What did you do today? Cobba's rebuild

[cobbadiggabuddyblooo]

Temps where no problem with the tyres as such.
Lakeside is about 25 minutes from home and mostly 60 kph stop start with Friday morning traffic.
I got there early and the bike sat in the shade for 2 hrs and set the pressures just before my group went out.
So the tyre temperature itself was pretty well at ambient temps.
I set pressures from the Dunlop recommended specs for the D211 GPR on the lower end of the threshold in cold pressure settings of 18*c rear and 30*c front.
No tyre warmers so my first 2 laps where accelerate and brake between corners to generate some heat and not working the tyre through the corners. The front was a little squirmy but by the end of lap 2 this had disappeared.
3rd lap exiting corners with a bit more throttle I could still feel the rear tyre squirming a little and all but disappeared through lap 4 so lap 5 all felt stable and was able to push things but by then I was overheating...

No tearing was evident in the morning 2 sessions and the debris field was pretty consistent over the edges and 1/3 and 3/3 of the tyre with the centre section being the 2/3. Come the early afternoon when things where at the hottest I could see what looks like a slight cold tear beginning to appear on the rear as Lakeside is predominantly a RH corner track. The LH rear side is still pretty uniform.
Maybe if I took a pressure reading while hot in the arvo straight after the session I may have needed to lose just a little tyre pressure.
Coming onto the straight is down hill RH corner so you enter it at 6000rpm in 3rd and hold it flat and wheel spin out which would have been doing the bulk of this damage



Starting to push things in the arvo as I managed my body temp challenge geometry wise the front behaved beautifully with the forks raised 11mm and level with the top tree and took away the slight oversteer I was getting coming into the corners which was more apparent in the slower bus stop section of the track.
The rear was doing just what I asked so with the raised ride height and more swing arm angle this time it stopped the slight running wide feeling I was getting last time out.



I'll post photos shortly..

Dunlop D211 GPR tyre pressure specs

FRONT cold ..... 2.1 bar ( 30.45psi ) and target hot specs .... 2.3 - 2.6 bar ( 33.3 - 37.7psi )

REAR cold ..... 1.2 bar ( 17.4psi ) and target hot specs .... 1.4 - 1.7 bar ( 20.3 - 24.6psi )

In hindsight I should have taken hot tyre temps in the afternoon straight after the sessions to factor in the hot track temps...

[cobbadiggabuddyblooo]

LEFT HAND SIDE FRONT WHICH DONE MOST OF THE WORK








RIGHT HAND SIDE

[cobbadiggabuddyblooo]

RIGHT HAND SIDE REAR DOING MOST OF THE WORK






LEFT HAND SIDE

[dandywarhol]

I don't think you do have a cold tear problem - the wear patterns look pretty conventional to me, considering the use and temperature. Careful not to confuse wear readings and debris picked up in the slowdown lap . What is impressive is the suspension is well set up, no evidence of edge puckering.

I find this guide handy https://lifeatlean.com/motorcycle-tyre-wear-guide/

[cobbadiggabuddyblooo]

Thanks Dandy, it actually felt the best it ever has.
It was doing everything I asked and expected to do.

When I rode back from the track and got home I parked the bike upright in the garage in a front wheel transporter stand and when in the garage today, I tried to remove the bike but the front tyre rubber had cooled and stuck itself to the stand...
I was dragging it along the ground for 2 feet until it finally broke contact and I could roll the bike out of it....
Look forward to the next track day in a few weeks time when the weather cools a little.

The bonus using the GPRacer instead of the Gp or Q2-Q-3's is they are non directional so I can reverse the rear tyre after a couple of track days

[coxylaad]

looks ok to me. I remember days when my tyres used to look like that.

add an extra 100bhp and they can get minced very quickly!

[cobbadiggabuddyblooo]

Swing arm angle from last track day settings and extra 30mm ride height over std with forks set flush with tops pf trees.

A lot better than 9.8* where it was previously and no doubt the rear squatting on the off chamber exit out of the carousel that caused my front end low side at last years track day. Steeper swing arm angle cured the understeer out of the corner and squatting issues.

Info on dog bone lengths / ride height and swing arm angles.


std trx 182mm = 00mm extra ride height

.........177mm = 16mm extra ride height ( 9.8* swingarm / 14mm forks exposed ) 22.5* rake Michelin Pures ( previous set up )

,,,,,,,,,176mm = 20mm extra ride height

yzf750 173mm = 30mm extra ride height ( 11.5* Swingarm / 00mm forks exposed ) 23* rake 98mm trail Dunlop D211

.........170mm = 40mm extra ride height (11.8* /00mm forks exposed )

The other point I should mention is this was all calculated with 5mm rear sag with the weight of the bike.


http://www.cleavebooks.co.uk/scol/calrtri.htm

[cobbadiggabuddyblooo]

I dusted off the cover on the TRX after a few months in hibination last week and she kicked straight into life at the push of the starter button.
I finally got around to sourcing some 170mm dog bones which are on their way so I can look at the impact on ride height and handling. Now I just have to find time over the next couple of months for a trackday...

[coxylaad]

I dusted off the cover on the TRX after a few months in hibination last week and she kicked straight into life at the push of the starter button.
I finally got around to sourcing some 170mm dog bones which are on their way so I can look at the impact on ride height and handling. Now I just have to find time over the next couple of months for a trackday...

now this is interesting to me. having a thunderace swing arm, the dog bone mount on it is in exactly the same location as the trx longitudal and laterally, however its 10mm lower vertically.
I am running 175mm dogbones and it 'looks' better, I have 170mm ones to try but I thought that might really jack the rear end up. even more so with a stock trx swing arm!

Do you think 170mm dogbones will be ok on your bike?

also how do you measure the swing arm angle? with the bike under its own weight or with you sat on it?

[coxylaad]

sorry just read you are measuring with the bike sat on its own weight.

I will do the same with mine.

[cobbadiggabuddyblooo]

You need to either place a chock under the side stand or have someone hold it so the bike is pretty well straight upright as such.
It's best to bounce the bike a few times beforehand to settle the suspension too.
I used a level app on my iPhone to measure the angle.
Measurements using the straight edge and square gave me pretty well the same result using the link.
First thing is to make sure you are on level ground so I placed a 6 foot long straight edge that ran beside the front and rear wheels and I calibrated my iPhone at 0* on the straight edge.
Next I needed to unbolt the LH footpeg.
Then I used the straight edge and lined it up to the centre of the swing arm pivot pin to the centre of the rear axle.
I then took my 2nd reading to gain the swing arm angle.
The thing to be mindful of too is if you change gearing at any time, this will also move the rear axle to a new position.
Let's say you currently have 17/41 like most Japanese imports, and you change to 16/39 the axle moves back, you swing arm angle will decrease so you'll need slightly shorter dog bones to keep you at your chosen angle.
Remember too the further back the wheel is in the swing arm you'll also need to increase the amount of slack in the chain otherwise it will bind and you suspension will not use the full stroke.
I'm currently looking into gearing options to see if I can get the rear wheel as far back on the swing arm to get as much wheelbase as possible.

[coxylaad]

Well i am currently 5.6 degrees with 525mm axle to axle.
Thats with 175mm dog bones.

Whats a good angle to go for?

[cobbadiggabuddyblooo]

Between 11-12 is what I'm shooting for

[coxylaad]

christ. looks like I will be fitting the 170mm dog bones over the weekend and checking that.

[Zero the Hero]

I do work with adjustable dogbones. My aim is the chain force of traction thing.
Kettenzugresultierende in german. The Points where the front and rear sprocket line meets the line of wheel and swingarm angle to ground and such things.
Using thunderace swinger, modified yzf ohlins shock, lowered front keeps me busy. This is a big issue. Hard to find a compromise. First you have to know what you want, easier than what you don't want.