disc brakes on grubee GT2A frame. Do i need a specific rotor size?

[Franky oh]

Hi all, I ordered my grubee gt2a frame and I want to put disc brakes on front and rear. I have never owned a bike with them so i am wondering if thers a specific rotor size that will work. What are some of you using on your gt2a frames?

Here's my build:
ported zeda 80
upgraded Intake, velocity stack, Air filter
rejetted carb (0.80)
sick bike parts expansion chamber
36 tooth sprocket
(soon: upgraded head)
Im looking for a used suspension fork on ebay (rock shox, manitou, fox-$$$, or maybe suntour)
Mag wheel set off of Bicycle-engines.com
bars, seat, seat post, pedals and sprocket, bottom and bracket will come from craccked huffy cranbrook frame hopefully.

 

[Will'smotobikes19]

203mm discs should be good they make 160mm 180mm and 203mm I have the 180mm. They make rock shox knockoffs that are like $70 That's what I have on my bike its a 1 inch threadless. You'd need a 1 1/8 fork. Also you could replace those wheel bearings with good ones after a good bit of miles or right from the start. SKF or Nachi are good brands and will last longer than the chinese if your'e planning on going on long rides/ commutes.

 

[Franky oh]

203mm discs should be good they make 160mm 180mm and 203mm I have the 180mm. They make rock shox knockoffs that are like $70 That's what I have on my bike its a 1 inch threadless. You'd need a 1 1/8 fork. Also you could replace those wheel bearings with good ones after a good bit of miles or right from the start. SKF or Nachi are good brands and will last longer than the chinese if your'e planning on going on long rides/ commutes.

how does it work with different sized rotors? In my mind, the calipers can only go so far from the hub (and so close) because of where they are mounted. Is there an adjustment with the calipers that allows the use of a larger diameter rotor?

 

[FurryOnTheInside]

how does it work with different sized rotors? In my mind, the calipers can only go so far from the hub (and so close) because of where they are mounted. Is there an adjustment with the calipers that allows the use of a larger diameter rotor?

I think (I don't have a gt2 frame) the caliper mounts are international standard (I.S.) on that frame. Most newer disc brakes are post mount (P.M.) so you'll need a PM-IS adapter for newer calipers which will be labelled with an appropriate size to match whatever rotor diameter you choose, or if you're getting calipers that are I.S. then you'll need the older style adapter for I.S. to I.S., IS-IS180R or IS-IS203R (sometimes labeled as the size increase such as "IS plus 20" or "IS plus 40" (even though it's 203).
This is assuming that the GT2 doesn't need spacing outboard (to the left of standard position) of course. If it does need to be spaced outboard then you can probably bolt the adapter to the outside of the I.S. mount instead of the inside.
It should be noted that the front I.S. mount is normally 20mm further from the axle than is the rear I.S. mount (but who knows on a GT2 frame, I don't have one). So an adapter is labelled with an F or an R, like this: PM-IS 203R, but plus 40 is just plus 40 from whatever the standard was on the frame or fork. Usually it's 160mm rear and 180 front these days.

Confused yet? Sorry. Just look at the adapters on ebay and you'll get the jist.

What brakes are you getting? The Avid BB7 are very popular here as they are a very decent cable operated brake. Cable operated is popular because many like to join both front and rear brakes to a single lever on the right hand side (much to my disapproval lol!).
The Avid BB7 is a P.M. caliper so you would need a PM-IS 203R adapter for most frames to be able to run a 203mm rotor which should be big enough to keep the caliper clear of a 36T sprocket.

 

[FurryOnTheInside]

Im looking for a used suspension fork on ebay (rock shox, manitou, fox-$$$, or maybe suntour)

Why no classic Marzocchi? Marzocchi is made by SR Suntour in Taiwan since 2006, and are just an overpriced Suntour fork lol, but prior to 2006 were made in Italy and many Marzocchi Bomber models have an open oil bath damping system which means that the weight is now so unfashionable that these forks are cheap, and all that oil is used both in the damping and also it lubricates the bushes and seals. This huge oil volume and circulation of the open oil bath system is just what we need in my opinion. It's a motorcycle fork for a bicycle. The oil is a heat sink and it transfers heat and keeps the fork cooled and the seals lubricated when the bike is ridden hard for prolonged periods of time which is what we do and mountain bikers don't.


Very early Marzocchi Bombers actually pre-date the I.S. disc caliper mount so try to avoid any from 1996/97

 

[Franky oh]

Why no classic Marzocchi? Marzocchi is made by SR Suntour in Taiwan since 2006, and are just an overpriced Suntour fork lol, but prior to 2006 were made in Italy and many Marzocchi Bomber models have an open oil bath damping system which means that the weight is now so unfashionable that these forks are cheap, and all that oil is used both in the damping and also it lubricates the bushes and seals. This huge oil volume and circulation of the open oil bath system is just what we need in my opinion. It's a motorcycle fork for a bicycle. The oil is a heat sink and it transfers heat and keeps the fork cooled and the seals lubricated when the bike is ridden hard for prolonged periods of time which is what we do and mountain bikers don't.
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Very early Marzocchi Bombers actually pre-date the I.S. disc caliper mount so try to avoid any from 1996/97

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[FurryOnTheInside]

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Figure out what steerer length you need, as used forks will almost certainly have been cut down by the original owner.
The GT2 frame uses a semi integrated headset. I don't think it comes with it already fitted. So you'll need to get that and find out what your stack height is and guess what length of steerer above the headset you'll want, add stem height and then try to find a fork that hadn't been cut way too short lol. It might be that you have to wait for the right one to come up for sale, but I believe it is worth it. There's nothing like the "plush" feel of a OOB Marzocchi Bomber.