Let's talk about brake dive

[kuksul08]

BMW telelever GS riders not included

Brake dive is a normal dynamic of telescopic suspension. When you put on the brakes, weight transfers to the front end and compresses the suspension.

However, if you've ridden many bikes you will notice that some bikes seem to dive a less than others or at least they feel like they do.

What do you think contributes to the feeling and control of brake dive. Is it low-speed compression damping? Spring rate? One of the best telescopic type bikes I have felt under braking was the S1000XR. It didn't feel like it was slamming down and didn't upset the chassis geometry.

 

[stangmx13]

the largest contributing factors to the feel of brake dive on conventional forks are:

brake application - squeeze it faster, itll dive faster
compression damping - controls speed of dive
spring rate - affects how far itll dive
fork oil air gap - affects how far itll dive
ride height / the bike's CG height - affects how fast & how far itll dive and what the rest of the chassis feels like while doing so.

there was a video i saw years ago comparing 2 WSBK Duc riders on corner entry. one rider liked the forks to compress slowly and not as far. the other rider liked the forks to slam down to the bottom very quickly. they had two different setups w/ two diff riding styles, yet they were both within a few tenths of a second per lap. ill see if i can find it.

 

[stan23]

All bikes will brake dive, that's physics, but if you're talking about what makes bike a) feel less than bike b)

I always thought a big factor was rake, the more rake the bike has, the more 'brake dive' feel you get.

 

[afm199]

Lots of variables here. Most stock bikes are valved and sprung on the soft side. Even the better ones. They tend to have too much shock spring and not enough fork spring, encouraging dive. As if they were set up to ride two up at the rear and a 135 pound rider at the front. The latest high end sportbikes have gotten pretty good.

 

[stangmx13]

All bikes will brake dive, that's physics, but if you're talking about what makes bike a) feel less than bike b)

I always thought a big factor was rake, the more rake the bike has, the more 'brake dive' feel you get.

rake makes a diff, but its not huge. move your forks 20mm and the change in rake is tiny. the change in brake dive 'feel' is affected by that, but more so by moving the bike CG height a bit. but lets say u take your compression adjuster from full hard to full soft. the change in brake dive there will be huge in comparison to the fork height change.

 

[kuksul08]

Interesting.

What about preload? Might it reduce overall brake dive distance, but also increase harshness?

 

[afm199]

Interesting.

What about preload? Might it reduce overall brake dive distance, but also increase harshness?

Unless you add preload to the point that there is no sag at all, it doesn't change overall dive, it changes ride height.

The proper way to deal with dive is proper springs and proper valving. Adjusting ride height should be about sag. It can be used as a band aid, but that's what it is for anything other than sag. Sag does change handling.

 

[stangmx13]

when using a properly designed spring under normal conditions (some preload, no coil bind, etc), changing preload does NOT affect springrate. so preload has no direct effect on harshness or the speed of the suspension. as such, preload's effect on dive is only a side effect of changing ride height. if u changed your preload and offset the sag change with ride height, the brake dive would feel exactly the same.

Unless you add preload to the point that there is no sag at all, it doesn't change overall dive, it changes ride height.

The proper way to deal with dive is proper springs and proper valving. Adjusting ride height should be about sag. It can be used as a band aid, but that's what it is for anything other than sag. Sag does change handling.

typo? what do u mean?

 

[afm199]

when using a properly designed spring under normal conditions (some preload, no coil bind, etc), changing preload does NOT affect springrate. so preload has no direct effect on harshness or the speed of the suspension. as such, preload's effect on dive is only a side effect of changing ride height. if u changed your preload and offset the sag change with ride height, the brake dive would feel exactly the same.



typo? what do u mean?

Preload's primary function is to set sag, after finding the proper spring rate. It changes ride height. Using it to stop bottoming out, change ride height to affect handling, or anything else remains a kind of band aid. Find the sag and set it. Leave the preload alone afterward. Sure it can be used to change handling but it's better to change spring rate or valving or shock length than use preload as a means of changing handling. ( I will certainly change preload to try and change a handling issue, but if it's a ride height issue, I'll change shock length or fork placement later.)

 

[kuksul08]

How do I know if I have the correct fork spring rate? I've found it hard to measure sag with the stiction in a fork.

 

[fast4d]

I don't mind brake dive as long as I still have travel left. in a straight line it's all good. it gives the front tire more traction.

of course the bike will dive 'more' if has has longer travel forks like my SMT.

 

[Smash Allen]

How do I know if I have the correct fork spring rate? I've found it hard to measure sag with the stiction in a fork.

You measure and net out the stiction by pushing forks down and letting them come up, then lift the forks and let them drop. Difference is your stiction

 

[stan23]

rake makes a diff, but its not huge. move your forks 20mm and the change in rake is tiny. the change in brake dive 'feel' is affected by that, but more so by moving the bike CG height a bit. but lets say u take your compression adjuster from full hard to full soft. the change in brake dive there will be huge in comparison to the fork height change.

I always thought moving your forks up and down the triple is not really an effective way to change rake.

 

[stangmx13]

Preload's primary function is to set sag, after finding the proper spring rate. It changes ride height. Using it to stop bottoming out, change ride height to affect handling, or anything else remains a kind of band aid. Find the sag and set it. Leave the preload alone afterward. Sure it can be used to change handling but it's better to change spring rate or valving or shock length than use preload as a means of changing handling. ( I will certainly change preload to try and change a handling issue, but if it's a ride height issue, I'll change shock length or fork placement later.)

i firmly disagree. this style of tuning kinda makes the assumption that there is a "correct" sag number. that is not true at all. a range works and there are times to run more or less. changing preload also offers some other advantages that cant be had with springrate, damping, or ride-height adjustments.

changing sag and changing ride height arent EXACTLY the same. the change in proportion of available travel def affects the feel when topped out. this is very important for the shock on corner entry and for the front end on corner exit. the diff btw changing sag and ride heights in those situations is noticeable. its very hard to describe, but its there.

preload changes allow for a much small change than springrate. a 1-2 turn fork preload change to stay off the bottom is MUCH smaller than changing springrate from .925 to .95. to get anywhere close to the same fork position under braking w/ that spring change, id need to reduce preload w/ the stiffer springs. that would affect the dynamic fork position everywhere else on track, which might be a bad thing.

using damping to adjust chassis position under mostly-static loads doesnt work. damping mostly affects how FAST the forks dive, not how FAR they go. so once your damping is within range, no one should be using it to control something like bottoming. if u show up at a new track and its hard braking zones create a bottoming problem... add preload.

Sag is the most overhyped bullshit measurement ever on a motorcycle. sag is a result, not something to shoot for. there is no right number. I havent measured sag in 4yrs because i know my springrates and total preload. Sag is useful for the trackday tuners that work w/ 20 riders, have no idea whats in their bikes, and need the setting to be OK without getting feedback. but after that, after u actually know what is in the forks, it doesnt matter one bit. drop in the right springs w/ a reasonable total preload and test ride it.

 

[stangmx13]

How do I know if I have the correct fork spring rate? I've found it hard to measure sag with the stiction in a fork.

springrate is a preference just like all other settings. most riders and bikes will work just fine w/ a range of 3 springrates. for ex, i can ride my race bikes with .90, .925, or .95 fork springs and 90, 95, or 100 shock springs. each one has its advantages and disadvantages. the softer springs generally offer more mechanical grip. the stiffer ones generally support harder braking and can load the tire more.

i know those springrates work best for me because ive tried other things and found they didnt work as well. i also paid the right people to give me recommendations on springrates, ppl that know my specific bike(s) and know what has worked for prev riders.

I always thought moving your forks up and down the triple is not really an effective way to change rake.

its not. but unless uve got really fancy triple clamps, how else are u going to change it on your bike to compare brake dive

 

[afm199]

i firmly disagree. this style of tuning kinda makes the assumption that there is a "correct" sag number. that is not true at all. a range works and there are times to run more or less. changing preload also offers some other advantages that cant be had with springrate, damping, or ride-height adjustments.

changing sag and changing ride height arent EXACTLY the same. the change in proportion of available travel def affects the feel when topped out. this is very important for the shock on corner entry and for the front end on corner exit. the diff btw changing sag and ride heights in those situations is noticeable. its very hard to describe, but its there.

preload changes allow for a much small change than springrate. a 1-2 turn fork preload change to stay off the bottom is MUCH smaller than changing springrate from .925 to .95. to get anywhere close to the same fork position under braking w/ that spring change, id need to reduce preload w/ the stiffer springs. that would affect the dynamic fork position everywhere else on track, which might be a bad thing.

using damping to adjust chassis position under mostly-static loads doesnt work. damping mostly affects how FAST the forks dive, not how FAR they go. so once your damping is within range, no one should be using it to control something like bottoming. if u show up at a new track and its hard braking zones create a bottoming problem... add preload.

Sag is the most overhyped bullshit measurement ever on a motorcycle. sag is a result, not something to shoot for. there is no right number. I havent measured sag in 4yrs because i know my springrates and total preload. Sag is useful for the trackday tuners that work w/ 20 riders, have no idea whats in their bikes, and need the setting to be OK without getting feedback. but after that it doesnt matter one bit.

How dare you disagree with me, dammit.

I always use a sag number on my initial setup, usually 35-40 front and 25-30 rear (mm.) It's a baseline that you need to start from.

Changing sag and ride height are the same.

Yes, small changes in preload affect handling and are important. If it's 1-2 mm, I use it. If it's 15mm additional, I look at making other changes.

I disagree, adding a bit of compression damping to a fork that is close to bottoming out does retard it enough to lessen the bottomout. As does adding spring rate or preload.

Adding or removing fork oil is one of the most effective ways to deal with bottomout. If you've got correct springs and like the handling, adding 10 cc of fork oil / cartridge fluid lessens bottomout.

 

[aminalmutha]

I always thought moving your forks up and down the triple is not really an effective way to change rake.

Probably depends on the bike, but I have absolutely noticed a difference in the way a bike turns when playing with fork height in the triples.

As for brake dive, I've found the best solution for excessive brake dive is stiffer fork springs. I've turned a few bikes into unbelievable nose-divers to very planted with fork spring swaps. Many bikes have very soft fork springs and in time, they can get sacked out anyways.

What exactly is the correct spring stiffness obviously depends on a lot of factors.

 

[stangmx13]

How dare you disagree with me, dammit.

I always use a sag number on my initial setup, usually 35-40 front and 25-30 rear (mm.) It's a baseline that you need to start from.

Changing sag and ride height are the same.

Yes, small changes in preload affect handling and are important. If it's 1-2 mm, I use it. If it's 15mm additional, I look at making other changes.

I disagree, adding a bit of compression damping to a fork that is close to bottoming out does retard it enough to lessen the bottomout. As does adding spring rate or preload.

Adding or removing fork oil is one of the most effective ways to deal with bottomout. If you've got correct springs and like the handling, adding 10 cc of fork oil / cartridge fluid lessens bottomout.

why did u pick 25-30mm on the shock? how is that any different than my 95 spring w/ 10mm installed preload? its not. they are both arbitrary baselines that we learned from others. they both assume the bike is known (diff travels work better w/ diff sag ranges). they also tend to both produce the same results on our bikes.

oh, one other bullshit part about sag i forgot to mention... its based on an imprecise measurement that is difficult to reproduce. its measurement can also be affected dramatically by temperature. but that isnt true of my baseline of springrate & preload. u prob know me well enough from these posts that theres no way in hell im going to favor an imprecise measurement over a precise one

changing sag and ride height are the same thing for 90% of track riders out there. but both u and i know that we arent in that 90%. play with it. with your pace and your experience, u should be able to notice the difference. try it on a bike w/ long topout springs for an even more dramatic effect.

fork oil changes are a good way to make a small change to the forks effective springrate in between fork springs. like all fork changes, it comes down to dynamic fork position. if i want less travel at the bottom and almost the same travel in the middle, add fork oil. if i want less travel everywhere and am ok w/ less at the top, add preload. for me, just like my springrates, i have a range that works on each bike. i cant stray outside of a 20mm range as i absolutely hate the feel on the brakes otherwise.

 

[afm199]

Good post.

 

[kuksul08]

Based on my research, 25-30% of the travel is supposed to be a good amount of "static" sag. So for a sport bike this is around 30mm, and for a dirt bike it's around 90mm. And you need sag to let the tire follow the road surface when it enters a dip.

Then there is the "free" sag amount, which is there to basically keep you from running a massively wrong spring with enough preload in order to get that target 25-30%. This is sag for the bike under its own weight, and many people recommend around 10%.


So where I'm getting at is - on my bike the fork can feel pretty harsh at times over small square edged bumps in the pavement. There is also a good deal of brake dive when I initially put on the brakes, but then it settles pretty quickly. Is it possible that stiffer fork springs would reduce the amount of brake dive as well as provide a smoother ride, because it limits the overall motion of the fork? I have .7 springs and the fork has 185mm travel, currently running about 50mm sag with 2mm preload.