Skip to main content ↑ ↓
Topic: Brake bias (Read 584 times) previous topic - next topic

Brake bias

Noob question - few weeks ago somebody posted that they had to replace the front rotors. And one member was surprised that they wore out. Question is do coaches rely that heavily on the rear brakes alone? Can the front/rear brake % be changed to even out the load? Seems backasswards to me. It would appear descending a steep grade more help from the front brakes would be beneficial, but maybe that's why the fronts are called steer tires on units w/o abs. Air brake actuator diaphragm size is probably a fixed setup, but are there boosters available?
Old Phart Phred, EIEIO
89 GV ored 36' #3405 300 hp cat 3208 ATAAC side radiator, mountain tamer exhaust brake

Re: Brake bias

Reply #1 –
Truck's chassis are built with brakes to match the weights of the axles. Before 1975 Big Rigs were not required to have front brakes. The theory was if the front brakes locked up you couldn't steer. With anti locks that no longer is an issue. The reason for the rear to do most of the braking on the motorhome is the rear axle weight is much more than the front. You wouldn't want to mess with what the manufacture's have designed, they are not race cars.
1999 40 ft. U-320 wtfe build 5563 Chuck & Lynda's "Rollin' Inn"  2030 watts solar
prev. mh's 71 GMC 5 yrs. 73 Pace Setter 1 yr. 78 Vogue 5 yrs 81 FTX 40ft all electric 18 yrs. 1996 Monaco Signature 3 yrs.
2014 Grand Cherokee Overland
Dream as if you will live forever. Live as if you will die today.  James Dean

Re: Brake bias

Reply #2 –
Let's back up a minute.  Brake/rotor failure is pretty rare AS LONG AS THE DRIVER KNOWS HOW TO DRIVE A HEAVY RIG AND THE ROUTINE MAINTENANCE IS PERFORMED.

The service brakes (brake pedal) is indeed to be used to stop the vehicle.  They are NOT designed to slow/maintain speed on long descents.  That is the purpose of the auxiliary brake system-- be it a transmission retarder, engine compression brake or exhaust brake.

Said another way, you need to choose a gear and "device" that will keep your speed in equilibrium on long descents without using the service brakes.  If you do need to use them, use them firmly enough and for long enough to allow you to drop to a lower gear that WILL keep speed in check.

Brake maintenance is the other cause of needing to replace brake components.  Using regular grease and/or greasing too often or not greasing at all can cause the brake pads to not retract, causing brake overheating and failure.  Same for slide pins.

So, if properly maintained and properly driven the brakes are good for way over 100,000 miles with no pad/rotor replacement.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Brake bias

Reply #3 –
Don't want to go off thread but that to me would be an advantage for the joystick retarder actuator over the rocker switch which
I have.I have to appy brakes to use the retarder.On long grades you slow and go as opposed to a steady retarder position.
96 U270 BUILD 4810
85 380SL
Drummonds TN.

Re: Brake bias

Reply #4 –
Just a warning re the retarder.  If you use it for long periods, such us down a long steep grade it will overheat the transmission and the engine will shut down leaving you with no power steering and whatever air you have left in the brakes!!  I found that out within a week of getting our coach as no-one at FT had told me that part???  That was several years ago and not to be repeated!!!
The advice so often given is to come down the grade in at least the same gear as you would need to go up it, and use the retarder, and brakes as and when needed to keep the speed down.  Here in the mountains that advice works well.

Speedbiord 1
2001 U320 Build #5865
Daihatsu Rocky Toad
VW Touareg
'82 F100 Stepside
Beech' Debonair

Re: Brake bias

Reply #5 –
On a Fortravel are the front and rear brake disks the same size, and part number?
John Duld
1995 U320C SE 40'

Re: Brake bias

Reply #6 –
Speed bird, that is not entirely true. By using the transmission, retarder and brakes, you can go down any hill, as long as you pay attention to the temps. It was a long learning curve for me but I can now go down any hill and keep retarder temp below 250 deg.
and eng RPM below 2000  without using the brakes.
Richard & Betty Bark & Keiko our Golden Doodle
2003 U320T 3820 PBDS
Build # 6215
MC # 16926
2016 Chevrolet Colorado 4X4 diesel

Re: Brake bias

Reply #7 –
Quote from: speedbird1 –
Just a warning re the retarder.  If you use it for long periods, such us down a long steep grade it will overheat the transmission and the engine will shut down
Speedbiord 1


This is the first time I have heard this.  My current coach has a exhaust brake, and I can use it all the way down a long steep descent to hold speed in check with no negative affect noted on dash gauges.  (EGT drops, coolant  and trans temp drops)

If using the transmission retarder to hold speed in check down a long steep descent is not viable, perhaps I don't want a Foretravel equipped with same after all?  There has to be more to this story?
Dan - Full timing since 2009
2003 U320 40' Tag 2 slide

Re: Brake bias

Reply #8 –
I don't doubt rbarks comments, but I was just giving a warning re the  continuous use of the retarder.  By all means, as I said, use a combination of all three methods to keep the speed under control and the temperatures as low as possible.
When I got this coach, I was not aware and had not been told, which is my purpose in getting the word out.  We don't all live or drive where there are long steep grades, but knowledge is a good thing.

Speedbird 1.
2001 U320 Build #5865
Daihatsu Rocky Toad
VW Touareg
'82 F100 Stepside
Beech' Debonair

Re: Brake bias

Reply #9 –
To reduce transmission temperature with retarder on:

Down arrow the transmission to a lower gear-- higher engine RPM turns the water pump faster and also offers a little more engine braking HP.

If temperature still climbing, downshift again-- takes less braking HP to descent a grade at, say 40 MPH than at 60 MPH.  You CAN reach a point where things are under control.  And if you can't, pull over and let the engine at high idle until the transmission temperature cools off.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Brake bias

Reply #10 –
We run in a lot of western states with long steep downgrades.

On current coach, with exhaust brake switch on, when I am zero throttle, the 6 speed Allison transmission preselects 4th gear (direct drive no overdrive), and downshifts when RPM safely allows (no RPM over speed).  At same time the exhaust brake kicks on.

I then can further downshift if desired/needed.  I use service brakes intermittently to keep rpm in safe range.

How does the Foretravel system work?  Any "automatic" preselect of transmission gear?  I have no problem manually selecting the appropriate gear myself, I just want to understand the system, and driving techniques that are best used for steep downgrades.

Dan - Full timing since 2009
2003 U320 40' Tag 2 slide

Re: Brake bias

Reply #11 –
The pre-select gear for engaging exhaust brake, engine compression brake or retarder can be programmed to any gear you want by any Allison dealer is about 15 minutes.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Brake bias

Reply #12 –
Quote from: wolfe10 –
The pre-select gear for engaging exhaust brake, engine compression brake or retarder can be programmed to any gear you want by any Allison dealer is about 15 minutes.

Our 1995 U320 is programmed to remain in 6th gear (or whatever the current gear happens to be) when the retarder is activated and it works well for me. I simply go ahead and manually select a lower gear when that's seems to be appropriate.

There are some long and steep downgrades on the interstates in Virginia and West Virginia and the retarder is a real blessing!
David and Carolyn Osborn
1995 U320C SE 40' Build 4726 Feb 1995
FMCA 147762
Motorcade 17186

Re: Brake bias

Reply #13 –
So when you have the retarder selected to on, does the system use a throttle position sensor to see zero throttle, preselect a downshift, then actually begin transmission braking? 

I have never driven a coach equipped with Trans retarder. 

I'll try the search function and look for a system description since we are off topic of OP post on  brake bias. 
Dan - Full timing since 2009
2003 U320 40' Tag 2 slide

Re: Brake bias

Reply #14 –
in my testing before the resonator and hydraulic fan motor controller change I intentionally ran the retarder down the tejon pass north of la from Lebec to Grapevine, ca at 55-60 and the unit on third or fourth setting to test its operation and to verify its warning system and how it may or may not affect the engine.

Did not manually downshift the engine at that time.

Vpms trans temp hit 250 as the road grade flattened out at the bottom and the warning light and buzzer came on as they should have.

Two miles of 65 mph in the flat ground at the bottom got the alarm and light to go off.

Dash gauge showed roughly the same trans temp. 

After seeing wolfe's posts about downshifting I do now follow that advise and have the engine at higher rpms than before if the grade is long enough and steep enough.

I was towing a 3,600 car so the combined weight was at roughly 36,000 pounds.

I need to retest the different setup I have now under similar conditions but occasional short times at 250 does not damage anything in the system I am aware of and my old info I remember said no load shedding on the retarder system until 300(?) degrees.

The engine/trans/retarder/radiator system as installed is very robust in capacity and has a lot of reserve in it.  Especially compared to other brands with higher weigh and/or rear radiators.

ALL owners have been extremely conservative(paranoid) about cooling issues since I started selling Rv's in 1984.  With good reason.

After driving hundreds of Rv's agressively in the western USA in summer I have demonstrated to customers countless times the coaches ability to safely absorb higher temps for short periods of time then some would feel might be ok.

That being said only do what you feel comfortable with.  But my point is that the systems are designed for much harder use and higher temps than we typically subject them to.


"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4