Just curious as to why the front brakes carry the load of stopping power for a car?

Is this because the weight of the vehicle, once braking begins, shifts foward?

If the vehicle is static, meaning weight distribution remains equal, then would all four wheels provide equal braking?

Well... there are many factors.

There really isn't a such thing as completely 'static' weight, even if it was possible to have a perfect 50/50 weight balance, either when moving or stopping, then you would still be shifting weight in some form or another at some point. The weight is a factor, but it's not the primary one. Notice how on every car the master cylinder is up front, even on Fieros? That's for a reason. The front brakes are the first to receive the pressure, then it goes to the rear (it's really almost instantaneous, but that's the best analogy I can give you). It's yet another reason why you bleed brakes in reverse order - you start from the rear, which is the furthest, and work you way forward.

One of the primary reasons why you would not want brakes with more rear balance in them for street driving is simply because of ease of driving. If you were cornering, and then apply more brake for whatever reason, you'd spin out every time or seriously lose control that would take a good amount of skill to correct and perform frequently. This is why there is more forward brake bias in most every single street car, as it's easier to control. There is also a weight shift factor here, as in pretty much every car there is a 'push' under braking. If you had a lack of dominance from the front brakes, then your car would be acting against this force - not very safe. With that said, there are many forms of motorsports that do have vehicles which can shift brake bias, but you have to remember that these guys are pro drivers, that take into account other factors that they can control such as downforce and spring rates, that allow them to alter brake bias, even to the point of there being more rear bias than front.

I think you have the heart of it. Imagine if you had an arrow shaped pendulum hanging from your headliner. Traveling at a consistant speed, it will hang straight down because only gravity is affecting it. WHen you brake however, the pendulum will swing forward. The force vector goes from straight down, to down and forward due to the inertia of your car slowing. This loads the weight on the front of the car. This increases the friction of those tires, allowing the brakes to work harder without inducing a skid. Even with a perfect 50/50 front to rear, the effect I so poorly describe would still be present.

I'm certain there are people who can explain this more clearly than I, but I hope this helps get you started.

#2... weight shift

because most drivers are skittish & panicy
when the front locks up or slides - people natural reactions are usually all that is needed to come back into control
when the rear locks up or slides - it is much more frightening, and the car ends up heading in strange directions, and most folk dont know what to do to regain control

the main scenario is: freeway entrance/exit ramps. classic "cloverleaf" design. coming off the freeway, going to fast. natural intinct: slam on the brakes. if the rears lock up first, the back of the car is gonna come around, and now there is all kinds of panicing going on. anyone driectly behind them is gonna have undies to change. if the front locks up - the car has a weight shift to the front, which allows it to grab a little more, and maybe save the car - if not - it shoots straight off, and the guy behind him just says "cool - see that?!"

quote Originally posted by RotrexFiero: Just curious as to why the front brakes carry the load of stopping power for a car?

Got a bicycle with hand brakes?

Do some braking tests using just the front brakes and just the rear brakes.

Report what you discover.

I have a motorcycle and if remember it was like 70% of the stopping power comes from the front brakes.

But, with a motorcycle you can modulate, or you darn well better learn how to modulate the the front and rear brakes, or you will wreck quickly.

Static weight would be not necessarily the cars weight distribution at rest, but the weight as it is distributed at rest. Of course once the car is moving all this changes since with acceleration and braking the weight load will shift.

So its partially by design, for safety sake, and also just the physics as the increased weight from stopping is moved forward?

Dont some of the higher end cars have braking that responds to different varying speeds, traction, and weight loads?

It's not rocket science. Due to weight being shifted forward during a stop, the rear brakes would lock up if the same clamping force was used front and back. Having the rear tires skidding every time the brake pedal was touched would be counterproductive to stopping safely.

The front brakes only supply a majority of the stopping power if you are going forward. If you are going in reverse the rear brakes will supply the majority of the stopping power.

quote Originally posted by dratts: The front brakes only supply a majority of the stopping power if you are going forward. If you are going in reverse the rear brakes will supply the majority of the stopping power.

That is actually incorrect. Almost all factory braking systems have a fairly signifigant front brake bias, meaning more clamping force is applied to the fronts no matter which direction you are travelling. Unless you're using a modified/custom braking system, or perhaps an abs system, most of your braking comes from the fronts no matter what, it's just how the system is designed. The front bias is there to save the squirrels who drive over their heads and don't know what they're doing behind the wheel. It's not the absolute optimal braking setup, but it is easier to control and recover for novice drivers if they get into trouble

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88blackchopv8

I understand the Fiero has almost 50/50 bias, but still a bit more front. Anybody know how much?

Where is this regulated? Just in the distribution block or also in the bore sizes of the calipers?

The front/rear bias is controlled in the proportioning valve which is right below the master cylinder. Even a Fiero has a good bit of front bias from the factory, but I don't know what the percentages are. The easiest way to change the bias would be with an adjustable proportioning valve available from most speed shops.

I ride a gsxr and most of the braking is done with the front brake. All your inertia is going to the front of the bike down to the ground where you can get the most grip. If the rear brake is applied with the same amount of force, the back tire locks up and skids because all the weight has transferred to the front. Stopping with just the rear brake on a motorcycle will yield double or triple the braking distance compared to using just the front. You can't get enough weight on that rear tire to stop quick like you would when inertia loads up the front wheel when braking.

If a car had the same brake bias front and back, during a hard braking situation, I would think that it would act like a bike where the back tires will lock up from the weight transfer to the front making the rear end come around.

quote Originally posted by Fiero84Freak: The weight is a factor, but it's not the primary one. Notice how on every car the master cylinder is up front, even on Fieros? That's for a reason. The front brakes are the first to receive the pressure, then it goes to the rear (it's really almost instantaneous, but that's the best analogy I can give you). It's yet another reason why you bleed brakes in reverse order - you start from the rear, which is the furthest, and work you way forward. .

Either way you look at it, due to weight shifting the front tires in most cases support the most weight and can therefore provide the most stopping power. There are exceptions where you will achieve a 50/50 balance (or even rear bias) during hard braking and in those instances front and rear could provide the same amount of stopping power (and if you have rear bias, then the rear brakes would obviously be capable of providing more stopping power than the front brakes), but in most cases there is a front bias dialed in from the factory to avoid the rears locking up first (and with a safety margin at that in order to have the car behaving similarly regardless of road conditions. just imagine being used to having perfectly balanced brakes on a dry road and then having to brake hard on a wet road where the same car would simply spin around).
It's all physics and general engineering.

The position of the MC has nothing, absolutely nothing, to do with which wheel will provide the most stopping power. nothing. It's all about packaging and keeping complexity to a minimum. It would be impractical to place the MC behind the driver as you'd need extra linkages or additional cylinders to control the MC.
If you remove any valves from the system, you will notice that there is no way the front calipers will engage before the rear calipers. Actually, on most cars, the rears would engage first as the smaller caliper pistons will move quicker. On the fiero, they would engage almost simultaneously as the caliper pistons are almost equal in size. The length of the brake lines has nothing to do with this, only piston sizes and distance between pad and rotor.
Imagine 4 water-filled jars all connected by piping positioned below the waterline. when you add water to one jar, the level will rise simultaneously in all jars (unless of course the pipes are very thin, but let's think of them as being of a large diameter as the brake lines on cars aren't really small enough to limit the fluid flow). This is a simplified representation of how a hydraulic brake system works.

quote Originally posted by RotrexFiero: I have a motorcycle and if remember it was like 70% of the stopping power comes from the front brakes. But, with a motorcycle you can modulate, or you darn well better learn how to modulate the the front and rear brakes, or you will wreck quickly. Static weight would be not necessarily the cars weight distribution at rest, but the weight as it is distributed at rest. Of course once the car is moving all this changes since with acceleration and braking the weight load will shift. So its partially by design, for safety sake, and also just the physics as the increased weight from stopping is moved forward? Dont some of the higher end cars have braking that responds to different varying speeds, traction, and weight loads?

I rode for 40 years, including some time on superbikes. 70 percent, at least, up front on hard braking. Modulating was never an issue. If you are on the rear brake at all, you can lean on that front as much as you like. It never tossed me.

Well, I started to quote a few responses but decided to be less personal. I’ll just say that there is more nonsense in the thread than any I have read.

Weight is transferred to the front during braking because the center of gravity of the car is higher than the contact patch of the tires which. This is the same reason that it is transferred to the rear upon acceleration. The higher the center of gravity is, the greater the transfer.

More front brake bias is designed into cars for another reason too. Locked brakes are less effective than nearly locked brakes because once the tire is not rotating at road speed, the friction between the road and the tire is dynamic friction. As long as the tire is rotating at road speed the friction is static friction. Static friction is always greater than dynamic friction.
Without ABS, and with the bias to the front, the front wheels lock first and loose their grip so then the rears are doing the work but the car is stable as if an anchor were thrown out the back. If the rears locked first, the fronts are doing all of the work but the locked rear tires are free to slide in any direction, allowing the mass of the car to push forward to one side or the other and the car will spin. That’s if you were going straight or trying to anyway. Try that in a curve and, well, at least you know which way you’ll spin.
Cars with ABS are still designed to operate safely when the ABS fails so the front bias remains.

The master cylinder is up front because the brake pedal is up front and that is all.

http://www.miata.net/sport/...Weight-Transfer.html

Do some research, pick up a paper and pencil, and stop throwing around vague terms like "majority", and "stopping power".

Last I checked (84-87), Fieros' front and rear rotors and pads are basically the same, that is, they have the same power capability.

Braking force is something else...

quote Originally posted by Patrick: Got a bicycle with hand brakes? Do some braking tests using just the front brakes and just the rear brakes. Report what you discover.

This is what I thought of instantly. Since I was about 12 years old and got a bike with handbrakes I realized that the fronts are the way to go when stopping.

Although bicycles have a danger that cars don't have and that is if the front locks up most likely you are going to go over the handlebars. Don't ask me how I know that.

quote Originally posted by RotrexFiero: Is this because the weight of the vehicle, once braking begins, shifts foward?

Bingo. Yup, it's just that simple. Sure, there's more to it than that, but essentially that's the main reason.

Cars like the Fiero, mid-engined cars, rear-engined cars can have outstanding braking with all four wheels because the rear weight bias helps keep weight more evenly distributed during braking.
Take a modern FWD car. There's no weight on the rear wheels normally, and when you hit the brakes, it's even less. Heck, sport bikes can get so much braking from the front wheel that they lift the rear wheel completely off the ground.

So, if you want better cornering would it not make sense to shift the bias more toward the rear wheels. I'm thinking that if you are going into a corner and asking the front tires to slow the car and also redirect the weight of the car they would easily be overwhelmed. I think its called understeer. If you had more rear bias then you could acquire greater cornering. Of course you would sacrifice some straight line braking.

I guess this is what is being programmed into the higher end cars that have ABS?

Well maybe it is rocket science!!! At least from a engineer's perspective there is a lot of physics and thought that goes into braking and desiging a efficient braking system.

Look here:

http://www.stoptech.com/tec...raking%20Systems.pdf

The brakes are set up with enough front bias to keep the rears from locking before the fronts under ALL road conditions. They are not set up to give ultimate stopping power in dry conditions. if you're autocrossing or roadracing in dry conditions your braking performance could benefit from more rear brake. This would help in any kind of braking in dry conditions, straightline or otherwise. If you're only using the vehicle in dry conditions sure you could benefit from more rear brake. Even under wet conditions you could benefit from a little more rear brake as factory systems have a wide saftey margin built in to keep inexperienced drivers from spinning out every time it rains. Be careful, too much rear brake under wet conditions and you'll have a car that wants to come around and swap ends every time you brake.

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88blackchopv8

[This message has been edited by av8fiero (edited 10-02-2009).]

quote Originally posted by jscott1: Although bicycles have a danger that cars don't have and that is if the front locks up most likely you are going to go over the handlebars. Don't ask me how I know that.

That's what I pictured as soon as I read his post.

Simple physics.

For ever reaction there is a equal and oppisite reaction. You stop the weight goes forward you take off fast the weight goes to the back.

If you stopped with more real brake they would hop and often spin the car out since they are unloaded.

quote Originally posted by gem1138: Locked brakes are less effective than nearly locked brakes because once the tire is not rotating at road speed, the friction between the road and the tire is dynamic friction. As long as the tire is rotating at road speed the friction is static friction. Static friction is always greater than dynamic friction.

What he's saying here is that you want to keep the static friction between the tire and the pavement and the dynamic friction between the rotors and pads turning the braking force into heat dissipating into the rotors decellerating the vehicle in a controlled fashion. If you lock the rotors your static friction is at the pads locked to the rotors, and your dynamic friction is at your tire skidding across the pavement out of control into the cornfield or wherever. Brakes are all about turning energy into heat that is dissipated into the rotors. Once you lock your brakes, you're no longer turning that energy into heat at the rotor. that energy has to go somewhere and it goes into your tire generating heat where it contacts the pavement as it skids across it.

It's all about car control. The front bias is there to keep the car easier to control for most drivers. It's also there to keep the automakers from being sued every time little johnny or suzie gets behind the wheel, does something stupid, and tries a panic stop. If you want ultimate braking from your braking system more rear brake can be dialed in, but you have to be careful and not go too far. Once you lock the rear brakes and move the static friction from the tire/road to the rotor/pad, and the dynamic friction from the rotor/pad to the tire/road, handling becomes dicey and it can be difficult to keep up with where the rear of the car wants to go

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88blackchopv8

Just to add to the posted info, many Ford & Dodge service vans have what looks like an adjustable porportioning valve in addition to the standard one below the master cylinder.
This valve is mounted on the body above the axle housing and as the van is loaded the lever on the P-valve is moved by the proximity of the body getting closer to the axle which modifies the pressure to the rear brakes.

Just my 2 cents

Spoon

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Two yeast spent their entire life "about 2 days" discussing what the purpose of life could be and not once did they even come close to the fact that they were making champagne. Quoted by: Unknown

quote Originally posted by jscott1: 88 Fieros have identical rotors and pads, front and back, but nevertheless you want the fronts to do the majority of the work, for the reasons stated earlier.

I even run front brake calipers on the rear of my 88 since I do not need a parking brake on the track. Although, I run a full race pad on the front and a good street pad on the rear.

Everybody always says the caliper bore size on the rear is slightly smaller than the front. Could someone be specific?
By HOW MUCH?

Is it the same for all years?

What's the difference in bias needed on wet, vs. dry?

When I first got my Fiero I drove it for a few months before it was inspected and then only found out the rear calipers were seized. The mechanic told me I had no rear brakes, but honestly I never felt a change in braking but then again I was not doing any high performance driving.

The question is well and truely answered already. If it was rear you would lose control like a handbreak turn whilst front breaking adds traction.

quote Originally posted by Australian: The question is well and truely answered already. If it was rear you would lose control like a handbreak turn whilst front breaking adds traction.

Not that it would but it could very easy. Either way it will not stopp well.

quote Originally posted by Austrian Import: Everybody always says the caliper bore size on the rear is slightly smaller than the front. Could someone be specific? By HOW MUCH? Is it the same for all years? What's the difference in bias needed on wet, vs. dry?

Firstly, I'll state that the best braking system locks up all 4 wheels at exactly the same time under any conditions. If say the rear is not locking up but the front is, the rear is not contributing what it could be. Stopping distance is longer than it could be.

Electronic brake force distribution is the best, but you can get an acceptable system by fiddling around with prop vales and different calipers.

There is no "ideal" caliper bore size ratio. What you're looking for is a bore size ratio that in conjunction with the proportioning valve will stop the car adequately at various brake pressures. Simply put, I recommend to get all calipers with the same bore. That's why I would not get a Corvette upgrade using Corvette calipers all around. Corvettes simply have different bias requirements. I should have known not to use 85 Cadillac calipers all around on my LeBaron brake swap, but I read good things about it, and decided to go against my better judgment. Now I am correcting the issue by purchasing aftermarket undersize bore front calipers.

Your bias requirements will be different depending on year, because the CG changes from year to year.

Look at the 84-87 brakes for example. They were designed for 84 Fieros with the iron duke. I saw a video of I think it was Motorweek, and they tested the 60-0 stopping distance.

Motorweek also tested the 85 GT the following year. Stopping distance was longer than the 84 model. Guess what, it now had the V6 in the engine compartment, shifting the CG rearwards. I think the GT they tested was also an automatic. The brake hardware never changed though.

It's not only front/rear CG location that makes a difference. The amount of weight transfer depends on the height of the CG from the ground. A lowered car needs more rear bias than a stock height car.

On wet asphalt, you need more rear bias than on the dry, because on wet you have less deceleration. Weight transfer depends on the amount of deceleration. Notice though, that the pedal pressure is basically related to the deceleration. You press harder to stop quicker.

Basically, you want to size the calipers to provide good stopping in the rain, say at 0.4 g for example. Then, you use a proportioning valve to reduce rear bias in the dry, like 0.9 g. How does the proportioning valve know if it's raining? It doesn't. It is pressure sensitive however, as pressure increases, so does front bias. In the rain, you press on the pedal lightly, bias is determined solely by the caliper bore ratio (assuming everything else is equal). When you press harder on the pedal in the dry, the proportioning valve reduces rear brake bias.

Read the link on weight transfer that I posted earlier in the thread.

It's always better to do your own homework than trust some internet bozo after all. There are a lot of bozos, and you have to be able to sift through the fallacies which are commonly spread.

[This message has been edited by pmbrunelle (edited 10-03-2009).]

Good info, pmbrunelle, but keep in mind your idea of "best" may not be the same as someone else's.

More rear brake bias for wet pavement may help you get more even braking, but it will also make the rear more likely to lock up if you do try to brake too much.
It's all a compromise between ultimate stopping power in any given condition, and driver preference on how they want the car to perform. Most OEMs set up a car to understeer because for the average driver that's safer. Many autox racers prefer neutral to slight oversteer since that gives them more options at the limit.

Two identical cars in identical conditions may still need different brake bias settings due to driver preference and driving styles.

quote Originally posted by pmbrunelle: Read the link on weight transfer that I posted earlier in the thread. It's always better to do your own homework than trust some internet bozo after all. There are a lot of bozos, and you have to be able to sift through the fallacies which are commonly spread.

I did read it. Either he is educated beyond his capabilities or he needs to learn how to articulate his thoughts better. Do more research from more credible sources. His comparisons are ridiculous. The weight of the car is g or gee, The ground pushes up, lift forces like an airplane, spelling and punctuation errors from a physicist.
There is some validity to what he is trying to state but his method of doing it makes it lose all credibility.

quote Originally posted by SCCAFiero: I did read it. Either he is educated beyond his capabilities or he needs to learn how to articulate his thoughts better. Do more research from more credible sources. His comparisons are ridiculous. The weight of the car is g or gee, The ground pushes up, lift forces like an airplane, spelling and punctuation errors from a physicist. There is some validity to what he is trying to state but his method of doing it makes it lose all credibility.

I agree on that part. The points are valid though, albeit brief.

I'm a bit confused about pmbrunelle's post about more rear bias needed in the wet. I thought it was the other way around.
So when people with proportioning valves drive a car, they adjust the valve when it starts to rain then?
Would I just mark two notches for optimal bias?

Something like this:
Proportioning valve for Dummies thread:

quote Originally posted by Austrian Import: JEGS Adjustable Proportioning Valve and Distribution Block

quote Originally posted by pmbrunelle: On wet asphalt, you need more rear bias than on the dry, because on wet you have less deceleration. Weight transfer depends on the amount of deceleration. Notice though, that the pedal pressure is basically related to the deceleration. You press harder to stop quicker. Basically, you want to size the calipers to provide good stopping in the rain, say at 0.4 g for example. Then, you use a proportioning valve to reduce rear bias in the dry, like 0.9 g. How does the proportioning valve know if it's raining? It doesn't. It is pressure sensitive however, as pressure increases, so does front bias. In the rain, you press on the pedal lightly, bias is determined solely by the caliper bore ratio (assuming everything else is equal). When you press harder on the pedal in the dry, the proportioning valve reduces rear brake bias. It's always better to do your own homework than trust some internet bozo after all. There are a lot of bozos, and you have to be able to sift through the fallacies which are commonly spread.

What you're saying here is basically correct, but what I think we are talking about is a max or threshold braking effort. I don't think what anyone was talking about was light braking.Sure the factory prop valve is pressure sensitive and will increase front bias with more pressure applied. Simply put a factory braking system provides more front bias at threshold braking than is needed whether it's wet or dry. The extra front bias is there to keep the average joe from spinning out everytime he slams on the brakes whether it's wet or dry. It's the extra front bias saftey margin that we're talking about reducing [not eliminating it completely] to increase braking effectiveness in a threshold braking manuever whether it's wet or dry. Certainly if you reduce said front bias too much you will negatively affect braking performance and car control, but as long as you don't go too far you will increase braking effectiveness. Reducing said front bias wouldn't really affect braking in a light braking instance anyway so this comment is really irrelevant to this discussion.

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88blackchopv8

[This message has been edited by av8fiero (edited 10-03-2009).]

quote Originally posted by Austrian Import: I'm a bit confused about pmbrunelle's post about more rear bias needed in the wet. I thought it was the other way around. So when people with proportioning valves drive a car, they adjust the valve when it starts to rain then? Would I just mark two notches for optimal bias?

In the wet, less forward weight transfer occurs under threshold braking (the only kind of braking I'm discussing in case that wasn't clear). Less front bias is desired.

Sometimes analogies are good...In other words, imagine you're on a bicycle on ice, and you brake. You'll just lock up the wheels without weight transfer and slide and fall. Ride the same bicycle on the dry. You can practically send yourself over the handlebars. All your weight is being transferred to the front wheel. The rear does no work. In fact, with a bike, the best stopping is achieved using only the front brake, and zero rear brake, because the rear wheel might even lift off the ground.

A car's CG is too low to make it somersault over itself, but it still undergoes the same phenomenon of weight transfer.

On a rainy day, the proportioning valve might never be activated. The pressure needed to cycle it could be above the pressure needed to lock the wheels. So the bias is determined by the calipers in this case.

On a dry day, you have a lot of traction. You can press really hard without lockup. This is when the proportioning valve reduces rear pressure.

If you have an adjustable proportioning valve, you should adjust the bias to be optimal for a dry day. On a rainy day, you won't activate the valve, so it doesn't matter.

That's the point of a proportioning valve, so the bias is more or less acceptable regardless of the conditions, without having to turn it to different settings depending on the weather.

If you had a dual master cylinder setup with an adjustable balance bar, then yes, you would need to fiddle with it depending on the conditions. But that's completely different.

quote av8fiero: Reducing said front bias wouldn't really affect braking in a light braking instance anyway so this comment is really irrelevant to this discussion.

I have only been discussing threshold braking. In a low traction situation, threshold braking happens to be light braking.

quote Originally posted by pmbrunelle: I have only been discussing threshold braking. In a low traction situation, threshold braking happens to be light braking.

There is still extra, for lack of a better word, front bias built into a factory braking system. Yes you're correct in saying said bias would be reduced under wet conditions, but it's still there. If you depress the brake pedal very lightly and slowly sure you can prevent the proportioning valve from adding some of the front bias, but that wouldn't be threshold braking. Even in the rain or wet when you're threshold braking the prop valve is providing some extra front bias to keep the car stable for the average driver. It's there to prevent lawsuits against the manufacturer. You can reduce this extra front bias and as long as you don't go too far you will have a better braking, controllable car wet or dry. If you go too far you'll be chasing the tail of your car, if you get it just right, you'll be rewarded with improved braking and handling no matter what the conditions are. Finding the proverbial sweetspot, in wet or dry conditions, requires testing in a safe controlled location such as an autocross/roadcourse, or a skidpad. I'll state it again, if you reduce this extra front bias you will improve your braking performance. Do a little road racing or autocrossing and you'll find this to be true. Even with stock pads you'll increase your braking performance, and reduce your lap times with less front bias than what is provided by the factory proportioning valve

[This message has been edited by av8fiero (edited 10-05-2009).]

quote Originally posted by av8fiero: That is actually incorrect. Almost all factory braking systems have a fairly signifigant front brake bias, meaning more clamping force is applied to the fronts no matter which direction you are travelling. Unless you're using a modified/custom braking system, or perhaps an abs system, most of your braking comes from the fronts no matter what, it's just how the system is designed. The front bias is there to save the squirrels who drive over their heads and don't know what they're doing behind the wheel. It's not the absolute optimal braking setup, but it is easier to control and recover for novice drivers if they get into trouble I still believe that regardless of the bias the rear wheels will do most of the stopping when going in reverse, because along with there being more weight in the rear, weight transfer would also go to the rear wheels during reverse braking. How could the front wheels do the stopping when there is little weight on them?