weights and measures

[seespotweld]

stock Ducati rear wheel ...worn out tire 35lbs.
Dymag wheel new tire 25 lbs.
both measured with disks and sprockets .
haven't done the front yet but I don't expect as big a savings maybe 5 lbs.

can't help but be a little excited she now weights 40lbs. less than stock ...who needs an 848.

cost per lb. lost was a little crazy but still far less than a new bike.

[throttlehead]

maybe you could provide some more important info.....like what OEM wheels from what Ducati Bike. What tires and what size were on the wheels.

i think you thoughts of saving 40 lbs might need a math recheck

[BC999S]

Originally Posted by throttlehead

maybe you could provide some more important info.....like what OEM wheels from what Ducati Bike. What tires and what size were on the wheels.

i think you thoughts of saving 40 lbs might need a math recheck

Yes, would be good to know which bike.

I'll guess that the total of 40 lb. lost would be cumulative of all mods done - not just wheels.

[seespotweld]

yes the total of all mods is 40 Lbs. the big one was the akro titanium exhaust..19 lbs. light battery and a few carbon bits.

[seespotweld]

2003 749s

[froggert]

which battery did you switch to and how much weight did it save? any problems starting the bike?

i've cut over 40 pounds (maybe more than 50) off my 749 and am still looking for ways to make it lighter.

[seespotweld]

battery :same one motowheels sells I think the savings was 3 and half or 4 lbs.
I have the euro light switch so this helps with starting . I think I can tell that the increase in compression with the 924 kit is not making things easier or it could be battery is getting old.
I'm always looking to drop weight ...what all did you do ?

[froggert]

here are the major mods, off the top of my head:

carbon dymags
carbon front fender
oversized carbon air intakes
fuelcel tank
replaced headlight assembly with al race fairing stay
replaced stock steel biposto subframe with al mono subframe
lightened rear rotor
sharkskinz upper and lower track bodywork, sebimoto tail section
termi ss/ti 54mm full system
bored out cylinders

removed the following:
lights
mirrors
left-side switch assembly
horn
fans
emissions cannister and hoses

[seespotweld]

yes you can do a bit more to a track bike can't you ...the battery thing should save you a few pounds as would getting rid of your starter altogether ...they actually bump start pretty good

[Miykl]

Originally Posted by seespotweld

...they actually bump start pretty good

Unless you have a slipper clutch.

[seespotweld]

another reason not to use one

[Ducati23]

The only sure check for the weight reduction is to weight it. It's really tough to argue with the scale.

[seespotweld]

agree ...it can be a little like talking mileage ...I worked on a new race bike all one winter drilling holes in everything except the tank ...checked it at the Daytona track scales to find that it was within two pounds of a stock bike . A good crap before the race would have made as much difference.

[Zillr]

[QUOTE=seespotweld;463024]stock Ducati rear wheel ...worn out tire 35lbs.
Dymag wheel new tire 25 lbs.
both measured with disks and sprockets .
haven't done the front yet but I don't expect as big a savings maybe 5 lbs.

Hi C Weld I was lookin in hopes of finding some mag and stock wheel weights to compare and do some spring calcs. ( failed to weigh my mags at time of change) Saw Shazam's response and I think he forgot to mention something regarding spring rate, and damping. The reduced weight of the unsprung wheel should require a stiffer spring to ctrl it's increased acceleration. I see confirmation of that in an chart, in an article by Ducati Up North, W Shazam's name attached. Shows a considerablly increased spring rate for a mag swing arm but the princiole still applies. Also changing the spring rate should require more rebound damping to overcome that increased wheel speed. I think it was the article by Shazam that went on to note a spring change of 2Nm would require new valving. I thought I understood your post to say you saw a 10lb reduction at the rear?

[seespotweld]

another fly in the soup ...not sure if I agree about the spring rates needing to increase ..for one thing I think the mag swingarm is longer than the stocker which would give the wheel more leverage making the heavier spring necessary . My understanding of the primary roll of the spring is to support the weight of the bike/rider ..this has not changed much . Most suspension setup articles I've read say the first thing you do is set the spring sag and then they move on to damping.I don't think the spring has much to do with controlling movement ,well of coarse it's part of it but the speed of said movement is handled by the damping.

[Shazaam!]

Seespotweld is correct. Changes to unsprung weight don't require changes to damping or spring stiffness. A longer swingarm will need a change to spring rate because of the longer moment arm and associated different linkage components.

CF wheels and damping

[JEC]

40 lbs is pretty significant, I shaved that much off my old SV and it made a very noticeable different across the board. Mostly the exhaust (16 lbs off) ditching the passenger equipment (9lbs) and paring down a variety of things. GSXR front end was slightly lighter, and didn't have the chromed-steel naked bars. Stock was 430-440 wet, I had it down to 390-400 wet (measured).

[Old Baldy]

[quote=Shazaam!;473366]Changes to unsprung weight don't require changes to damping or spring stiffness. A longer swingarm will need a change to spring rate because of the longer moment arm and associated different linkage components.

I assume you mean to retain the same overall suspension rate/inch of travel?
Simply adding swingarm length will NOT result in any difference to overall resistance to bottoming, or any difference to the stroke of the shock and thus damping and spring rate can remain constant. The same 200lb load applied to 2 identical bikes with identical shocks and springs but one with a longer swingarm, will result in the shock compressing exactly the same distance on both bikes. One simply has more travel and a softer rate/inch, but the overall compressed % of available travel is almost identical (I say almost only because of the angular change to the swingarm axle - pivot angle relative to the vertical plane).

So, if the prime aim is to retain the same resistance to bottoming, then no changes are required for different lengths of swingarm, but if the prime aim is to retain the same effective suspension rate / inch of travel, and thus increase the overall resistance to bottoming , then a spring (and damping) rate change is required.

Agree?

[Shazaam!]

Suppose you have a 996 swingarm measuring 470mm and it takes a 7Kg/mm spring, what spring stiffness would you need if you extended the swingarm out to 495mm (to compensate for the extra leverage.)


The solution is to first determine the effective stiffness at a point on the the hub axis based on the geometry. Then we need to figure the spring stiffness needed to preserve this value for the longer swingarm.

Let,

L1 = the distance from the swingarm pivot to the spring attachment point
L2 = the distance from the swingarm pivot to the hub axis (i.e. the swingarm length)

X1 = the distance that the spring compresses (i.e the movement of the spring attachment point)
X2 = the movement of the hub that occurs when the spring compresses a distance X1

F1 = the force in the spring
F2 = an equivalent force applied to the hub that would give the same displacement X1

K1 = the spring stiffness = F1/X1 (Eq.1)
K2 = the equivalent stiffness of a spring placed at the hub that would give the same effect as K1 = F2/X2 (Eq.2)


from symmetry,

X1/L1 = X2/L2 (Eq.3)

summing moments about the pivot point,

F1 x L1 = F2 x L2 (Eq.4)

such that

K2 = K1 x (L1 squared/L2 squared) (Eq.5)

Case a

L1a = the distance from the swingarm pivot to the spring attachment point
L2a = the distance from the swingarm pivot to the hub axis

K1a = the spring stiffness
K2a = the equivalent stiffness of a spring placed at the hub

Using (Eq.5)

K2a = K1a (L1a squared/L2a squared) (Eq.6)


Case b

L1b = the distance from the swingarm pivot to the spring attachment point
L2b = the distance from the swingarm pivot to the hub axis

K1b = the spring stiffness
K2b = the equivalent stiffness of a spring placed at the hub

K2b = K1b (L1b squared/L2b squared) (Eq.7)


In order for the Case a and Case b systems to have the equivalent stiffness,

K2a = K2b (Eq.8)

combining (Equations 6, 7 & 8 )

K1b = K1a [(L1a squared/L2a squared) x (L2b squared/L1b squared)] (Eq.9)

>>>> (NOTE: This is the general equation that you need) <<<<<<<<<<<<<<


Assuming that the spring attachment point isn’t different for the longer swingarm,

L1a = L1b (Eq.10)

and combining (Equations 9 & 10)

K1b = K1a (L2b squared/L1a squared) (Eq.11)

Solution:

L2a = 470mm

L2b = 495mm

K1a = 7Kg/mm

using (Eq.11)

K1b = 7Kg/mm x (495 x 495mm)/(470 x 470mm) = 7 x 245,025/220,900 = 7 x 1.109 = 7.76 Kg/mm

So, assuming that the spring attachment point distance from the pivot is the same in both cases, you need to replace a 7Kg/mm spring with a 7.8Kg/mm spring when you change the swingarm length from 470mm to 495mm.

(Sorry about the awkward notation, and yes, I know that there’s a more straightforward proof)

[beaugator]

Now I remember why I hated algebra!