Brake dive

I ran across this youtube channel today and this video reminded me of the many claims on tacoworld that shock absorbers are important to brake dive. Even if you don't spend the time to understand all the physics in the video, take note that the author never mentiuons shock absorbers.

https://www.youtube.com/watch?v=HeclLbsRHbc

 

Dont have time to watch it, but lifting the front elimnated my brake dive.

 

Does he say springs are the thing? Cause that's what I learned while auto crossing.

 

Yes springs matter but other design decisions are about suspension geometry and weight distribution. If you had no springs at all, there would be no brake dive. That's not true of shocks.

On the flip side, if you have ever endo'ed a bicycle under heavy braking, you know there are still rotating forces to deal with despite the lack of a suspension.

 

If you increased your spring rate, then you likely also reduced your brake dive. Springs work against the force pushing the front end down. The downside of that solution is that you also tend to reduce other aspects of your ride quality, e.g. it gets stiff in the front. Engineering is about trade offs.

 

Yup, and thankfully Bilstein 5100's took care of that solution. best mod I've done to the truck (to eliminate brake dive).

 

Ok, but my point was that shocks have little or nor effect on brake dive.

 

It absolutely does matter. Source: am engineer.

Shocks resist movement. That’s why they are called shock absorbers. They absorb shock. Slamming on the brakes introduces shock.

 

If you are engineer, why are you not using engineering terminology, or at least not using it correctly?

 

You disagree that shock absorbers resist movement or something? Would you prefer we talk about bounce and jounce? Cause we aren’t going to get there for a while.

If your whole argument is “shocks don’t prevent dive, springs do”, it greatly oversimplifies. Dive is caused by the combination of suspension geometry, spring, and shock. In theory you could design an anti-dive suspension without a shock absorber. But it would be terrible in other ways. Also the design of the braking system itself can have an effect.

If you need to work within the existing geometry, you are going to need to work with the shock absorber FOR SURE.

While I’m not a suspension expert I do understand the basics. My specialty is in automatic transmissions and internal combustion engines.

 

Soft shocks will make the dive more noticeable. 5100s with a raised perch effectively increases pre-load which reduces dive.

Dive is just a byproduct of trucks, they are intended to haul weight, so more weight in the rear reduces nose dive in its own way.

 

You just may put a dent into "oem coils are 700+lb/in" statement. Can you explain from "lab" observation why 590 lb/in w/5100 dives less than oem low grade shocks with "700 lb" coil?

 

At the risk of repeating myself. The force a spring applies is proportional to the displacement. The force a shock absorber applies is proportional to the velocity (1st derivative). If the suspension has zero velocity, the shock absorber is not applying any force.

Therefore the steady state amount of suspension compression during braking is not affected by shock absorbers. Shock absorbers do however affect the transient behavior, ie the period of time before the suspension stops moving

I really don’t think I am disagreeing most of you. I think it’s just a difference in terminology.

More info here:

https://en.wikipedia.org/wiki/Harmonic_oscillator

Note there is no mention of shock absorbers in this video:

https://www.youtube.com/watch?v=HeclLbsRHbc

 

My 5100s 100% solved my brake dive. When first put them on, I was still on stock springs. The force causing the nose to dive must push the oil though the oil ports of the front shocks/struts, and with 5100s they are tune for a firmer ride, so it takes longer for that oil to transfer in the shock tube (the oil is either thicker, or ports are smaller, or the shocks tuning stack (that restricts oil flow) is stiffer.

Ultimately what is changing is time. It takes longer, with the same given force, for the oil to transfer, which gives more time for the vehicle to recover its stance, and rebalance itself, hence less brake dive.

The same is happening in the rear, where the shocks must extend to accommodate the brake dive. The rear 5100’s also have more restriction that the OEM shocks.

The firmer the shocks, the closer the vehicle is to having “locked out suspension”. Certainly there is no brake dive in a car with no suspension, none at all. So as we trend towards a hypothetical “solid plank with wheels” we get less body roll and less brake dive. If you could completely block your shock’s oil ports, this would be the affect. No dive at all, ever. This is the way some adjustable or electronically adjustable shocks work.


So if you wanna cure brake dive, and firm up your ride, get some 5100’s. They do this for a reasonable cost.

 

You're partly right, and partly wrong.
The video you posted is specific to anti-dive and anti-lift. It doesn't mention shocks because they are not part of those specific topics. They ARE however an important part of the whole bigger picture (controlling the dynamics of such).
When you first stab the brake: with lighter (or no) shocks, the front actually dives down lower than it would otherwise be for a moment, then bounces back up (oscillating) slightly before settling to a final position (equilibrium) during the braking event (this makes a vehicle feel "sloppy" during braking).
Stiffer shocks reduce (or eliminate) those oscillations by slowing the rate at which the suspension dives, leading to a much tighter feel to the vehicle's braking. No, shocks do not change the final amount the suspension has dived down to, but they do drastically change your perception of it because the dynamics of it are much more tightly controlled (and is why we often see such comments that better shocks help with brake dive).

Similar dynamics come into play for body roll also. Shocks do not change how much the body rolls, but they do change how fast the body reaches it's final roll amount (again giving a much tighter feel to the vehicle's handling).

 

We aren’t completely disagreeing. By transient behavior (physics/engineering term) I mean the behavior between the time you hit the brakes and the suspension stops moving. The steady state behavior is the time between that and the truck comes to a stop assuming constant steady braking that occurs during people’s daily driving.

I think I used static and dynamic behavior (the terms you used) above as well to mean similar things.

The shocks (aka dampers) have zero effect on the compression of the front suspension during the steady state period. That’s all geometry and springs.

So basically, unless you are jamming the brakes like you are in a race, the shocks aren’t doing much of anything. Any decently working shock will do during daily driving. There should be a noticeable oscillation as you mentioned.

Keep in mind too that OEM shocks, although not published, likely have very similar damping constants compared to stock shocks for the same reason they have similar extended and collapsed lengths. Where they more likely differ is their ability to maintain the damping constant under stress, eg dissipating heat and preventing oil cavitation while racing down a desert trail.

In more general terms, steady state is a state in which the system is no longer changing. A transient state is in which the system is transitioning between two steady states. A steady state is static. A transient state is dynamic.

 

I'm just going to say it.

Considering my previous truck ran 'sticky' torsion beams, and my other vehicle is a very tight sports car, I really love how my truck bounces around between acceleration and brake dives.

It makes a boring commute so much more visually dramatic. No one honks at the guy making his truck lurch 3" vertically on sport mode gear changes...

 

Yes, shocks (aka dampers) apply a force proportional to the velocity (aka speed) of the suspension in the opposite direction. The oil has viscosity (internal friction) and the faster it flows through the shim stack, the faster kinetic energy (movement) is converted to heat. So yes they will slow the rate at which the front suspension compresses after you hit the brakes. But they have no effect on how far the suspension compresses during a typical stop during daily driving because the suspension is not moving fast. On the other hand, springs and anti-dive geometry do affect the how far the suspension compresses during a typical stop during daily driving.

Seriously guys, how hard do you brake on the way to the mall? My front dips maybe 1/4" during a normal stop. It's almost imperceptible to me and my passengers.On the flip side, my dirt bikes had a great deal more brake dive because I road them very aggressively, but still brake dive was not an issue even talked about with my dirt bike buddies.

 

I have to apologize. There are two different online threads on brake dive i've been involved in recently and I got them mixed up. When I said, "At the risk of repeating myself", I thought I was replying to the other thread. oops.

 

The video is about anti-dive geometry, but yes springs are a poor man's way to attack dive, lift, squat, and roll. Without springs none of the above would happen, but you would lose your teeth.