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Brake Bias and Weight Transfer

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7.6K views 22 replies 13 participants last post by  hvymtlmcn  
#1 ·
Recently purchased the the BOE brake kit with the tilton dual masters and balance bar so I've been reading about setting the bias. I have a few questions...

1. Does brake bias affect weight transfer?
2. Does suspension travel (appreciably) affect weight transfer?
3. Before the lockup threshold, does brake bias affect handling?

Details -

I could imagine braking mostly from the front or mostly from the back could influence how the center of gravity is pushed or pulled which could affect weight transfer, but ultimately it seems like it's mostly a function of center-of-gravity and G force, correct? Also I've read threads where people claim springs/shocks and suspension travel control how fast weight is transferred, but again... isn't that all about G forces? Go-karts have no suspension yet plenty of weight transfer!

With brake bias tuning, there seems to be an emphasis on tuning the rear for maximum G-force, and therefore all braking forces before lockup are indirectly front biased. This formula changes if you've got a proportioning valve or electronic brake distribution where you intentionally overbias the rear, but I believe proportioning valves are unnecessary unless there is some advantage to having even brake distribution before lockup. I know that pad wear would be more even, but would there be any handling advantage to well-distributed brakes that have not locked yet?
 
#2 ·
Here's a quick diagram of the forces on CG during braking. If you add the tire forces, they'll sum up to mass*g. The forward arrow depicts acceleration due to braking and adds load to the front tires by the same amount it reduces load on the back tires. Increasing braking force (due to better brake distribution, for instance) will mostly just exaggerate all this due to the larger decel rate. Add in rotation due to spring compliance, and you'll get additional forward transfer, but, like you noted with go karts, you still get transfer without compliance, too. This sketch also ignores aero effects which opens a whole can of worms. I am pretty sure aero is why I ground off my front splitter under braking from 100mph since I doubt it would've hit the ground without the help of air.

1276523
 
#3 ·
@cyow5 got it written up faster :) anyway...

  1. Does brake bias affect weight transfer? Yes, because of added deceleration (rears closer to threshold braking)
  2. Does suspension travel (appreciably) affect weight transfer? I'd say yes, with reasoning that vertical change in CG will change weight transfer
  3. Before the lockup threshold, does brake bias affect handling? I'd say yes because you'll get more rotation under trail-braking because the slip-angle on the rears is greater
 
#4 ·
I generally agree with the info above.

1. Marginally, yes. If you increase the braking deceleration from 1.0 to 1.1G, the amount of extra weight transferred is a function of CG height, wheel base and mass. Given an approximate CG height of 18", it's roughly a delta of 3% more weight on each front tire and 5% less weight on each rear tire. The braking torque bias that yields peak braking deceleration shifts from 57% front to 61% front, assuming ~23" front tire diameter and 24.7" rear tire diameter.

2. Marginally, yes. It's a cos(pitch rotation angle) change. There's probably a more notable transient impact from damper tuning than from steady state pitch of the vehicle. For example, cos(1deg) = .9998. Given the wheelbase, you just aren't going to experience a huge pitch angle change, even with soft springs. Even cos(5deg) = .996.

3. Yes, more significantly. If you're asking the rear tires to use all of their friction capacity to brake, you don't have much left over to handle lateral forces building during turn in. @thebuzzard is absolutely correct.
 
#6 ·
I'm old school. I set-up bias by doing threshold braking in a straight line and getting both front and rear to lock-up nearly simultaneously (front slight before the rears). Then, I add a bit more front bias so in a straight line they are locking consistently before the rears. My preference is to have stability under brakes in corner entry, especially true if you're trail braking (even a small amount). I'm not a big fan of brake induced oversteer on corner entry, it's unstable and keeps me off the throttle until the car has settled.

Just, my 2-cents and reflective of my driving habits and preference.

Cheers,
Kiyoshi
 
#7 ·
I’m with Khama. You can talk theory all day, but I adjust brake bias by feel- during threshold braking slightly favoring the fronts. Beyond that, everything effects everything else. That’s why suspension tuning is so tedious. And don’t forget, speed efficiency in the corner is all about brake release and that can only be controlled by your foot.


Sent from my iPhone using Tapatalk
 
#9 ·
I finally got my bias adjuster working well with a well setup car at COTA the other day and then it was raining, and I was able to adjust the TC and the bias as I drove around and the session dried up, it was so much fun!

In another mostly agreeing with the others answer

1. Bias only adjusts those things a little in that it allows the car to brake more or less.
2. Bias DOES affect the rotation into a corner as you will be changing the available grip in the rear. If you would normally turn in with the rear sliding a little, then you add that the rear is locked up 5% say (which would be close to ideal), now you are asking the rear to do "too much" and you induce rotation without "locking up".

That said if you are not driving the car at all in the "sliding" regime then it won't matter much.
 
#11 ·
@realkiran

When you get the brake bias cage installed, the first question is:

Where do I start?
Carroll Smiths Drive to Win explains in the Braking section 2-28 how to set initial bias. His method works great if you have cooperative help.

If you usually don't have help, like me. I use this method on my Formula Ford.

Put the car on jack stands with the wheels on.
Cut a stick long enough to place on the front edge of the driver's seat and hold itself gently on the brake pedal so the pads gently contact the rotors but the front wheels will still just barely turn by hand.

Then go to the rear wheels and try to turn them. If they turn easily, turn the bias knob/bar so until the rear wheels are as hard to turn as the fronts. If they won't turn at all, then move the bias towards the front until they do.

As Carroll Smith says, "you should be within one turn on the adjuster every time."

I do this check every time I work on the brakes.
 
#12 ·
Wow, okay, great answers, as always everyone!

1. So brake bias effects weight transfer insomuch as good brake bias will induce more weight transfer, and bad bias less. I was mainly concerned with debunking (my own) ideas that braking from the rear or braking from the front, within the limits of grips has any effect on how much weight ends up in the front or back. Seems like that is not the case unless we're approaching the limits of grip where a better distribution does induce more transfer. I'm happy with this answer.

Along those lines It seems like there might exist some kind of bias curve, where at lower G's you want more rear bias to induce more weight transfer but at higher G's you want less rear bias, but all within the same braking event! It would seem a proportioning valve could achieve this with smooth brake application, and perhaps this is "the point" of smooth brake application on a proportioned system, but it also seems a bit of chicken and egg - could a front biased system really achieve more braking G's than it would have otherwise allowed with lower G's (more static, rearward) weight distribution? A fascinating question I have trouble wrapping my head around!

2. I agree depending on how the suspension rotates the car there'd be some effect to the center of gravity, but I don't think there's a clear formula on this. An extreme example - abruptly jamming the front brakes on a bicycle causes an "endo" where the center of gravity is lifted :)

I poorly stated the question but I was mainly concerned with how softening or tightening a suspension could change how fast weight transfers. I believe this answer to be no. Although a softer suspension seems to "slow down" weight transfer, that is not actually the case. Please tell me if I'm wrong.

3. I had an instructor tell me to get better laptimes I need to start trail-braking, so understanding how bias affects handling is the meat and potatoes of what I'd like to grok here. My other questions were of the "eliminating variables" variety.

Bias must always be set frontward to whatever the ultimate level of grip is. But does, say, having it more front biased than necessary affect anything other than minumum braking distance? Similarly, does biasing forward a system for sticky rubber incur handling penalties at lower braking G's? And (as a point of understanding) could a proportioning valve which keeps the rear bias in a "sweet spot" actually offer handling benefits? Or perhaps I'm too focused on weight transfer here and I should be concerned with the F/R distribution of available grip (ie understeer/oversteer characteristics)?

^^^I'm trying to understand your answers from that perspective so maybe someone could provide an intuitive example? Like say, I'm getting understeer on trailbraking, which way do I move the dial and why. Or should I not touch the dial (rely on shock adjustment) and why not?

As an aside - I'm almost certainly going to set the balance bar by doing panic stops as mentioned, maybe dial it forward a touch for wiggle room. I'm wondering then if there's more fine tuning to be done to control trail-braking.
 
#13 ·
It's not that complicated. Too much front bias and the front end will push (understeer) when trail-braking. Too much rear bias and the rear will want to swap ends. Just think of the friction circle (google "friction circle"): any grip used for braking reduces the amount of grip available for cornering. Pushing is more benign than spinning, so better to start with too much front bias then gradually shift the bias rearward. With the bias right, trail-braking will help rotate, but not over rotate, the car during turn-in. You can definitely feel it when you get there. Also keep in mind that the "right" bias will vary with track conditions, corner geometry, tires, and suspension setup (to be safe, dial-in more front bias when things get slippery).