What did you do today? Cobba's rebuild

[cobbadiggabuddyblooo]

If you want to run 5.5" rim 180 tyre just modify a 5.5" rim to fit TRX swing arm.
I did this with a yzf 750 rim but sold it to Udaysen who still has it fitted to 1 of his TRX's.

Complete YZF 750 rear end is only adding extra unecessary weight.

Plenty of light weight late model 5.5" rims will work with a little thought to get them modified so you have centred and chain alignment and 20mm axle mod.
Most Yamaha 5.5" rims have the hub centred the same on brake side but hub is 6mm wider on sprocket side so you can machine sprocket carrier for sprocket alignment and make a new top hat spacer.

If it's early 5.5" rim they usually run 245mm disc and you can use trx caliper mount.
Dymags I have now where made for TRX so used the trx brake mount and disc but I found a early YZ dirt bike wavey disc with same bolt pattern and centre as trx but only 220mm disc.

If you went with a modified 5.5" 750 rim plus the savings made with the thinner smaller 220mm wavey disc gets you around the same weight as the std 5" trx rim once again. So you have a wider rim with no extra weight...

Early GSXR 220mm calliper and mount worked with very simple modification.
Calliper Mount is same width for wheel spacing/centring but needed to install a bush to decrease axle size to 20mm and I removed 1.5mm from mounting point where caliper bolts on to centre the calliper over the disc.
Better still use a later model R6 rim and 220mm calliper set up and gain a wider lighter set up than std trx 5" plus the 5 spoke rims look great too.

[coxylaad]

I fitted the 170mm dog bones on the bike and I am up to 7 degrees. Looks like I need something like 160mm. Won't the shorter dog bones muck up the rising rate linkage.?

[cobbadiggabuddyblooo]

That all depends on the angle of the relay arm compared to the std TRX set up on how it sits and how that will impact on rising rate (the banana shaped item your shock and dog bones bolts to)
The ratio will get steeper near the end of the stroke.

[coxylaad]

I have just picked up some 162 and 168mm dog bones to try out.

I think over the winter I will have the swing arm modified so the dog bone mount is in the same position as the trx swing arm.

[cobbadiggabuddyblooo]

Information on Ohlins rear spring rates.. Ohlins shock springs are generally a 57mm spring diameter
Fitted and sprung to weight, mine is a 01092-34/100 L0613

01092 refers to the unladen spring length
01092 is an Ohlins specific coding for a 170mm spring length

-34/100 is the spring rate
-34 is an Ohlins specific coding /100 is 100Nm (10.19 kg/m usually noted as a 1.2 kg/mm)

L0613 is the production batch number

Spring deflection is around the 80mm on my spring.
Preloading the spring is proportionate to this value.
if 17mm pre load is added to the spring this will reduce my travel to around 63-65mm which is the stroke of the shock. (shock should bottom as the spring binds)
So if say 22mm pre load is added, it impacts on the actual shock travel because the spring will bind before the shock compresses to the full stroke and reduces your rear wheel travel.
If you have an incorrect spring fitted, you may get your rider sag set ok with the spring fitted to your bike but your free sag numbers will be out due to either your shock bottoming out (too stiff spring before bind) or your spring binding before shock bottoms out. (too soft a spring)


You can see from the photo of the original spring fitted when I purchased the shock was too light being a
01092-21/75 498
Same spring length by the first part of the coding but an Ohlins 21 which is a 0.75kg/mm
The 498 is the production batch.

[Racer-unk]

sorry to jump on the bandwagon here...but if i just replace the standard shock with another unit that is 25mm longer (and keep dogbones standard).....will the rising rate etc stay the same as standard bike?

[cobbadiggabuddyblooo]

The main issue if replacing with another shock is the stroke length of 63mm from memory on the trx.
Pointless putting on a shock with a similar eye to eye length when the stroke is 20mm longer. Or even worse shorter...

Even if you get within cooee of the eye to eye length you want to get around your ride height, then the damping settings and Spring rate may be way off due to the possible different rising rate relay arm on the other shock and swing arm length which all impact on damping shim stack and characteristics in the shock.
Cheapest and best option is to just get the std shock rebuilt and sprung to your weight.( around $250 )
Good chance the other shock will need a rebuild anyhow.
Most will weep the gas and turn your oil to mud after 2 yrs anyhow.
Plenty of positive feedback from those with rebuilt std shock.

R6 shock is a common bling mod with extra coloured buttons because of the similar stroke length but an adaptor is needed. Plus the R6 shock has a heavier 0.98 Spring fitted but damping setting are not correct due to the weight of the bikes and longer length but shorter 120mm travel of the R6 swing arm and different rising rate profile which is more linear.

[cobbadiggabuddyblooo]

Note to self

7N01

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5459202/

https://www.google.com.au/url?sa=t&rct= ... LeFdEYCfpA

[Racer-unk]

They say no question is a stupid question, and you cant gain anything if you don't risk.

that said, I have already paid and received for the 2013 Sachs BMW shock before I got your good advice. ( witch I may still end up doing and refresh standard shock)
BMW has a short stroke, cant find precise data anywhere....maybe a secret??? 2013 shock has better valving than 2010 unit that had limited adjustment.

My question is that:.... if i reverse the relay arm (seeing the shock mount is off centre), it will suit the shorter stroke shock. (i'd have to take note of the before and after ride height and adjust with dogbones)
i know it is shot in the dark....but you never know what you hit.

your thoughts please sir!

[Racer-unk]

addendum to my thoughts above. I see there is a second position where relay arm can mount onto the frame....looks like shock might touch swing-arm if you change the position without changing the relay arm for a shorter unit.

Thought maybe i must make a cardboard/wooden suspension to scale and measure the percentage of change. (roughly require 80% of original travel).

[cobbadiggabuddyblooo]

This all will impact on how the damping set up/ shim stack within the shock is set up.

Note on tyre slick compounds

Dunlop and Pirelli
SOFT is for hot track

Bridgestone and Michelin
SOFT is for cold track

[Racer-unk]

I made a cardboard "full-size" model and it gave me good encouraging results. Next i am going to made a wooden one with full size swing-arm to determine accurate dog-bone lengths (for the same travel as standard setup), and also determine the ride height. This i will do after my race on the 19th.

Currently I have Diablo Rosso Corsa tyres. front runs lovely on a cooler track, but works harder on a hot surface. Rear will tell me on Saturday coming.
I am not allowed to use slicks (Not sure why, because Its not a championship class. They just allow us to ride with the Power sport classes)

Changed Needle jet and Jet needle today along with Carb rubbers and new K&N filter....Breathed fire in my garage after I warmed it up, but I will practice with it like that on Saturday. Will try and get Carbs hooked onto a fancy machine next week to sync, and get an opinion on my current situation.

[cobbadiggabuddyblooo]

Note to self...
Tank air filter fitted

Jetting change for winter...
Up 2 sizes in main jet to 167.5
1-3rd gear Runs low/mid 12's (5500 - hi 11's ) and back to mid 12's over 6000 to hi 12's at peak.
4th gear runs hi 12's to mid / hi 13's at peak HP, 5th- hi 13's peak torque

[cobbadiggabuddyblooo]

Track day temps 1st day of spring..
12-22*c
32*c front with warmers
25*c rear with warmers
Hottest part of the day session 3 and 4 needed to go up 1 psi in the rear.
then back to 25*c for the remaining 2 sessions.
I can feel the torsional flex in the rear swing arm now so might look at a little bracing sometime down the track.
Lap times where averaging 1.09's due to traffic but get a free run I would find the 1.06's

https://www.youtube.com/watch?v=YLPuZksfBSY

[dandywarhol]

Dirty looking track in places

Weird "not harmonic" imbalances going on with yer camera monnt versus chassis vibration