[Vincelp]

Hello

The owner's manual tells us that the right tyre pressure for our Scrambler is : 2.5 bars for front & rear tyre.

I had a look at the load indication on our tyres :
- 58 for front tyre, which allows 236 kg on the tyre
- 73 for rear type, which allows 365 kg on the tyre

For a 186 kg bike, even with a 85 kg driver, we are really far from these values, so I think we've got over-sized tyres on our bike and the indicated pressure is, imho, too high.

So, today, I performed a test : set the pressure to 2.2 bar in rear tyre and 2.0 bar in front tyre and I rode about 350 km like this.

The bike is lot more comfortable on small bumps, I got no shock in the handlebar any more. The bike remained as easy to ride as before. The tyre temperature remained normal.

So, I think I will keep these new values for my tyre pressures, the main disadvantage being a quicker wear of my tyres.

 

[tonyjuliano]

MUCH quicker...

Keep in mind those specs come from Pirelli themselves, they were not arbitrarily set by Ducati.

Pirelli recommends those pressures - for that tire - for asphalt riding on any bike.


Sent from my iPhone using Tapatalk

 

[Vincelp]

'lut

Pirelli recommends those pressures - for that tire - for asphalt riding on any bike.

Yes, as you say, for any bike... Some other tyre manufacturer have recommendations for each bike.
Read carefully the Pirelli web site about MT60 RS tyre. It's been developed for hypersportive bike to use on dirt... Is the Scrambler an hypersportive bike ? With the same weight and same power than an hypersportive bike ? Producing the same efforts on its tyres ?

What about a bike which weight is 186 kg, versus another one which weight is 240 kg ?

You can do what you want and respect Pirelli recommendations. What I wanted to tell here is that, with little bit lower pressures, the Scrambler becomes a lot more comfortable without any change in the road handling behaviour.


Finally, I came to 2.2 bar in each tyre and it's perfect for me.

 

[jadatis]

Registered to this forum to give you an answer.
Once got hold of the formula and system that European car tyre makers use to determine the advice pressures for radial tyres, and went running with it.
Gathered a lot of information about tyres in time and dare to call myself tyre-pressure specialist nowadays.

For motor tyres its a bit different but also pressure needed is all about to give the tyre a deflection that does not give to much heatproduction for the speed you drive,so sertain area's of the tyre get to hot and harden and damage in next bendings of the rubber so deflections.

If you calculate that pressure with the formula for car tyres you sertainly wont get to low answers .

So if you give weight on rear and front and your tyre data , I can calculate a save lowest pressure for the speed wich you maximaly use and wont go over for even a minute. So your used maximum speed give it too, even if its faster then allowed by law.
Weights mostly can even be weighed with a persons weight scale, with a little help of someone to prefent the motor from falling sideward , because you have to weigh with persons and load on it.
Motors weight can vary more in percentage then cars weight.

What do we know already

Front tyre 236 kg maxload and probably ( but check it) 2.5 bar/36psi AT pressure.
Rear 365 kg maxload also AT 2.5 bar/36 psi
This AT pressure is not the maximum pressure of tyre , as they only give on Standard load and XL/reinforced/extraload car tyres , motor tyres mostly this AT-pressure given.
AT-pressure/referencepressure/maxloadpressure is the pressure the tyre maker used to calculate the allowed maximum load up to 99m/160km/h so it wont give to much heat production.
for higher speed there is a system ( also used for motor tyres) to highen up the AT//-pressure with a system depending on speedcode of tyre ( probably for motors between V ( maxspeed 240km/h) , W ( 270km) or Y ( 300km). so I also need you to give that from sidewall.

Weight of your motor I make of your last message 186 kg and driver 85 kg.
Total weight about 271 kg make it 280 , and this weight is about 2/3th on back and 1/3th on front tyre, so front about 95 kg and rear 190 kg.
If you take a passenger only the weight on rear gets that 85 kg more , because he/she sits above or even behind the rear wheelaxle.
so front can be verry low , roughly calculated (95-25)/(236-25)x 2.9 = 1 bar even for XL///tire for up to 99m/h ( 2.9 is AT pressure for XL// -tyres.
That low... yes but only for the savety of tyre so it wont get to hot at sertain spots. For riding quality you yust have to try out .
so you sertainly wont have to be affraid to destroy your tyre by the pressure of 2.2 bar you newly use.

Rear (190-25)/(365-25) * 2.9 bar=1.4 bar same speedlimit and if XL//-tyre.

But give the exact weights ( weighed) and tyre data ( read from sidewall) and maximum speed of tyre and using, and I will do a better calculation.

I now calculated with the on the road methode, that asumes a part of the load to be carried by the construction of the tyre and the rest by the pressure times surface on the ground calculation. That construction load Lc I assumed to be 25 kg .

Greatings from a Dutch Piggheaded Self-declared Tyre-pressure Specialist.

 

[Vincelp]

Thanks a lot. Your message quite confirms what I thought : our bike has got over-sized tyre, in terms of load acceptance.


I already had that with my previous bike : an '80 XT 500 with Bridgestone BT45 that I needed to inflate only at 1.5 bar, instead of 2.2 bar (Bridgestone recommendation), to get a good bike and suspensions behaviour on road.


The Scrambler tyres have got these information on their sides :
- front : Pirelli MT60 RS - 110/80 - R18 - MC 58H
- rear : Pirelli MT60 RS - 180/55 - R17 - MC 73H


For the weights :
- bike weight : 186 kg
- driver weight : 85 kg (I never ride with a passenger)


For the front/rear weight sharing, I would use more an 40% front - 60% rear, due the position of the motor in the bike. In fact, I'm not sure it doesn't change that so much, regarding the low values you got with 33% - 66%.

 

[jadatis]

The H in tyre specifications means maximum speed of tyre of 210km/h so you r max speed of motor will not be above that , or else these tyres would not be allowed by law.

I agree with your 40/60 FR weight division , you will know better , but weighing would make it sertain.

So new calculation
Front 40% of 280 kg= 112 kg Ok make it 120 kg
Rear we keep on the old calculation so its never to low. so 190 kg .
Then for maximum speed you have to highen up the AT//-pressure but I am busy with a simpler system mayby even better and that is lowering the loadindex by 1 for every 10 km/6.5m/h higher speed .
Here 210-160= 50km higher speed so 5 Li steps lower maxload.
So front 58-3=53 LI/206kg/454lbs maxload.
Rear 73-5=Li68/ 315kg/694lbs maxload .

Then still the on the road calculation.
Front (120kg-25kg)/(206kg-25kg) * 2.9 bar=1.52 bar rounded up 1.6 bar.
Rear ( 190kg-25kg)/(315kg-25kg)*2.9 bar=1.65 bar rounded up 1.7 bar.
Then assumed still AT pressure to be 2.9 bar for XL because I could not make that of your tyres given specifications.
And with this pressure allowed to drive up to 210km/132m??/H.
So all the reserves build in I could imagine.

So as long as you stay above that , your must be save for the tyres, and are free to experiment.

 

[Vincelp]

Thanks a lot.
My tyres pressure are now : 2.2 front & rear, instead of 2.5.

 

[tonyjuliano]

'lut
You can do what you want and respect Pirelli recommendations. What I wanted to tell here is that, with little bit lower pressures, the Scrambler becomes a lot more comfortable without any change in the road handling behaviour.

The "comfort" issue lies with the stock suspension and seat. Altering tire pressure to compensate is not the correct way to fix the problem, IMO.

But hey, it's your bike, do what you wish.

These tires - and I absolutely love them - are way too soft for me to be running that low of pressure, they wouldn't last 6 months for me.

 

[Vincelp]

The "comfort" issue lies with the stock suspension and seat. Altering tire pressure to compensate is not the correct way to fix the problem, IMO.
But hey, it's your bike, do what you wish.
These tires - and I absolutely love them - are way too soft for me to be running that low of pressure, they wouldn't last 6 months for me.

On any bike, the tyres are entirely part of the suspensive system, aside the suspensions and the seat. Just imagine what would be our bike behaviour if it hadn't air inflated tyres, but no tyres or entirely gum filled tyres like on very old cars ?
I agree with you that these tyres look like being soft tyres, but not from a structure point of view, but in terms of gum quality. They are really efficient although their ground contact surface is lower than usual, so that can't be achieved except by soft gum, that will wear quicker than usual. These tyres are bi-gum tyres, with a hard center zone and soft sides.


Be sure that, if I realize that my tyres gum disappear quicker than usual, I will let this know here and I will change my mind on the tyre pressure.

 

[Karlo]

I was not paying too much attention to my tires until last weekend

front was at 29.5 and back 34
my dealer told me front 36 and back 38 so I set them like that this weekend using the this
https://www.aliexpress.com/item/201..._7&btsid=cc4f5167-847d-40e6-9e1d-037fca7c7436

I hope is accurate

 

[Karlo]

by the way guys

yesterday I was riding alone and I had too much handlebar wobble any idea? my new pressure? 38 back - 36 front