CB"R" 250

[Skub]

Sounds like a perfectly valid explanation to the missus for buying new wheels when you need new tyres.

Skillz.

[Count Steer]

Sounds like a perfectly valid explanation to the missus for buying new wheels when you need new tyres.

Skillz.

Or, 'Well, I have these wheels but they really need a bike to put them on'.

[A_morti]

Those arrived yesterday from UK.

Pretty
I still don't know if they'll fit. Need to grab our shared ABBA stand from Jules's garage and have a play.


The tyres are dated 1322 and 1822, which in tyre years, is about 5 years younger than what's on the CBR.


The tyres don't have 100 miles on them and that sprocket has never seen chain lube spray.
I reckon someone spent a few weeks pocket money on maintenance at the garage (you'd clean the sprocket carrier if you were doing it yourself) then crashed pretty much the next day.

[A_morti]

Buckle up, it's about to get technical.

Front wheel first:

Step one:
Jack up your motorcycle.


Step two:
Weigh the wheels to see if there's any performance benefit to all this, or if it's just for vanity.




Yes, that's 8.65kg Vs 10.48kg.

Nearly 2kg difference between the wheels with bearings and central spacers, tyres and discs.

That's a 4lb difference in rotating weight for our American members.

The CBR wheel has a Pirelli Diablo SM tyre, the CB wheel has a Dunlop gpr300. Both are 110/70-17 and while they won't weigh the same, certainly most of the difference is in the wheel and disc.

I wonder how much you'd pay for that kind of weight reduction if you were running superstocks?

The CBR disc just about shows a sliver of daylight between the disc and the tyre:


The CB disc is approx 2mm further inboard, which is about the same as the thickness of the ABS ring which I removed. I am pretty confident that the caliper sliding pins will pick up that slack, but if not I could either put the ABS ring back or use some M8 washers as shims either under the disc bolts (are they flat enough?) or between the caliper and fork leg.


The CB wheel is 2mm wider across the bearings. It'll go in the fork easily, but it's 2mm (2 turns of thread) closer to the fork leg on the non-brake side:




That could be easily corrected by replacing the stock 15*22*23mm spacers with 15*22*22. With a lathe you could skim 1mm off the factory spacers, but then they'll want to rust on the machined face.

Looks good, right?


The rear is more of a headache.

[A_morti]

Mentioning tyre age: the tyres on the CBR wheels are dated 2119 and 5019, so they are about 7 and 6.5 years old respectively. I forget which was which.

The front had a puncture patch and the guy who did it refitted the tyre back back to front. That was longer ago than I'd like to admit.

The CB wheels have tyres dated 1322 and 1822, so about 3 and 2.5 years younger.

Rear wheel:
There's also a decent weight saving to be had, at 12.28kg Vs 10.84kg = almost 1.5kg (3lb) difference for the wheel, tyre, disc, and bearings.



The width across the bearings in the new wheel is about 3mm more and it seems to be all on the brake side. If I fit the new wheel with the old sprocket carrier and sprocket spacer on the left, then the brake side spacer won't fit due to the +3mm width, but I can tighten the assembly without that spacer to about the same stack height and the wheel is centered and the chain runs correctly. The disc side would need a 3mm shorter spacer to equal out the stack height which seems doable. The disc is still a few mm too far inboard, again I'm hoping either the sliding pins will eat the difference or maybe it takes a washer under each disc bolt - TBC.

The sprocket side gets more complicated though, and I do have some questions.

The part numbers for the cush rubbers are the same, so I felt confident the sprocket hubs would cross over. They do, however the o-ring marked 27 in the diagram below is about 53mm on the new wheel and about 58mm on the old wheel.


That means while I can use the old sprocket carrier on the new wheel, it doesn't engage that o-ring at all. I'm not certain what the purpose of the o-ring is. If it's just to prevent dust and water ingress to the bearing, I would either live without it or find a thicker o ring to take up the gap. If however it has some function in keeping the sprocket hub centred, then it may be essential. Does anyone know?

I would like to use the old carrier because the new one has a cb125r sprocket on which is for a 428 chain, so I'd need to buy a cb300r sprocket (520). The cb300r sprocket only comes with 36 teeth and my bike has 38. Considering where the bike lives, I don't need lower revs at high speed! So I'd probably also have to buy a smaller front sprocket to correct that, and I don't want to because the chain kit is fine.

Even then, the new carrier places the sprocket about 2mm too close to the swingarm. So either the studs would need removing and the sprocket mounting face skimming, or maybe an offset front sprocket is available. I don't know if there is one matching the engine splines and even if there is, I don't know if there's enough clearance to move the chain outward 2mm before it hits the frame.

[A_morti]

I found a pair of front wheel spacers in stainless from uncle Ali for under €4. So that's the front done, as soon as the slow boat arrives.

Rear is still a headache. I reckon that o-ring interface is for centering the sprocket to the wheel and is required. Can't be a coincidence that pretty well every wheel with a cush drive has that.

I reckon that rear sprocket was fitted inside out; the offset should face the other way. There's no reason the branding would be on the inside. Maybe the lad's new chain came off causing his crash?!

[Rockburner]

I can't see an o-ring being used for positioning purposes - that's what a bearing or bush is for. An O-ring won't resist any forces that are trying to move a sprocket carrier off-axis. My bet would be that it's just a "seal" to reduce water/dust ingress.

[A_morti]

During assembly, the carrier can move a few thou due to clearance between the spacer and the spindle.
The o-ring would at least line that up.
But yeah - during operation if the spacer can move enough that it needs an O-ring to catch it, the spindle nut isn't tight enough.
Only other thing I can think is for redundancy. If the carrier bearing collapsed, the o-ring would at least vaguely keep the chain in line, maybe enough to stop a complete disaster.

[A_morti]

After getting frustrated with the bike and my back last time, I went back to it today with an idea to be more scientific.

I realised I was trying to compare sprocket alignment using an offset 428 Vs a flat 520. That's pointless.

The mounting face of the new sprocket hub is about 24mm from the table, using the original spacer.


For the life of me I couldn't get the sprocket nuts on the original carrier undone, that'll be one for a rattle gun. The old sprocket is about 18mm off the table, and the 520 sprocket about 6mm thick.



The bearing and spacer in each carrier have the exact same part numbers, so it's about the same 24mm height from swingarm to sprocket mounting face whichever sprocket carrier I use, and as a little bonus the new one is a few grams lighter.

I used a similar method to check where the wheel is in relation to the swingarm.


Both rims are 17*4" and they measure the same across the lips. The new wheel needs to come left about 2mm, so reduce the 18mm thick spacer to 16mm and job's a goodun. To accommodate the new wheel being 3mm wider across the bearings, the brake side spacer would want 1mm taken off.

That puts the sprocket 2mm too far left, but JT Sprockets has a step on their cb300r sprocket which can help. On the cb125r 428 sprocket I have it's 3.5mm on the thin part. If the 520 is the same, then running it upside down moves the sprocket about 2.5mm in board.


The chain line is then correct within a millimetre, and that's me being generous with potential tolerance stack.

Ideally I would then slap it together and ride into the sunset. But the new wheel's disc is 5mm too far inboard and while the sliding pins might make it work, the disc won't pass through the slot in the carrier where the pads sit on the spring shim.

So, looks like it would mean spacing the disc out 5mm, but I'm not sure what that looks like since the bolts are Hondas "M8 with 10mm shoulder" thingies.

Or maybe the better idea with more swapping but less sketchiness is to find a cb125r caliper. I know the two bikes use identical spacers between the disc and caliper carrier but I don't know how thick the caliper carrier is as a spacer, and whether it then jives with everything else.

Fun times

[A_morti]

This thread is currently a stream of consciousness to myself, but that's ok. It helps me get thoughts in order.

Thoughts return to alignment of the rear brake disc.

The CB125R/CB300R caliper has an overhang to it such that the caliper would line up with the disc. The spindle hole is the same size but the torque reaction slot won't line up with the CBR250R swing arm, so that's a negative.

I've also realised all this talk of sliding pins picking up misalignment is nonsense. There's really not much difference between the width of the pad slot in the caliper carrier and the disc itself - that has to line up pretty close.


So: how can I safely space the disc out from the hub to meet the existing disc slot?

I have disc screws like the 22mm model here:



The 3mm high head is non-negotiable; it needs that to not crash the carrier. Regular bolts are closer to 7mm head height, and won't work.

Half of the 6mm thick shoulder is for centering in the disc, half for centering in the wheel. It's not a very tight fit, so it is also possible the thicker section is mainly to give enough depth for an internal hex. The shoulder doesn't bottom out in the counter bore, so it's not like a fairing screw where it's for stopping the plastic panel getting crushed.

If I change for the 33mm screws (as used on GSX1400 and similar age Suzukis) I get 10mm of shoulder. With that, I can add a 4mm thick spacer between the disc and the wheel, so it will align with the pad slot. I think.

So to make the back wheel work, my shopping list is:
1 Cb300r rear sprocket, 4 longer disc bolts and 4 spacers to go under them (I need to measure that thickness a couple more times), and 2 shorter wheel spacers (this is still open, I need to measure at least twice more then find who can make them).

[A_morti]

I've ordered those 33mm disc screws and after measuring again also some 5mm spacers.

A preliminary shopping list for anybody who wants to do this wheel swap is:

• CB300R / CB125R wheels with bearings, centre spacers, sprocket carrier, front disc (rear disc optional). If you can get the CB300R wheel spacers then do so; they're identical to CBR250R spacers and then you'd have them available to modify.

• 1* JTR1224 rear sprocket, this is stepped and flipping it should correct the chain line to within 1mm. Note it only comes with 36 teeth, CBR250R stock is 38. It may be (necessary/possible) to add a thin M10 shim under the sprocket to get it perfect, I'll measure again when I have the sprocket in hand.

• 4* M8x33mm disc bolts, as used on gsxr-srad models. I've paid about €15 for them in titanium on AliExpress.

• 4* 10x16x5 spacers for the rear disc (spacers are all given in millimetres as inner diameter X outer diameter X length). I've paid under €5 for them in stainless on AliExpress.

• 2* 15x22x22 spacers for the front wheel (you could remove 1mm from the stock spacers but they'd need re-zincing). I've paid under €5 for them in stainless on AliExpress.

• 1* 17x30x16 spacer for the rear wheel chain side (you could remove 2mm from the stock spacer).

• 1* 17x30x22 spacer for the rear wheel brake side (you could remove 1mm from the stock spacer).

• TBC if the front disc needs spacing out or caliper spacing in due to removing the ABS ring. I think it'll work as-is.

It's not the hardest project in the end, especially if someone has done the measuring for you, and double especially if you have a lathe or a friend who does.

The prize is much more modern looking wheels weighing about 4lb less on the front and 3lb less on the rear.

[A_morti]

I realised there's no picture with both wheels on. Even if they're not properly on yet, now there is.


The 22mm front spacers arrived yesterday, so it was time to cross fingers and toes and offer that up.


Close but not quite a ceegar; it wants 0.5mm taking off the disc side spacer to get the wheel dead-nuts centred between the fork legs. It's probably close enough honestly, but it'll bother me knowing it's there.

Actually as I write this I realise I don't know that the original wheel sits exactly centred between the fork castings. It'd be weird if it didn't but I'd better measure that before getting spacers trimmed.

Questions regarding the best way of aligning the front brake:
With the ABS ring (2mm thick) as a spacer the disc aligns nicely in the caliper carrier slot. I don't want to use the ABS ring though, it's ugly considering it's non-functional. Would repair washers be flat and of a consistent enough thickness for replacing that, or do I need to fetch some machined spacers?
I could also space the caliper in by 2mm with a couple of basic M8 washers, but the caliper bolts come out relatively frequently and I can just see the washers dropping out and rolling away on a future owner who doesn't notice, so it doesn't seem smart. Or it'll be fine?

[A_morti]

I put the original wheel back in, and confirmed it sits centred between the forks.
Not exactly the same centre though, since it's a 2.75" rim versus the new one which is 3".
But both bikes had a 110/70-17 as stock front tyre size.
I think 2.75" is a new one on me.

[KungFooBob]

The important question is how much more fasterer is it going to be?

[A_morti]

The important question is how much more fasterer is it going to be?

About 6-7 per cent.

Any thoughts on how best to align the front brake?

More I think about it, washers under the caliper bolts seems the way to go. It's less critical that they're perfectly flat, since not-flat washers under the disc will feel like a warped disc, but a floating caliper won't care too much if it's 0.1mm higher at one sliding pin than the other.

[Rockburner]

The important question is how much more fasterer is it going to be?

About 6-7 per cent.

Any thoughts on how best to align the front brake?

More I think about it, washers under the caliper bolts seems the way to go. It's less critical that they're perfectly flat, since not-flat washers under the disc will feel like a warped disc, but a floating caliper won't care too much if it's 0.1mm higher at one sliding pin than the other.

weeelll - you might not get the pads flat to the disc.... <sucks teeth>

But - if you grab a handful of washers and a micrometer you should be able to find 2 the same thickness.