Upgrade to big disk, better on front or rear?

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jag   10 kW

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Upgrade to big disk, better on front or rear?

Post by jag » Aug 10 2010 9:53am

My Cannondale Jekyll has 6" discs front and rear. With a geared hub (no regen braking) and sometimes pulling a trailer, I was wondering if it's a good idea to upgrade brakes?

I have an 8" disc I could fit to either front or rear. At first I thought that naturally the front should have the biggest brake, then I was not so sure.

Argument for front:
During hard braking most force is on the front.

Argument for back:
When braking with the trailer, too much front braking might jacknife the bike/trailer (b.t.d.t. on a car and boat trailer)
When braking steady, but not hard down a long hill, most weight is on the rear tire, hence rear brake would heat up most.

The newer Cannondale Prophet has 6" discs on both front and rear, but they are different:
The rear has a better vented disc
The front has fewer vents, and thus a bit more mass in the disc.
Thus one might suppose that the front can take a bit higher peak force, but the rear would be better at long sustained braking.

Any advice?

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Re: Upgrade to big disk, better on front or rear?

Post by battman » Aug 10 2010 7:19pm

Is braking performance mostly a problem with the trailer attached, if so could you add brakes to the trailer instead?
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Re: Upgrade to big disk, better on front or rear?

Post by John in CR » Aug 11 2010 1:18am

If you use more rear brake for braking down the hills, and you frequently do hills, then I think you should put it on the rear. Downhill is when you need the additional cooling of the larger disc the most, so whichever one has to dissipate more heat is where it should go. That's of course assuming that you already have sufficient braking power.

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Re: Upgrade to big disk, better on front or rear?

Post by Pablo_1985 » Aug 11 2010 1:48am

I would say in the rear wheel, just because experience, i have crashed with my bike for not having a big enough brake in my front wheel. when i brake hard in any conditions the rear wheel is hardly on the air.

Regards

PS: look for any bike who has bigger brakes in the rear wheel...strange
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Re: Upgrade to big disk, better on front or rear?

Post by LI-ghtcycle » Aug 11 2010 1:47pm

How heavy is your trailer?

If you are hauling seriously heavy stuff, do you have the best bike & trailer combo for the job?

If you answered yes to those questions,

I would default to say bigger on front as 75% of your breaking force on any 2 wheeled vehicle is accomplished at the front, all things being equal, but if you have break fade from heat, put the bigger break on the back and see what happens.

Since the front break is going to be loads more efficient no matter which disc is on it, I would say put the bigger one on the break that is the hottest or fading the most going down hill, and I would guess it will be the back, but ultimately, you might want to have 8" front and rear if this is the case.

I'm sure there is a way to get brakes on the trailer, but maybe you need to consider my first 2 questions first and either adjust your equipment type, or reduce the load your carrying.

Just my 0.02$ :)
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Re: Upgrade to big disk, better on front or rear?

Post by johnrobholmes » Aug 11 2010 2:22pm

I say front, cause all the braking power is there! One can't stop any faster than an endo. The only exception would be if you tend to use the rear brake mostly. In that case, put the large on the rear first. I also agree on trailer brakes.

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Re: Upgrade to big disk, better on front or rear?

Post by jag » Aug 11 2010 4:18pm

Thanks for the tips. Looks like the majority of evidence points to putting the bigger disk on the front. Bummer is that my standoff is for rear mount. Of course two big disks would be best, but I'd have to wait until they go on sale again...

I also realized that my argument that front braking would increase the risk of jackknifing with a trailer is probably flawed.

My trailer load is just one kid, so not that great. I don't think I need a separate trailer brake, but I was still curious how that would be done on a bike. Didn't find examples on ES or google. Anyone has links?

Should I ever need to carry a lot on a trailer (maybe I should joins auraslips line of business and start an all electric lawn mowing company.) then I think it would be just as good to build a powered trailer, and make sure to get a regen controller motor setup that can be activated with just an extra button on the handlebar.

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Re: Upgrade to big disk, better on front or rear?

Post by Hillhater » Aug 11 2010 6:31pm

Can some one point me to the data that says disc brakes are better than "V" rim brakes ?? ( in the dry)
simple mechanics suggest that rim brakes should have a huge mechanical ( and heat dissipating) advantage.
..and they eliminate the major (braking) force on the spokes !
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Re: Upgrade to big disk, better on front or rear?

Post by jag » Aug 11 2010 7:11pm

Hillhater wrote:Can some one point me to the data that says disc brakes are better than "V" rim brakes ?? ( in the dry)
simple mechanics suggest that rim brakes should have a huge mechanical ( and heat dissipating) advantage.
..and they eliminate the major (braking) force on the spokes !
That is a very good question. I have not seen good experimental evidence. Has anyone else?

Just did a quick google search. Most anecdotal opinion only claim more predictable all weather performance for disc brakes, not universally better stopping. One just stated "half the stopping distance", but didn't say under what conditions or give any other evidence.

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Re: Upgrade to big disk, better on front or rear?

Post by Sturdly » Aug 11 2010 8:49pm

If you think about it V brakes are in principal a type of disc brake, only difference being with a V brake the rim is the disc. Far different than comparing to a roller, band or drum brake.

AS to the OP's question, I agree with putting the biggest stopping force on the front wheel.

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Re: Upgrade to big disk, better on front or rear?

Post by johnrobholmes » Aug 11 2010 9:34pm

The best V brakes I have/do use are Shimano XTR with the parallel push. Magura HS33 hydro canti mount are the best stud mounted rim brakes I have/do use, and they are so powerful that it can crush rims and bend frames without brake boosters. Indeed, the rim brake does have better leverage against the wheel turning as compared to disc. I would gladly run a hydro 8" instead of any rim brake on a faster ebike, however (18mph bikes just don't need them with my light weight). The fade resistance and modulation of modern pad compounds makes the disc far superior, and nice disc brakes use less finger force than typical V brakes. Rubber rim brakes get hot quick, they can't shed heat like a sintered pad, caliper and piston setup.

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Re: Upgrade to big disk, better on front or rear?

Post by John in CR » Aug 11 2010 9:55pm

You left out the biggest advantage to disc brakes, especially hydro's (at least for me), which is not having to fiddle with adjusting brakes all the time. I need bigger braking forces than most here on the forum since I'm bringing 360-400lbs to a stop from 40-50 mph. Maybe my tune will change once I start relying on regen braking for the bulk of my non-emergency stopping force, but until then it's discs and hydro calipers for me. Since my change to that, my choice was either fiddle with brake adjustments almost daily, or be without good brakes till I adjusted them. Now it's been reduced to brake pad changes every 6 weeks to 2 months with zero adjustments in the past 6 months. My attitude is that EV's should be as close to 0 maintenance vehicles as possible as a prime selling point. I have a feeling that regen braking + hydraulic discs brakes will get me pretty close to that ideal, especially after I got to a motorcycle front tire too, so even air leakage thru rubber will all but go away.
Last edited by John in CR on Aug 13 2010 11:01am, edited 1 time in total.

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Re: Upgrade to big disk, better on front or rear?

Post by Hillhater » Aug 12 2010 12:49am

jag wrote: Just did a quick google search. Most anecdotal opinion only claim more predictable all weather performance for disc brakes, not universally better stopping. One just stated "half the stopping distance", but didn't say under what conditions or give any other evidence.
as you say...not very analytical . :roll:
most "V" brakes can lock the wheels (front or rear) with nominal effort !
Pad wear is not dramatic if some thought is put into the correct selection.
The real downside to all rim brakes is the wear imposed onto the expensive alloy wheel rim.
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Re: Upgrade to big disk, better on front or rear?

Post by amberwolf » Aug 13 2010 1:57am

And that any distortion of or damage to the rim can require adjusting the brakes outward to complete a ride, leaving one with significantly reduced braking force in some (many?) cases. Sometimes if it's bad enough the brakes must be disconnected completely to prevent rubbing on the bent rim in order to get home. :(

With disc the trueness of the wheel doesnt' really matter, and are unlikely to be affected much (if any) by something that bends the rim only enough to ruin rim braking but not destroy the wheel entirely.

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Re: Upgrade to big disk, better on front or rear?

Post by Hillhater » Aug 13 2010 4:49am

Very true Amberwolf, but likewise if you snap a few spokes, the disc brake is more likely to destroy the wheel than a rim brake is. ! :?
.. how often do you bend a rim without puncturing the tyre also ??
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Re: Upgrade to big disk, better on front or rear?

Post by amberwolf » Aug 13 2010 5:35pm

Hillhater wrote: .. how often do you bend a rim without puncturing the tyre also ??
Constantly. :roll:

Potholes around here eat rims for snacks, on heavy bikes like mine, especially on my unsuspended rear wheels. (one reason I'm working on a new bike that will fix that).

Plus, apparently it's very easy to laterally bend a rim on a hubmotor that uses heavy-gauge spokes, as it does not seem possible to tension them sufficiently. Someone else (JRH? cant' remember) recently pointed out that if you really tensioned them good that they'd destroy the typical rim supplied with them. I'm probably going to try lacing up the Fusin with regular gauge spokes for that reason, once I stick it in a non-bent rim.

I bent it by skidding in gravel, where the wheel/bars got turned almost perpendicular to my direction of travel, trying not to crash. If it had been tensioned properly I doubt it would have bent like that (it was so bad I almost had to disconnect the brake arms from the cable, just so I could ride home, and did have to let out the cable some, even after standing on the rim and pulling up the bent part).

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Re: Upgrade to big disk, better on front or rear?

Post by FMB42 » Aug 13 2010 6:10pm

John in CR writes:
"You left out the biggest advantage to disc brakes, especially hydro's (at least for me), which is not having to fiddle with adjusting brakes all the time."

Exactly.

Anybody who is old enough to remember the constant hassle of adjusting twin leading shoe front drum brakes on motorcycles can attest to this.

This, along with the tremendous force multiplier effect of hydraulics, are why cable operated motorcycle drum brakes are largely a thing of the past.

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Re: Upgrade to big disk, better on front or rear?

Post by Hillhater » Aug 13 2010 9:44pm

If you have problems with spokes on hub motor rims its worth bearing in mind that a disc brake will put far more load (torque transmission) on the spokes , than any hub motor ever will !
I like disc brakes, and would not consider a car or moto bike that did not have them fitted. Likewise, as someone pointed out, a bike rim brake is one of the largest diameter disc brake set ups you can see anywhere in common use. So big infact that there is no need for the use of hydraulic actuation, which has its own issues and cost factors to consider.
Bikes and Ebikes are ment to be a simple light weight , low cost means of transport. "Keep it simple" should be a motto !!
Adjustment? ....is that really an issue ?...if necessary, I can adjust my cable "V" brakes as i ride. :wink:

PS.. why dont road race cycles use disc brakes ? :wink:
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Re: Upgrade to big disk, better on front or rear?

Post by amberwolf » Aug 13 2010 10:20pm

If it werent' for the rim bending problems I have (regardless of type of wheel or spoke, it still happens), I'd rather use rim brakes simply because they're easy and cheap and stuff. But since I may be out and about with a really heavy (100lbs+) load, and/or pulling the trailer (another 200lbs+, potentially), and have a rim get bent, yet still need to brake hard, I need to have all my braking power still available even with that bent rim.

Right now the only way I can do that is to let the brakes rub on the rim in the bent spot (which can be so bad sometimes that it can actually jam against the wheel on the rear for DGA, because of it's side panels outside the triangle, unless I take the pad completely off). That robs me of forward power and wastes battery power and my own energy, both of which I need really bad when hauling such a load.

I'd rather deal with the stress on spokes using disc than not having brakes at all which is what has sometimes happened on a bad day.

Sometimes the rim problems are not really a bent rim per se, but rather a broken spoke (or two, etc) because of the potholes, which lets the rim pull to the unbroken side. In those cases I can sometimes just loosen the other side spokes a bit to retrue it temporarily, but that weakens the wheel a lot, and has caused me to bend rims just going around a corner sometimes (whereas leaving the spokes alone but disconnecting the brakes does not bend the rim in the same situation, so far).

In the latter situation, the disc brakes are only going to exacerbate the spoke problem, but at least they will still function just as well as they did before the breakage, unless of course they break enough spokes to taco the wheel. :lol:

Sometimes I think just ejecting a drag anchor behind me when trying to stop would work best, so it can catch a pothole and yank me to a stop. :lol:


All that said, I suspect if I had the money for really good rims and spokes, the problems I have would be minimized. But I don't, so these other things are an interim solution. :)

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Re: Upgrade to big disk, better on front or rear?

Post by will_newton » Aug 15 2010 8:05am

It is easier to ask if you can lock up your rear brake at any time as is. If so, then you do not need a more powerful rear brake- stick it in the front. Rear wheels will lock up much sooner, due to weight shift during braking. As the brakes are applied the rear wheel lifts and losses traction while the front wheel absorbs more weight and gets better traction, so is harder to lock up.

If you can already lockup both brakes, then don't worry.

Bigger isn't necessarily better, but hydro disc brakes will brake better than cable discs. You didn't mention what you had currently, you might just need an upgrade in type of brake, not size.
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Re: Upgrade to big disk, better on front or rear?

Post by amberwolf » Aug 15 2010 2:05pm

Regarding rear vs front, remember that not everyone has the same weight balance on their bikes, or even normal bikes. Some bikes like my CrazyBike2 have so much weight low over the rear and such a long wheelbase that the rear could take a LOT more braking power than "normal", before I'd even need to try using the front brake. :)

The same will probably be true of the newer version I'm working on, enough so that I am likely to try to add both rim and disc to it.

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Re: Upgrade to big disk, better on front or rear?

Post by MitchJi » Aug 15 2010 4:52pm

Hi,
jag wrote:Of course two big disks would be best, but I'd have to wait until they go on sale again...

My trailer load is just one kid, so not that great.
Even if you were not towing your kid I think you should go with two large hydraulic discs. The simple cost benefit analysis is you have a lot more to gain by having excellent brakes than the few extra dollars for another disc (about $30) and related parts.

Check this link:
http://www.google.com/products?q=bicycl ... hl=en&aq=f
Best Wishes!

Mitch


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Re: Upgrade to big disk, better on front or rear?

Post by nuvola » Aug 17 2010 11:02am

Hi,
I've an ebike with a trailer.
my weigth it's more than 100kg and my trailer carry 48V 60ah so the total weigth of trailer it's about 35-36kg
bike it's 15kg and emotor about 8kg
totale 100+35+15+8=158kg!

When I use bike on flat, a 6" disc it's enough, but when I use ebike on mountains big problems starts!
note that due to my 9C wheel I can mount only front brake
a downhill of only 5-10% fired my 7" shimano brake! (XTR disk and m485 brake)
well the front disk was so hot, that changed color from alluminium to black and now they are still black!
so I upgrade to 8" I bougth a Shimano Saint (top shimano idraulic brakes 300€ for rear and front 8" disk).
I still have to repeat the same downhill, but I'm sure front is more and more important than rear.
with a rear powerfull brake, if you put too much brake power, rear wheel can only screech and I think you could skid with your trailer...
all the weigth it's in front so put 8" on front.
Remember that if you trailer is heavy like mine and you do mountains, you need good brake like shimano saint,Formula one,Avid Code and so on..
bike 1) 9C Rh205 28" Rear, controller 48V 27A with 51V 60ah battery, watt's up
bike 2) 5305 26" Rear, controller 72V 40A with 72V 15ah battery, CA

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