REAR BRAKE ROTORS

[patrick flowers]

>
> Could someone explain why only 11 inch rotors are used on the back wheels,
> when there are 13" available? It seems to me if you are going to install a
> new braking system on the rear wheels 13" rotors could have been used and 80
> mm calipers.
> The larger the rotors and calipers the more surface areas and leverage.
>
> I'd love to have discs on mine, even 11" ones, but I just wanted to throw out
> that question.
>
> Scott Nehoda Adohen

Scott,

That's what Leigh Harrison uses on his disk brake conversion - on the
front bogie, which does the majority(some say all) of the rear braking.
To accomplish this he makes a spacer to backspace the rotor away from
the wheel. Otherwise the caliper doesn't fit inside the wheel. Check
the pic's on my site from the Marion rally. I've got a couple shots of
his setup there. The braking his setup provides is awesome and the
quality is first rate, and it's priced to match - but then, you get what
you pay for, eh?

Patrick
- --
Patrick Flowers
Mailtoatri63

The GMC Motorhome Page
http://www.gmcmotorhome.com

 

[thomas g. warner]

Scott I would love to have disks on the back also but am not willing to
convert unless all of the components are off the shelf and readily available
for replacement. The best brakes in the world are worthless if something
goes wrong a long way from home and parts are not available. Looks like
someting I will research this winter during my rebuild.
]
Just my paranoia about breaking down more than 10 feet from my garage.

>
>Could someone explain why only 11 inch rotors are used on the back wheels,
>when there are 13" available? It seems to me if you are going to install a
>new braking system on the rear wheels 13" rotors could have been used and 80
>mm calipers.
>The larger the rotors and calipers the more surface areas and leverage.
>
>I'd love to have discs on mine, even 11" ones, but I just wanted to throw out
>that question.
>
>Scott Nehoda Adohen
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach

 

[donald w. miller]

Hi Scott,

Here is a different slant from a weight fanatic.

If 11" disks easily lock up a rear wheel, 13" disks add unnecessary weight.

Lower unsprung weight is an excellent reason to add either rear disk system.

I heard the 13" disk system reduces unsprung weight by about 14 lbs.. per
wheel.

An 11" disk would shave off a few more pounds where it is important to ride,
handling, shock life, etc.

Anyone know how rear disks affect brake system balance in a panic stop ?

Wouldn't a properly proportioned system lock all six wheels up about the
same time ?

Don Miller
Shenandoah Valley of Virginia

- -------------------------
>
>Date: Fri, 11 Dec 1998 00:00:06 EST
>From: Adohen
>Subject: GMC: REAR BRAKE ROTORS
>
>Could someone explain why only 11 inch rotors are used on the back wheels,
>when there are 13" available? It seems to me if you are going to install a
>new braking system on the rear wheels 13" rotors could have been used and
80
>mm calipers.
>The larger the rotors and calipers the more surface areas and leverage.
>
>I'd love to have discs on mine, even 11" ones, but I just wanted to throw
out
>that question.
>
>Scott Nehoda Adohen
>

 

[patrick flowers]

>
> If 11" disks easily lock up a rear wheel, 13" disks add unnecessary weight.

The larger rotors will give you more swept area and more heat capacity.
Keep in mind that the idea is not to "lock up" the wheels at all. Once
the brakes lock you're along for the ride.

> Lower unsprung weight is an excellent reason to add either rear disk system.

Yep - as well as simplicity and the ability to better shed heat(less
likely to fade).

> Anyone know how rear disks affect brake system balance in a panic stop ?
>
> Wouldn't a properly proportioned system lock all six wheels up about the
> same time ?

Equal braking force on all six wheels would be the ideal, but on a
vehicle like the GMC it would be nearly impossible to obtain. The
combination valve limits front brake action until the rear line pressure
is up to around 100psi(IIRC) and weight transfer on the bogies means
that the front bogie does the lions share of the braking. Trying to
achieve that balance with mechanical components alone would be a
nightmare, and then it would only be good for one specific weight
loading/bias. To acheive it under all conditions would require a 6
wheel electronic ABS system.

My opinions - worth the price
Patrick
- --
Patrick Flowers
Mailtoatri63

The GMC Motorhome Page
http://www.gmcmotorhome.com

 

[thomas g. warner]

Don just get an adjustable proportioning valve and adjust it to the coach.
they are readily availble and cheap. Hot rodders use them all the time.

>Hi Scott,
>
>Here is a different slant from a weight fanatic.
>
>If 11" disks easily lock up a rear wheel, 13" disks add unnecessary weight.
>
>Lower unsprung weight is an excellent reason to add either rear disk system.
>
>I heard the 13" disk system reduces unsprung weight by about 14 lbs.. per
>wheel.
>
>An 11" disk would shave off a few more pounds where it is important to ride,
>handling, shock life, etc.
>
>Anyone know how rear disks affect brake system balance in a panic stop ?
>
>Wouldn't a properly proportioned system lock all six wheels up about the
>same time ?
>
>Don Miller
>Shenandoah Valley of Virginia
>
>
>
>-------------------------
>>
>>Date: Fri, 11 Dec 1998 00:00:06 EST
>>From: Adohen
>>Subject: GMC: REAR BRAKE ROTORS
>>
>>Could someone explain why only 11 inch rotors are used on the back wheels,
>>when there are 13" available? It seems to me if you are going to install a
>>new braking system on the rear wheels 13" rotors could have been used and
>80
>>mm calipers.
>>The larger the rotors and calipers the more surface areas and leverage.
>>
>>I'd love to have discs on mine, even 11" ones, but I just wanted to throw
>out
>>that question.
>>
>>Scott Nehoda Adohen
>>
>
>
>
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach

 

[thomas g. warner]

Or double proportioning adjustable valves!

>>
>> If 11" disks easily lock up a rear wheel, 13" disks add unnecessary weight.
>
>The larger rotors will give you more swept area and more heat capacity.
>Keep in mind that the idea is not to "lock up" the wheels at all. Once
>the brakes lock you're along for the ride.
>
>> Lower unsprung weight is an excellent reason to add either rear disk system.
>
>Yep - as well as simplicity and the ability to better shed heat(less
>likely to fade).
>
>> Anyone know how rear disks affect brake system balance in a panic stop ?
>>
>> Wouldn't a properly proportioned system lock all six wheels up about the
>> same time ?
>
>Equal braking force on all six wheels would be the ideal, but on a
>vehicle like the GMC it would be nearly impossible to obtain. The
>combination valve limits front brake action until the rear line pressure
>is up to around 100psi(IIRC) and weight transfer on the bogies means
>that the front bogie does the lions share of the braking. Trying to
>achieve that balance with mechanical components alone would be a
>nightmare, and then it would only be good for one specific weight
>loading/bias. To acheive it under all conditions would require a 6
>wheel electronic ABS system.
>
>My opinions - worth the price
>Patrick
>--
>Patrick Flowers
>Mailtoatri63
>
>The GMC Motorhome Page
>http://www.gmcmotorhome.com
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach