Why Higher Pressure In Front Tyre?

[Mr Flannel]

Newb question (tried a search but my search-fu is weak)

 

Why is it that the front tyre (particularly on track) is run at a higher pressure than the rear tyre?

 

If, generally speaking:

• lower pressure = more traction, and
• higher pressure = more stability,

wouldn't you want more traction on the front, given a front end slide is a harder proposition to save? Probably a bit simplistic but there you go..

 

I note that for street riding, manufacturers (both bike and tyre) often recommend a higher rear tyre pressure vs front regardless of whether it is one-up or pillion riding.

 

Cheers, Justin

 

 

[FieryRobot]

I think this largely depends on the particular tires. They aren't all that way, and if they are, it's usually by a lb. or two max compared to rear. I think part of it is that you need the front tire to not push when you turn—you need it to turn. An under-inflated front is going to make turning... interesting.

[rchase]

That's a really good question. I have never really thought about why. My "guess" would be because of carcass deformation. In the rear a low pressure and lots carcass deformation gives the bike a lot of grip due to the heat generated and larger contact patch. The same would be true in the front however since that front wheel is a wheel that does the steering the deformation of the carcass would make the front end resist steering the bike. The larger the contact patch the more physical effort it takes to turn the bike. That's just a guess though.

 

Any experts out there on those round black things?

[Hotfoot]

Different tires have different characteristics but generally speaking a front tire with pressure too low will make the steering feel sluggish and vague. Too high can make it harder to warm up and make it feel harsh over bumps.

 

I normally associate the recommendation of front pressure a little higher than rear with high horsepower bikes that have a wider rear tire than front. On the smaller bikes where front and rear tire are the same size the pressure is sometimes recommended to be the same for both front and rear.

[Mr Flannel]

Ok. I'm reading that under-inflating the front can lead to steering issues, for a variety of reasons - a wider contact patch in front leading to understeer makes sense.

 

So, here's my next question - why doesn't the higher inflation of the front hold true for street riding? It seems the bike manufacturers (who presumably test their OE tyres extensively) recommend a higher inflation rear even without pillion. Take the Pirelli Supercorsa SP for example. The current Pirelli tech recommendation for track riding is (cold) 23-26 PSI rear (180/190/200) and 30-32 front (for a 120/70). However, on my Panigale which uses the same tyre as OE, Ducati recommends a front tyre pressure of 30.5 to 31 PSI and a rear tyre pressure of 32 to 32.5 PSI. I see BMW recommends a higher rear/lower front for the S1RR (or at least on the one I looked at). I understand the reason for generally higher tyre inflation on the street - less chance of rim damage, etc. but it still doesn't explain the reversed front/rear inflation thing-o..

[rchase]

The manufacturers "recommended" pressure is just that. A recommendation. I think their interest is reducing tire wear for somewhat "normal" street use. I don't know many people who ride a high performance Superbike in a "normal" fashion all the time but that's another conversation.

 

I recently put my Dunlop Q3's at the BMW recommended pressure and made the bike very harsh. I run the pressure now at just over 1-2PSI over the track recommended pressure. That works really well for me. I do know some people that run track pressures on the street with the logic of having the maximum grip available. That makes a lot of sense to me. The only reason I run a slightly higher pressure is due to center wear that I see when I do a lot of interstate riding. Generally the higher the pressure the lower the wear but also the lower the grip.

[Mr Flannel]

I don't know what you're talking about. I ride my superbike in an entirely lawful manner, your honour...

 

I run my street tyres similarly - above track pressures but I do bring the rear up quite a bit. I suspect running the much lower track pressures common on rear tyres like the Supercorsa will lead to potential rim damage and fast tyre wear on your average street and country road asphalt...

 

I still wonder about the reversed polarity between front and rear pressures for track vs street recommended by the tyre and bike mfgs....

[rchase]

I always try to keep it sane on the street myself. The corners at the track are much more fun than any road. We all of course occasionally get lost in the moment.

 

I'm wondering if the Dunlop folks might have a bit of technical insight into the differences in pressure and the why behind it. I'm interested now as well.

 

Hotfoot provided some good insight. She touched a bit on the different geometries of bikes and tire sizes. Powerful 1L machines have different geometry setups because of their power and weight while smaller bikes like 250's are optimized to take advantage of their low weight and power output. I suspect a lot of the pressure recommendations relate directly to the geometry of the machine and the tire sizes it's equipped with.

 

If you compare the S1000RR and your Panigale they are drastically different designs. One bike has an inline 4cyl engine that's 4cyl wide and the other has a 2cyl engine that's only 1cyl wide and it uses the engine as part of the frame. I don't have the specs handy but I bet the geometries are completely different.

 

Here's an interesting reference to motorcycle geometry.

 

http://sportbike.natkd.com/motorcycle_geometry_101.htm