[dexterq20]

It seems that 8 inch rotors are the most common size for rear brakes on DH bikes, but are they really needed? I currently have Hope brakes, and Hope doesn't even make an 8 inch rear rotor; 7 inches is as big as they get in back.

My thinking on this is that 8 inch rotors only increase your tendency to skid the rear wheel. When braking, the majority of your weight (and traction) is on the front wheel, making the front brake the most useful/powerful. Since the rear wheel has less weight on it (and less traction) during braking, it becomes easier to lock up. It seems like a 7 inch rear rotor would be more effective because it would be more difficult to lock up the rear brake, effectively keeping the wheel rolling and giving the rider more control.

So my ultimate question is: why go with 8 inch rotors instead of 7 inch rotors in the rear?

 

[seismic]

I dont think it matters....the question may be if you feel you ought to go 8"....if not, then dont....

 

[Red Bull]

I for one like being able to lock up my rear brake. It makes throwing the rear end around in steep sections and corners so much easier.

 

[downhill66]

not nessicary but very usfull

 

[jon-boy]

For a long time if you ran Hayes it was 8" or 6". I ran that configuration for a while on my hardtail. Then I had Hopes with the setup that you mentioned, 203mm front, 185mm rear (8 and 7"). Now I'll be running 8" f&r again (Hayes) but I see that they are offering a 7" disc now (finally), so maybe you'll see more and more people running 7" rear disc's, now that there's an option.

 

[skinny mike]

well i have noticed that a larger rotor also helps with dissapating heat, which may or may not be a good thing depending on how long the runs that you typically do are and how often you use the brakes.

 

[Cave Dweller]

well i have noticed that a larger rotor also helps with dissapating heat, which may or may not be a good thing depending on how long the runs that you typically do are and how often you use the brakes.

Yep.

I tired running a 6inch on the rear of my turner for a while just to see if there was a differance. As far as braking power it was about the same.

However, i ended up scortching the rotor, it went black, and the pads glazed as well. Too much heat build up. Larger rotors are better for DH.

 

[RaID]

Well not too sure about 7" rear discs.

But I was really suprised last year when my 6" disc rear brake over heated on a second run at whistler. I didnt have any problems with the same brakes with an 8" rear disc. As much as you dont really need the extra braking power of the 8" rotor it definately helps with heat dissipation.

 

[Msisle Dad]

DH yes, MTX and DS no

 

[joelsman]

I also like having the same size rotors so i can have 1 spare, not 2.

 

[haromtnbiker]

I have a 210mm front rotor and a 190mm rear rotor with my Magura Liouse Fr brakes. I dont really care about rotor size much. I just think small rotors look bad on dh bikes.

 

[bballe336]

8 inch is neccessary for DH. It provides lots more power and better heat dissipation. I feel that a 6 inch disk (with hayes) is very underpowered. And 6 inch rotors over heat very quickly which leads to a fade in power. I say it's 8 inch rotors in the back all the way.

 

[MOTODH]

Just curious-
Would 6 inch mean more modulation but higher temperature and 8 inch mean more power and better heat dissipation?

 

[dhbuilder]

better heat dissapation for brakes that are just as strong at the end of a smokin run as at the beginning.

 

[afromandookie]

8 inch is neccessary for DH. It provides lots more power and better heat dissipation. I feel that a 6 inch disk (with hayes) is very underpowered. And 6 inch rotors over heat very quickly which leads to a fade in power. I say it's 8 inch rotors in the back all the way.

Totally true because in long runs (Whistler is a prime example), even with 8" rotors your brakes get really hot so it is necessary to run 8" rotors in the rear. But if you’re not doing long runs or DH riding you won’t need them to be that big in the rear, plus you will save a little weight.

 

[Daver]

Yep.

I tired running a 6inch on the rear of my turner for a while just to see if there was a differance. As far as braking power it was about the same.

However, i ended up scortching the rotor, it went black, and the pads glazed as well. Too much heat build up. Larger rotors are better for DH.

Yep, did the exact same thing on my 8" juicy at thredbo. The pads were pretty shot anyway. 8" on the rear just feels a lot more comfortable on a your hands on long downhills anyway, the 6" ones on my 4x bike seem to change to quickly as the heat builds up faster.

 

[Mike Buell]

Bigger Rotors should give your pads a longer life too, since you wont be overheating them.

 

[T3h1337r]

I think it is nessessary with some calipers, and not with others. All depends on braking efficiency. Hopes and juicy 7s run alright with 6's. But if your a mechanical nut like me (God i hate hydros. The constant yearly fix, bleed, seals going, ripping my bf goodrich line off on a tree and having dot fluid spray in my face) then run 8's. I have 10inch in front and 8 in the rear. Dont ask where I got the 10 kuz I have no clue who makes a 10. I made it myself in the machine shop. I felt it was needed as my brakes were dialed in but still worthless at high speeds. They heated bad also and warped my 6inch rotors. Id go with 8's mandatory for mechanical and 6,7,8 for hydros. (All depends on which hydros tho) Alot of hayes need 8's because older models heat up etc, Some can get away with 6's.

I just miss the days of the v brakes. Noone worried about brakes or stopping, you just ran whatever over, and prayed you didnt end up in the basket of a chopper being taken to the local er.

 

[llirttoc]

I weight 195 and have warped 2 6" rear hayes rotors on my bullit. the first time going fast down dirt road switch backs at the Wild 100 in WVA. the second time doing DH at Snow Shoe MT. not so fast but just on the breaks all day, and yes the rear got so hot it was smoaking. BTW have an 8" on the front had no isues.
hope this helps.

Later
llirttoc

 

[DoubleDown]

I have been running a 7" rear rotor with my Juicy's for 2 years now. I have been to Whistler with this set-up and also routinely do runs at Keystone (10 minute runs while riding with your buddies) and have never over-heated it or burned a rotor.

For me, there is no reason for an 8" rotor in the back. I didn't notice a huge difference in stopping power between the 8" and 7" set-up, so why not use the smaller, lighter one.

 

[DHS]

Hope doesn't even make an 8 inch rear rotor; 7 inches is as big as they get in back.

Are you sure about that?

i have the mono6 Ti's and run 8" front and rear. hope even makes a 9" and 10" rotors. yes going from 7" to 8" makes a difference

 

[bballe336]

I think it is nessessary with some calipers, and not with others. All depends on braking efficiency. Hopes and juicy 7s run alright with 6's. But if your a mechanical nut like me (God i hate hydros. The constant yearly fix, bleed, seals going, ripping my bf goodrich line off on a tree and having dot fluid spray in my face) then run 8's. I have 10inch in front and 8 in the rear. Dont ask where I got the 10 kuz I have no clue who makes a 10. I made it myself in the machine shop. I felt it was needed as my brakes were dialed in but still worthless at high speeds. They heated bad also and warped my 6inch rotors. Id go with 8's mandatory for mechanical and 6,7,8 for hydros. (All depends on which hydros tho) Alot of hayes need 8's because older models heat up etc, Some can get away with 6's.

I just miss the days of the v brakes. Noone worried about brakes or stopping, you just ran whatever over, and prayed you didnt end up in the basket of a chopper being taken to the local er.

Welcome to the future! Here in the future we choose to use hydrualic disk brakes so our hands don't fall off. Mechanical disk brakes are wothless in any DH application.