If you have, or if you were to have, a dirtjumping-specific mountain bike (24 or 26 inch wheels here), would you run a front brake?
Do you run a front brake on your DJ'er?
- Thread starter Shibby
- Start date
i always run a front since i commute on my biek and its scary with just a rear brake, well at the mo i've not got any brake since my c2 striped the thread and one of the pads on my dh4 fell to bits (brakeing material separated from the metal backing which isn't a 1st)
priss
priss
i run a front brake on my Dj rig(specialized p3) but i use it for everything, not just DJ, although i find that you dont usually need a front brake if your just riding urban and DJ
I will always run a front brake, even given the weight penalty, and this is why.
On a 20" wheel it's effect is marginal, but one thing to consider when using larger diam. wheels (24-26) is the gyroscopic effect, and rotational leverage that comes from the spinning front wheel.
When you can brake the front wheel you are able to control this effect, reducing it, or translating it into a motion that is desireable.
I will explain it this way (hopefully a simple expl.!!)
If you are in the air, and your front wheel is spinning, it's rotational inertia (and the corresponding gyroscopic effect it produces) resists any changes in direction, meaning that if you were to attempt X-ups or bar spins (possible with pots mod), it would resist the spin, requiring more force to start it moving,and slowing the speed at which it rotates when it is moving..if you were to brake the wheel at take off, it would not have this effect, and would spin quite easily...to test this easily yourself, take your front wheel off, hold the axle in your hands and have a friend spin it, now while it is spinning try to turn it, you will see that it resists it, and infact it tries to turn 90deg. to the direction you tried to turn it (gyroscopic precession).
This does not mean that every time you jump you want to stop your wheel, if you are trying to make the bike turn relative to the front wheel (tailwhip, lookback) this resistance is beneficial.
but there is another benefit to stopping your front wheel in the air.
if you are in the air, and your attitude is nose high, and your are having difficulty pushing the nose back down to meet the tranny of the landing, (in other words you will land manualing), applying the front brake will leverage the frame upwards relative to it..(the same will happen with rear brake applications, but instead of frame leveraging upwards, the nose leverages down, same net effect)..
You can observe this phenomenon in action whenever you watch the X-games high jumps...watch tommy clowers immediately after his front wheel reaches over the bar, he grabs the front brake, pitching the rear of the bike upwards and over the high bar, that is why he can go so high, he understands the physics behind it.
On a 20" wheel it's effect is marginal, but one thing to consider when using larger diam. wheels (24-26) is the gyroscopic effect, and rotational leverage that comes from the spinning front wheel.
When you can brake the front wheel you are able to control this effect, reducing it, or translating it into a motion that is desireable.
I will explain it this way (hopefully a simple expl.!!)
If you are in the air, and your front wheel is spinning, it's rotational inertia (and the corresponding gyroscopic effect it produces) resists any changes in direction, meaning that if you were to attempt X-ups or bar spins (possible with pots mod), it would resist the spin, requiring more force to start it moving,and slowing the speed at which it rotates when it is moving..if you were to brake the wheel at take off, it would not have this effect, and would spin quite easily...to test this easily yourself, take your front wheel off, hold the axle in your hands and have a friend spin it, now while it is spinning try to turn it, you will see that it resists it, and infact it tries to turn 90deg. to the direction you tried to turn it (gyroscopic precession).
This does not mean that every time you jump you want to stop your wheel, if you are trying to make the bike turn relative to the front wheel (tailwhip, lookback) this resistance is beneficial.
but there is another benefit to stopping your front wheel in the air.
if you are in the air, and your attitude is nose high, and your are having difficulty pushing the nose back down to meet the tranny of the landing, (in other words you will land manualing), applying the front brake will leverage the frame upwards relative to it..(the same will happen with rear brake applications, but instead of frame leveraging upwards, the nose leverages down, same net effect)..
You can observe this phenomenon in action whenever you watch the X-games high jumps...watch tommy clowers immediately after his front wheel reaches over the bar, he grabs the front brake, pitching the rear of the bike upwards and over the high bar, that is why he can go so high, he understands the physics behind it.
okay, i getcha. we all have our reasons for running certain things, or not.
this question came into my mind after a few consecutive weekends of DJing... i found that the only place where i was using the front brake (when i was going to case anyway; i'm weird like that, i'll pull the brakes) was not necessary, and my P.3 is really only used for street and jumping.
so i took my front disc off and put it out back (avid mech) and did away with the hayes mech that was back there.
after a week of street on it, i can say:
-having one brake puts a lot more force on the brake - it's the only one you've got! loose hub, loose rotor, quickly wearing tire, etc
-it's (now) easier to do balancing moves (wheelie, manual) because i'm keeping more fingers on the bars and i'm not stopping the front wheel (rotational inertia again, plus gyroscopic procession - helpful in balance)
-i go for stuff cause i can't stop at the last minute (big plus for me - i've got more skill than ball, but my skills aren't that great anyway)
-still need to really go DJing on it, but so far, i'm loving it. not to mention the fact that i've subtracted a lever, cable, caliper, and rotor from the front of my bike. less moving parts better!
time will tell.
btw - i think that grabbing the brake on a bmx while in the air still has a pretty good size effect; we're talking about a 2-3" radius difference. i'd look up the formula in my physics book, but i left it at school -- i don't need that thing the night after my AP exam.
this question came into my mind after a few consecutive weekends of DJing... i found that the only place where i was using the front brake (when i was going to case anyway; i'm weird like that, i'll pull the brakes) was not necessary, and my P.3 is really only used for street and jumping.
so i took my front disc off and put it out back (avid mech) and did away with the hayes mech that was back there.
after a week of street on it, i can say:
-having one brake puts a lot more force on the brake - it's the only one you've got! loose hub, loose rotor, quickly wearing tire, etc
-it's (now) easier to do balancing moves (wheelie, manual) because i'm keeping more fingers on the bars and i'm not stopping the front wheel (rotational inertia again, plus gyroscopic procession - helpful in balance)
-i go for stuff cause i can't stop at the last minute (big plus for me - i've got more skill than ball, but my skills aren't that great anyway)
-still need to really go DJing on it, but so far, i'm loving it. not to mention the fact that i've subtracted a lever, cable, caliper, and rotor from the front of my bike. less moving parts better!
time will tell.
btw - i think that grabbing the brake on a bmx while in the air still has a pretty good size effect; we're talking about a 2-3" radius difference. i'd look up the formula in my physics book, but i left it at school -- i don't need that thing the night after my AP exam.
Yes you are correct, it does still have quite a bit of effect even on a 20" rim, (I have ALWAYS run both brakes, and use them quite a bit in the air when riding a halfpipe, but not so much DJing).
Though if you use the formula I=MRsq
(sq means squared), which is the SIMPLEST rotational inertia formulas, with I= inertia, M=mass, R=radius
(there are better and more accurate ones to use, but this will suffice for what I am trying to show)
And you base the mass of the wheel/tire/tube combo on lets say a sun DBLwide and average tire/tube you will come to approx 1500g for a 24" wheelset (just the outside circumfrence mind you, that is the only mass we are concerned with when using I=MRsq), and then for simplicity you convert everything into metric, that equals a mass of 3.9g per linear centimeter of tire/tube/rim assy.
Which corresponds to these relative masses (numbers have been limited to 2 dec. places)
20"=1205g
24"=1500g
26"=1617g
and then you input these into the rotational inertia formula I=MRsq, you get these inertia numbers (expressed in Kg/cm because the numbers are very large)
20"- I=614.55kg/cm-sq
24"- I=915.00kg/cm-sq
26"- I=1067.22kg/cm-sq
Now you can clearly see that the rotational inertia of the 26" wheel is nearly twice that of the 20" wheel.. in fact the 24" wheel has nearly 50% greater rotational inertia than the 20" wheel.
these are significant differences, which is why I basically dismissed the 20" wheel in my first dissertation..
It is effected by rotational inertia and gyroscopic effect, but the effect on larger wheels is much greater.
(hey everyone sorry for the general physics lecture!!)
Though if you use the formula I=MRsq
(sq means squared), which is the SIMPLEST rotational inertia formulas, with I= inertia, M=mass, R=radius
(there are better and more accurate ones to use, but this will suffice for what I am trying to show)
And you base the mass of the wheel/tire/tube combo on lets say a sun DBLwide and average tire/tube you will come to approx 1500g for a 24" wheelset (just the outside circumfrence mind you, that is the only mass we are concerned with when using I=MRsq), and then for simplicity you convert everything into metric, that equals a mass of 3.9g per linear centimeter of tire/tube/rim assy.
Which corresponds to these relative masses (numbers have been limited to 2 dec. places)
20"=1205g
24"=1500g
26"=1617g
and then you input these into the rotational inertia formula I=MRsq, you get these inertia numbers (expressed in Kg/cm because the numbers are very large)
20"- I=614.55kg/cm-sq
24"- I=915.00kg/cm-sq
26"- I=1067.22kg/cm-sq
Now you can clearly see that the rotational inertia of the 26" wheel is nearly twice that of the 20" wheel.. in fact the 24" wheel has nearly 50% greater rotational inertia than the 20" wheel.
these are significant differences, which is why I basically dismissed the 20" wheel in my first dissertation..
It is effected by rotational inertia and gyroscopic effect, but the effect on larger wheels is much greater.
(hey everyone sorry for the general physics lecture!!)
A
a13x
Guest
I used to run a front brake but I jumped into a tree one day and bent my fork to bajeesus and the V'brake didn't like that. So I just keep jumping that bent fork and now have no front brake.
I do have a new fork now and a disc wheel laying around but wouldn't cha know! My g/f has 'borrowed' my front disc brake.. :angry:
So just a 8" rear disc for me
I do have a new fork now and a disc wheel laying around but wouldn't cha know! My g/f has 'borrowed' my front disc brake.. :angry:
So just a 8" rear disc for me