Changing disc rotor size

[Pete D]

I have recently received a Cytronex front wheel electric conversion kit which I ordered to be fitted to a Giant Anyroad bike. The front wheel is currently equipped with Avid BB5 cable operated discs and a 140mm rotor. However, when attempting to fit the hub-motor wheel supplied by Cytronex, the hub rubbed against the red adjuster knob on the inside of the forks.

I rang Cytronex who suggested a workaround of putting a 160mm rotor on to the wheel, along with a new post mount to fit the bb5 to.

I am not the most mechanically competent of people. In fact, if you imagine the least mechanically competent person you know, I would make them look like Sheldon Brown in comparison. I know someone who will be able to do the actual installation for me, but my question is what type/size etc. of post mount exactly do I need to buy? Also, I presume any 160mm rotor with the right number of bolt fixings would do, as the ones on Wiggle all say they are for hydraulic disc brakes, but surely it’s just a piece of metal and so the fact they are cable operated discs won’t matter?

Any advice (in terms a technical numptie like me would understand) gratefully accepted.

Thanks

[slowster]

Take a look at your fork and front brake to confirm the mount type. You've asked "what type/size etc. of post mount exactly do I need to buy?", but post mount refers to the type of mount. You probably do indeed have a fork with a post mount fitting, but make sure.



There is also another standard, 'flat mount', which is more recent and used on road bikes. It positions the caliper closer to the fork leg than post mount.



If you have a post mount, your brake caliper will be bolted directly onto it if using a 140mm disc. Increase the disc to 160mm and you need a 20mm adapter in between to position the caliper further away from the fork. The key thing is that the adaptor must be the right type, e.g. if you have a post mount fork an adapter designed for IS mount will not fit, and the right size, i.e. 20mm (160mm-140mm).

I think there is usually no problem mixing and matching components, such as using a Shimano adapter with an Avid caliper, or a Shimano disc with an Avid caliper, but I prefer to keep to the same brand wherever possible to avoid any nasty surprises, and I think the bulk/shape of the Avid mechanical brakes can foul on some non-Avid adapters (see photograph of an Avid adapter below with a bend which I think the adapter has in order to accommodate the shape of the BB5 and BB7).

Since availability/price of Avid/SRAM discs does not appear to be good, in your shoes I would get a basic Shimano disc. There are two types of disc mount: six bolt and Centrelock (which uses a splined mount), and you need to buy the right one (unless you want to spend more money on an adapter to mount a six bolt disc on a Centrelock hub).

So, assuming a post mount fork and six bolt hub, I would buy the 20mm "post to post" Avid/SRAM adapter from Planet X (£1.99 plus postage), and a 160mm Shimano RT66 disc from Wiggle or Decathlon.

https://www.wiggle.co.uk/shimano-rt66-6-bolt-rotor

https://www.decathlon.co.uk/p/brake-dis ... mc=8383761

https://www.planetx.co.uk/i/q/BSSDBM/sr ... nt-adaptor

I know someone who will be able to do the actual installation for me

Don't get them to do it for you. Get them to tell you how do it and watch over you while you do it. You will never learn otherwise.

[slowster]

I've just noticed that Planet X do not include the bolts and (CPS) washers with the adapter. The washers are essential with BB5 calipers to align them properly. However, you should already have a set of washers on the caliper fitted to your bike which you can use.

The price of the adapter from Planet X is so low that it is probably worth buying it and buying suitable length bolts separately, rather than finding somewhere that sells the adapter with bolts and washers.

The relevant technical document from SRAM is here:

https://www.servicearchive.sram.com/sit ... unting.pdf

It shows that for a 140mm post fork with a 160mm disc, you need adapter 20 S or 20 P (the latter being what Planet X is selling), and the bolts are 43mm and 37.5mm. If you buy bolts separately they should be the same (or slightly longer) length to ensure adequate bolt engagement, i.e. bolts come in 5mm increments so get 45mm and 40mm (or buy longer bolts and shorten them). I think SRAM/Avid use stainless bolts rather than the high tensile steel bolts used by some other brands, and I have similarly purchased generic stainless bolts to use with SRAM/Avid adapters when the adapter was supplied without bolts.

[Pete D]

Many thanks for that in depth reply, Slowster. I'll double check the mount on the fork and may well be back on here with photographs to confirm what I think it is is what it actually is, when I get a chance.

Unfortunately, getting someone else to do it is borne of necessity. I have limited dexterity in my right hand due to a ripped tendon and associated referred pain which means I'm unable to do a lot of fiddly things and tightening of various bolts. I will stand over and watch and get him to explain, though, so I can learn for the future when hopefully my gammy fingers heal.

[slowster]

For your information I would add that what I have described above is what I did myself when fitting a 180mm disc to a 160mm post mount MTB fork on a bike with Avid BB7 brakes, i.e. I bought that very same 20mm adapter from Planet X and bought some longer stainless steel bolts separately to use with it.

Unfortunately, getting someone else to do it is borne of necessity. I have limited dexterity in my right hand due to a ripped tendon and associated referred pain which means I'm unable to do a lot of fiddly things and tightening of various bolts. I will stand over and watch and get him to explain, though, so I can learn for the future when hopefully my gammy fingers heal.

I would suggest you read/watch the following articles and videos before you/your friend tackle the task.

https://www.parktool.com/blog/repair-he ... stallation

https://www.parktool.com/blog/repair-he ... -alignment

https://www.parktool.com/blog/repair-he ... adjustment

[Pete D]

Thanks for all your advice and links above, slowster. Very much appreciated.

[TheBomber]

Also, I presume any 160mm rotor with the right number of bolt fixings would do, as the ones on Wiggle all say they are for hydraulic disc brakes, but surely it’s just a piece of metal and so the fact they are cable operated discs won’t matter?

For completeness, in relation to this point, I’d stress that for BB5s you need a rotor with a bit of flex in it which typically means that it has been made from a single sheet of metal - like the models Slowster has suggested. Some rotors feature a much stiffer large central section that will not work well with brakes where only one of the pads moves (like BB5s). The one in the photo of a ‘flat mount’ brake up thread is an example of the type to avoid.