stand. Possibly...



When it comes to my MOT's, they are shall we say... just a 30 minute chat about motorcycles



I also think the 251 is a bit small for me too (I have a 1990 metal rear mudguard too!). The steering is more direct certainly and sure this can be nice in the city, the 250 though is a nice longer bike that somehow seems to hold more confidence with me. The seating position all seems the same, and although I feel like I am sitting 'in' the bike on the 251 owing to the way the tank is shaped, I do feel more bolt upright almost whereas the 250 seems lower.

[Puffs]

Yes you're right about the MOT, I forgot. I once had my YZ 250 MOTed by a guy who tested it by making a stoppie, and after a chat it was all done. But not every tester is the same, and that guy stopped doing MOTs shortly afterwards.

The more I think about the idea to change the rear wheel to 18", the less I like it. The ETZ250 seems more attractive, but it takes more than changing a rim to get that geometry. That said, if anyone tried this change I would still love to hear about it.

[Eric Frith]

On my ETZ125 I converted it to 18" rear wheel by lacing an ETZ250 rim onto the ETZ125 hub. The fit was then straight forward but to make a nice job of it I moved the swing arm cross piece forward buy an inch, this works well and looks like a factory finish.

[Eric Frith]

18" Rear wheel

The reason for the 18" rear wheel is to give the bike a higher ride & seat height as I'm just over 6' 2"

[breakwellmz]

Hi

If it`s any help many years ago i converted a Supa 5 for off road use.
I had my local MZ/Jawa/CZ dealer replace the original rear 16" rim with an 18" one from a CZ 250 and fitted a 4.00 x 18 trials tyre, i don`t recall any clearance problems.

Cheers

[Puffs]

Eric, thanks for that pic & sharing your experience. It looks like when the bike on the centre stand, the tyres do not have much clearance, but it still works. The bike looks better with the bigger rear tyre (3.50x18 ?)!

You say you moved the swing arm cross piece forward by an inch; does that mean you took the swing arm completely apart, and re-welded the lateral bar 1" closer to pivot point? That would sure help & make clearance for the tyre, but it might weaken the swing arm a bit (the two lateral connections are then closer together).

breakwellmz, thanks for sharing that!

[Eric Frith]

That's exactly what I did, but I used a slightly bigger section of box steel rather than the original piece of tube, I'm guessing at about 40mm x 25mm, placed a piece of large studding through the pivot end and the same at the other end to keep everything in place. Cut, ground and then welded, gave it a little tweak in the vice and then painted it.

[Puffs]

Ta, kudos. And as long as you don't mount a side car or use it on an MX track, strength is not much of an issue, certainly not if you use a wider rectangular bar & with the shocks mounted in the farthest position (as you have).

If you want to keep a useful chain adjustment range, seems it's either moving that bar forward (and doing something with the front side of the mudguard too, at least on my bike), or extending the swing arm backwards (use ETZ250 one?). The latter should also make the bike 1" longer.

[Puffs]

Look at that, with a 4.50"x18" MX tire:

ETZ251 '89, 4.50x18, RH.jpg (28.4 KiB) Viewed 126 times

Maybe not that big. This one doesn't actually fit inside the mudguard. And looks daft.

[Blurredman]

I always wondered whether that extra hole at the back of each side of the swingarm was to change shock angle?

What kind of effect does it give?

[Puffs]

Re: 18" rear wheel in ETZ251?

Postby Puffs » 23 Jan 2018, 20:19
I think it's an alternative mounting position for the lower eye of the shock. It's also M10, and if the bike's on the centre stand, both mounting holes are at pretty much the same distance from the upper mounting stub. So unladen the bike has the same height in either position.

But that hole is 5cm further back, typically directly above the rear axle (when in the middle of the adjustment range). In that position the wheel axle is 45cm from the swing arm axle.

If you use the backward mounting position, the rear wheel will have the same stroke as the shocks, and suspension will be slightly stiffer (will spring inward slightly less at the same load, compared to the forward position). So if you sit on it, you'll be a little bit higher: you'll have a slightly higher ride height. Seems advisable with a pillion.
In the (default) forward position the wheel stroke is 45/40 x the shock stroke, and you'll get a slightly more responsive suspension, and slightly longer stroke. And stress the swing arm a bit.

Always use the same mounting position left & right.

I have actually decided to stick with the standard 16" rim, just for simplicity & cost. When I need to change the tire, I could put a bigger one. On the 2.15" rim you can go up to 120/90-16", which compared to the standard 3.25x16" gives a 1" height gain, same as mounting a 3.25x18". Of course on an 18" rim you can gain an extra inch by using a bigger tire, but for now I can't be bothered with the mods & costs.