LX brakes > GTS brakes, NOT!
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Mon Feb 22, 2010 10:08 pm quote
I've been doing a lot of comparison between the GT/GTS and the LX in order to get the best combination in project rosebud. Over the weekend I compared the front brakes on the LX, GT/GTS, and even looked at the MP3. Tonight I have crunched the numbers on the GT vs LX and while I suspected as much I was still shocked at the magnitude of the outcome.

First the data


Model disk diameter contact width caliper piston tire diameter
GT/GTS 220mm 25mm 2 X 25mm floating 473mm
LX 200mm 30mm 2 X 30mm 414mm
MP3 2 X 240mm 30mm 2 X 30mm 473mm
GTS Brembo 220mm 30mm 2 X 30, 2 X 34 473mm


I first looked at the ratio of the braking torque produced by the GT to the LX. By looking at the ratio things like the hydraulic pressure and friction coefficients drop out and it boils down to the relative size of the pistons and disks. I used the contact width to get the average radius of the disk.

Result: The brakes on the GT only produce 80% as much torque as the LX.

Next I looked at the stopping force applied to the ground, again working with the ratio of GT to LX. Due to the larger wheel on the GT (or smaller on the LX depending on the point of view) the force finally applied to the ground is only 70% that of the LX.

Last edited by oopsclunkthud on Sun Mar 07, 2010 9:40 am; edited 4 times in total
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Mon Feb 22, 2010 10:17 pm quote
Damn you and your fancy "mathematics!"

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Mon Feb 22, 2010 10:21 pm quote
dahaym!
Molto Verboso
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Mon Feb 22, 2010 11:08 pm quote
Just as I suspected!
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Mon Feb 22, 2010 11:20 pm quote
That is interesting but... will that help people put a disk brake on the back of an LX ????? Has anyone done that kind of thing?
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Mon Feb 22, 2010 11:48 pm quote
Copper Dolphin wrote:
That is interesting but... will that help people put a disk brake on the back of an LX ????? :P Has anyone done that kind of thing?
Other than "it would look cool" why would you want to do that? All the stopping power is needed at the front and the drum on the rear is lighter and has plenty of power to lock up the rear as it is. Plus there's just no room for it.

Now if anyone wants to put a disk on the rear of a Fly... that is doable, though again not needed.
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Tue Feb 23, 2010 3:58 am quote
I prefer the feel and control of a rear disk brake over the rear drum on the LX, that would be the reason I would want a rear disk. I just find I can modulate a disk better and can feel when I have pending lock up better.
Molto Verboso
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Tue Feb 23, 2010 6:48 am quote
Benito wrote:
I prefer the feel and control of a rear disk brake over the rear drum on the LX, that would be the reason I would want a rear disk. I just find I can modulate a disk better and can feel when I have pending lock up better.
Idem ditto!

The feel and performance of a disc is better.
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Tue Feb 23, 2010 7:31 am quote
So Patrick, the real question is if "math" can "prove" if the Brembo is better/worse than the LX...
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Tue Feb 23, 2010 8:44 am quote
TheO.Z. wrote:
So Patrick, the real question is if "math" can "prove" if the Brembo is better/worse than the LX...
Don't even have to do the math to "prove" that one, though give me a day and I will. The brembo that jettin sells has 2 X 30mm pistons and 2 X 35mm pistons. The brembo on the GT/GTS should have about 2 times the stopping force of the LX.

The real question with the brembo is is it overkill and is it worth the extra un-sprung weight? It would be overkill on an LX, it may not be on a GT. It does look and work great, it's just a mater of balance.

Back to the disk on the rear, I agree that a disk has a better feel and control. Still wouldn't bother on an LX but would on a Fly.
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Tue Feb 23, 2010 8:47 am quote
disc brakes and want better braking...... fit a bigger pistoned master cylinder
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Tue Feb 23, 2010 9:03 am quote
maver wrote:
disc brakes and want better braking...... fit a bigger pistoned master cylinder
How so? With all other things being equal, that just reduces your mechanical advantage (less braking force for the same lever input).
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Tue Feb 23, 2010 9:10 am quote
maver wrote:
disc brakes and want better braking...... fit a bigger pistoned master cylinder
Other way around Smaller piston in the master gives higher working pressure. But good point, there are other ways to adjust the over all force than just at the caliper end.

For my LX I'm currently looking at ways to keep the 11in rim and 2 X 30mm caliper pistons while moving up to the 220mm disk. Then make any additional adjustments at the master.
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Tue Feb 23, 2010 9:23 am quote
Question - can you lock up the front brake? If you can the brakes are as good as they need to be already.
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Tue Feb 23, 2010 9:39 am quote
jimc wrote:
Question - can you lock up the front brake? If you can the brakes are as good as they need to be already. ;)
On the GT, no!
On the LX, not quite.
On my smallframe, yes.

That really is the goal though, Just enough to be able to lock the front.
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Tue Feb 23, 2010 9:42 am quote
oopsclunkthud wrote:
jimc wrote:
Question - can you lock up the front brake? If you can the brakes are as good as they need to be already.
On the GT, no!
On the LX, not quite.
On my smallframe, yes.

That really is the goal though, Just enough to be able to lock the front.
huh. I swear I've locked the front on the GT at ~20 doing some stopping exercises one day while bored. The front end pogo'd a few times in a stutter.

Had to basically clamp the levers to the grip though.
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Tue Feb 23, 2010 10:00 am quote
On a side note, I've noticed what I perceive as frame torsion under heavy front braking.
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Tue Feb 23, 2010 5:40 pm quote
Hey Oppsklunkthud
The drum brake on my LX150 has little or no stopping power, and I do not want to count on the front disk brake as that has crashed me once already. In a real easy stop coming up to an intersection I had the front and back brake applied gently and because I had anything on the front brake when I hit an oily, pebbly streach of asphault. I went down in less than a blink of an eye, no time to react or keep myself upright. Since then I have been appling the rear brake 1st and only using the front for additional stopping once I have slowed down. The rear drum is proving to have little or no stopping power. Unlike the rear disk brake on my GTS250. In those harry spots of stoping in rain, snow or oil, you have to use the back brake only, no front. It would be nice to be able to stop the scoot in less than the distance of a football field, using the back break only when and if I ever have to.
When it is dry and you have a clean road surface then the front is your main stopping power... and I use both brakes evenly. It would be nice if the LX150 could stop it's self with rear brake only. The only way that could happen is with a disk brake.
I thought I had seen on a thread here where someone had fitted a disk brake on the rear of an LX or maybe it was a S.
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Tue Feb 23, 2010 5:44 pm quote
Ran the numbers on the Brembo and it produces 2.5 times the braking torque and 2.25 times the stopping force compared to the LX.
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Wed Feb 24, 2010 8:34 am quote
My MP3 and Vespa GTV stop all the time and I check my pads, rotors, fluids and tire wear/pressure at every service. It does not take math to figure that out.

Jon
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Wed Feb 24, 2010 8:47 am quote
scooterjon wrote:
My MP3 and Vespa GTV stop all the time and I check my pads, rotors, fluids and tire wear/pressure at every service. It does not take math to figure that out.

Jon
Roll your eyes all you like... it's how EFFECTIVELY you stop that is being discussed here, and whether that effectiveness can be improved. It takes math and engineering know-how to systematically approach that topic. Deal with it.
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Wed Feb 24, 2010 9:08 am quote
Effectiveness on stopping power is based on the reaction time of the rider (urban environment). Having good judgment and riding defensively are important not only do these things count but servicing of the of the brake pads, rotors, brake fluid and TIRES will make that much of a difference. If you have none of the above on a two wheeled vehicle then if does not matter how many pistons you have a hydraulic braking caliper. Sorry just pointing out the obvious things to check and hope everyone relies on better judgment and servicing of the said parts. Other then that ride safe.

Jon
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Wed Feb 24, 2010 9:12 am quote
scooterjon wrote:
Effectiveness on stopping power is based on the reaction time of the rider (urban environment). Having good judgment and riding defensively are important not only do these things count but servicing of the of the brake pads, rotors, brake fluid and TIRES will make that much of a difference. If you have none of the above on a two wheeled vehicle then if does not matter how many pistons you have a hydraulic braking caliper. Sorry just pointing out the obvious things to check and hope everyone relies on better judgment and servicing of the said parts. Other then that ride safe.
I think it's safe to say that Patrick does, in fact, understand those points, and is attempting to push the boundaries well past the "usual" riding conditions.
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Wed Feb 24, 2010 9:20 am quote
The MP3 has better brakes than the LX and given it's got two of them up front it's probably close to the Brembo. I updated the table above and will crunch the numbers tonight.

Agree that to be effective the brakes (and the rest of the scooter) must be well maintained and operated with care and skill. This is true of just about anything in life, even making or even drinking coffee.
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Fri Feb 26, 2010 6:33 am quote
Here's a combination of parts that will work with minimal modifications. The caliper has the same 30mm pistons as the LX but will allow the hub and 220mm disk from the GT to be used. The best part is that the LX rim still fits with about 5mm clearance. The result would be a 12% increase in braking force over the stock LX setup.

IMG_2129.jpg

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Fri Feb 26, 2010 7:42 am quote
Re: LX brakes > GTS brakes
oopsclunkthud wrote:
I've been doing a lot of comparison between the GT/GTS and the LX in order to get the best combination in project rosebud. Over the weekend I compared the front brakes on the LX, GT/GTS, and even looked at the MP3. Tonight I have crunched the numbers on the GT vs LX and while I suspected as much I was still shocked at the magnitude of the outcome.

First the data


Model disk diameter contact width caliper piston tire diameter
GT/GTS 220mm 25mm 2 X 25mm 473mm
LX 200mm 30mm 2 X 30mm 414mm
MP3 2 X 220mm 30mm 2 X 30mm 473mm
GTS Brembo 220mm 30mm 2 X 30, 2 X 34 473mm


I first looked at the ratio of the braking torque produced by the GT to the LX. By looking at the ratio things like the hydraulic pressure and friction coefficients drop out and it boils down to the relative size of the pistons and disks. I used the contact width to get the average radius of the disk.

Result: The brakes on the GT only produce 80% as much torque as the LX.

Next I looked at the stopping force applied to the ground, again working with the ratio of GT to LX. Due to the larger wheel on the GT (or smaller on the LX depending on the point of view) the force finally applied to the ground is only 70% that of the LX.
Interesting stuff. What formulas/equations are you using? Do they take into account the weight of the ride, master cylinder diameter and brake lever 'leverage?'

I've no expert in design of brakes for a vehicle, but one would expect that the goals are to design brakes such that a certain reasonably-high amount of force on the brake lever is just enough to lock up the wheel, and that lever travel is also in some reasonable range.
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Fri Feb 26, 2010 8:23 am quote
Those are some crazy but interesting mods oops. Are you also calculating the different area of tire that touches the ground of the LX vs. GTS?
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Fri Feb 26, 2010 8:35 am quote
oopsclunkthud wrote:
Here's a combination of parts that will work with minimal modifications. The caliper has the same 30mm pistons as the LX but will allow the hub and 220mm disk from the GT to be used. The best part is that the LX rim still fits with about 5mm clearance. The result would be a 12% increase in braking force over the stock LX setup.
Did you only changed the position of the caliper then?

How? pictures?
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Fri Feb 26, 2010 1:18 pm quote
All these comparisons are done with the assumption that the lever force and master are all the same. I've now added the weight of the bike to give deceleration. By making all of this relative to the stock MP3 (changed my baseline to the best stock setup) it saves having to know a lot of other data about the system. Even though it's counterintuitive, the tire contact patch has no influence on the stopping force. Friction is funny that way.

Relative Braking Torque
Torque: torque generated by the braking system.
piston: caliper piston diameter (assumes same number of pistons)
Rdisk: disk radius at the center of the pad width.
Torque1   piston1^2   Rdisk1
------- = --------- * ------
Torque2   piston2^2   Rdisk2
Relative Braking Force
Force: force generated at the wheel assuming the tires stick
Dwheel: rolling diameter of front wheel
Force1   Torque1   Dwheel2
------ = ------- * -------
Force2   Torque2   Dwheel1
Relative Braking Deceleration
Deceleration: deceleration assuming the tires stick
Mass: mass (or weight) of the scooter
Deceleration1   Force1   Mass2
------------- = ------ * -----
Deceleration2   Force2   Mass1
Results


Model Relative Torque Relative Force Relative Deceleration
MP3 100% 100% 100%
LX 40% 46% 86%
GT/GTS 32% 32% 44%
GTS Brembo 103% 103% 142%
Rosebud 45% 52% 96%


Last edited by oopsclunkthud on Sat Feb 27, 2010 6:05 pm; edited 1 time in total
Veni, Vidi, Posti
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Fri Feb 26, 2010 1:30 pm quote
Excellent analysis, Patrick!
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Fri Feb 26, 2010 5:10 pm quote
oopsclunkthud wrote:
Here's a combination of parts that will work with minimal modifications.
So which caliper did you end up using? It's not entirely clear from the thread & I'm curious to know. Sounds like a great setup in any event!

On a side note, anyone who thinks this is overkill has clearly never boiled their brake fluid before...
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Fri Feb 26, 2010 8:24 pm quote
bagel wrote:
oopsclunkthud wrote:
Here's a combination of parts that will work with minimal modifications.
So which caliper did you end up using? It's not entirely clear from the thread & I'm curious to know. Sounds like a great setup in any event!

On a side note, anyone who thinks this is overkill has clearly never boiled their brake fluid before... ;)
I will share the details of this setup as soon as I've tried it out. There are a few details (like the speedo drive) to work out before installing on a scoot. With luck I may try it out this weekend. There is a small bit of dremel work to do on the caliper mount to clear the screws for the larger disk.
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Sun Feb 28, 2010 8:04 pm quote
I've done a dry fit of all the parts needed to put the GT disk onto the LX (or the lx style caliper onto the GT but given this was moving more into a project, instead of gathering data, I created a thread in the projects section.

Rosebud Brakes
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Sun Mar 07, 2010 9:55 am quote
Crap!
Um, I think I've made a big error regarding the GT/GTS brakes. The GTS has a floating caliper with the two pistons on one side. While it's generally accepted that floating calipers are a crime against all that is holy there's been this nagging doubt hanging in the back of my mind regarding one little advantage that they do have. The clamping force of a pair of opposing pistons is no greater than that of a single piston. This means that I've under estimated the GTS by a factor of 2. Oops.

Corrected resultes


Model Relative Torque Relative Force Relative Deceleration
MP3 100% 100% 100%
LX 40% 46% 86%
GT/GTS 64% 64% 89%
GTS Brembo 103% 103% 142%
Rosebud 45% 52% 96%


Last edited by oopsclunkthud on Sun Mar 07, 2010 10:01 am; edited 1 time in total
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Sun Mar 07, 2010 9:58 am quote
So what did you crash into? Got pix of the damage?

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Sun Mar 07, 2010 10:05 am quote
jess wrote:
So what did you crash into? Got pix of the damage?

hah. No crash, the setup works ok on the GT, just not as good as stock. It's still a good improvement for the LX.

Dreading heading down to the garage to switch it all back...
Sir Frets-A-Lot
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Sun Mar 07, 2010 10:07 am quote
jess wrote:
So what did you crash into? Got pix of the damage?

Rosebud isn't running, is it..?

I had a dream that included Rosebud last night. There were some interesting... "structural accommodations" that were made in my mind, and it looked quite curious. But was pretty cool nonetheless.

funny, dreams about someone's unfinished bike. Hah.
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Sun Mar 07, 2010 12:24 pm quote
jimc wrote:
Question - can you lock up the front brake? If you can the brakes are as good as they need to be already. ;)
Switched the GT back to stock and went out looking for front wheel skids. Found them! Not in all conditions but cresting a hill, on a steep down hill...
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Wed May 04, 2011 10:09 am quote
Credit to the Piaggio Engineers
In my last entry on this I had made the assumption that the piston in the master cylinder was the same for the GT/GTS and LX, it is not.

Thank you bluecloud for pointing this out in your project S -- a hotrod modern smallframe

If the numbers on the housing are to be believed the GT has a 12.7mm piston and the LX has an 11mm piston. A smaller piston in the master creates a higher working pressure and more force on the brake pads. After taking this into account we find that the GT and LX have exactly the same deceleration capability.

Corrected corrected results

Model Relative Torque Relative Force Relative Deceleration
GT/GTS 100% 100% 100%
LX 84% 75% 100%
GTS Brembo 160% 160% 160%
Rosebud 94% 107% 144%
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Wed May 04, 2011 11:20 am quote
scootermarc69 wrote:
On a side note, I've noticed what I perceive as frame torsion under heavy front braking.
You know that the front wheel dips during braking, due to load transfer, right?
This is a normal function of vehicles with telescoping forks.
It would probably take a lot more force to actually torque the frame, since it's a unibody construct, than what is applied in normal braking.
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