Front tire in rear position opinions

[Rough Edge racing]

  Cliff notes ;I run old Triumphs in land speed racing and do quite well. I'm now building a dual 650 engine Triumph LSR bike. If all goes well the bike will be running 150 MPH  plus...In the one mile standing start.
The tire speed rating needs to exceed the class record so a Z rated tire is required. These tires are mostly available in wide modern sport bike sizes.Wide tires are unsuitable for this type of bike.. But 120/780/18's are made by several manufacturers that would be a better fit on the LSR bike. The bad is they are front tires.
Some guys do run fronts on the rear and the tech inspectors seem ok with this so long as the tire is less than 10 years old and mounted so the directional arrow is not forward . They claim the only difference between front and rear for the purposes of LSR is the siping for water drainage. LSR bikes don't race in the rain and go in straight lines.
I race on paved tracks ,they don't allow drag strip starts but the machine accelerates as fast as 120 HP in a 420 pound bike can go for a mile or mile and a half.
Tires manufacturers shy away from questions like this most likely because of product liability..

What do you all think about this ? Is the only difference between front and rear just siping?

[nighthawk]

Wrecked a rear tire on the way home from AZ this spring on a Sunday evening. Next day, could only find a 120/90x18 front in Amarillo and said mount it backwards and I'll head for home. Used it w/no problems for 800 or so miles but replaced it when I got home. Not much help I know, but unless someone can offer other information, I think the directional siping is the primary difference. In my youthful Firestone tire wrangler days, we always changed the directional rears rotation for fronts, but that was a bit too agricultural compared w/speed trials. 

[Arizona Wayne]

Riding 2up years ago from Seattle, Wa. to Sacramento, Ca., our Conti. Blitz rear tire got blisters on the tread.  Was lucky to find an independent bike shop that had an Avon tire of the right size and had it mounted there. We made it home but the bike felt a little weird.  That's when I realized my new rear tire was a front tire!  I called Avon and they asked if we made it home OK.  I said yes, and they told me to find a local bike shop and get a new rear tire and they would cover the cost and ship the Avon tire to them.  We did. Front tires are not made the same as most MC rear bike tires and generally should NOT be treated the same as is normal for car tires unless the MC mfg. says it's OK. 

What tires work on the salt flats doesn't mean it works on street paved roads with curvy roads, etc. too.

[kirby1923]

I was always told that the construction difference on front and rear was they had cord/plys that were laped for (max for speed rating) stress in one direction.(if there was an arrow). In braking for the front and acceleration for the rear thus the reversal of the direction  arrow (in mounting) was required to insure that integrity of the tire was maintained if the tire was swapped.
Tires without direction arrows are good in any direction of force.

I suppose that could be a myth but worth checking out. Been a long time since I have seen any data on this but seem to remember it was true. It does make sense.

mike

[Arizona Wayne]

I was always told that the construction difference on front and rear was they had cord/plys that were laped for (max for speed rating) stress in one direction.(if there was an arrow). In braking for the front and acceleration for the rear thus the reversal of the direction  arrow (in mounting) was required to insure that integrity of the tire was maintained if the tire was swapped.
Tires without direction arrows are good in any direction of force.

I suppose that could be a myth but worth checking out. Been a long time since I have seen any data on this but seem to remember it was true. It does make sense.

mike

10? years ago there were Taiwan(Cheng Shin) tires that could be used either front or rear but they were rated safe (S) only up to 112 mph.  Most of us use street tires now (H) rated safe to 130 mph. Don't know if you still can even find new MC tires S rated any more.  You can for scooters.  For safe riding it's not only the tire construction but the tread pattern too.

Otoh, I was talking to a Norge high mileage rider who was thinking about lowering his bike.  I said, 'if you do that you will lose corner clearance.'  He said,  'most my riding is on straight roads.'   So to him corner clearance for going thru curves is a non issue.    Whereas I literally  LIVE to ride thru curves! 

[leafman60]

I've done it but mostly on vintage bikes that aren't running all-out in a racing situation although I have pushed them very hard on roadways.

Yes, the tread designs are designed to provide best traction and displace water based on the direction of travel for the tire. Additionally, the cord construction of the carcass is designed to handle braking force and acceleration force based on the mounting position of the tire.  A front tire receives tremendous stress from braking that is in an opposite direction on the tire from the stress incurred by the rear tire that deals with stress from acceleration.

Fact is, in some situations, the size rear tire you desire (or vice versa) is available only in a front-specified tire. In those situations, as long as you orient the directional arrow opposite to the front application, from my experience, things work okay.

My only caveat is that I recommend using a premium tire.  Don't experiment with cheap tires in a situation like this.

[Rough Edge racing]

  The bike doesn't a front brake and the rear brake isn't used very hard because there's plenty of distance to just sit up into the wind and use engine compression to slow down.
  Although the bike accelerates as fast as any 120 HP 430 pound machine the whole run only takes about 25 seconds. So the tire will never heat up very much

[charlie b]

Don't think it will be a problem, but, check the load rating too.  Front tires are made for less weight than rear tires.

Front tires on the rear will probably wear a lot faster.  Their 'longer wear' band is not on the center section like a rear tire.  Also probably not an issue for your use.

Other than that you should be fine. 

[Bisbonian]

Come on.

Everyone knows that all you need for a high speed tire is to cut off the tread blocks with a carving knife.

I saw it in a movie so it must be true.

[kirby1923]

  Front tires are made for less weight than rear tires.
Other than that you should be fine.

Think of a ZX14 in hard braking from 120+ on a 120/70/17 and doing a "stoppie" for the last 50' or so.

If anything the front's have to handle allot more weight than the rear.

Just say'in

mike  :-)

[charlie b]

But only temporarily.  Not the same thing.  The rear takes more load on a continuous basis.  Heat generation over time is the enemy of the tire.

Now, that brings up another advantage to fronts on the rear for purpose built bikes.  The front may weigh less than the rear type.  Might help performance.

[kirby1923]

When I ride hard I abuse the front allot more than the rear due to heavy braking and far exceed the load on the rear repeatedly but the point is that the tire can take it.
 
On the salt just for 25 sec at a time could be defined as temporary I think.

mike

[mtiberio]

A 120/780/18??? What rim width? Conti makes some new narrow rim radials. Avon plenty of universal front/rear.

[kirby1923]

I have been running Cont Classic Attack radials on my CX. Been thru a couple of sets and I like them.
18" sizes were 110/90/18, 120/90/18, and 90/90/18. I like the 110/90 on the back and the 90/90 on the front. Using 3" wide wheels on the back and I think its a 2.15 on the front??

They also have a 100/90/19 in that series.

Stick good and last longer than the demons.

[jetmechmarty]

I accidentally got a Pirelli front tire on the back of my liter+ old Japanese bike.  The new tire was completely worn out in 1800 miles.  The dealer (Rider's Hill) put the correct tire on for nothing.  The correct tire has 7000 miles on it.  The front tire on the back seemed to work fine, but I ate it up in a heartbeat.