Considering using an automotive tire...

**forfortoo** · 05-25-2011, 08:22 AM

I know this will likely become a safety discussion, so here it is. I am considering using an automotive tire on the back of my bobber build. I think I should be able to work out any clearance problems with the chain, and if I can't find a way to make it work in that respect then I will scrap the idea, but otherwise this is my intention. BF Goodrich has a 155 / 80-R15 size Radial TA. The circumference is close to the stock tire. The section width is close to the 6" fender I plan to use. Does anybody foresee a problem with me using this tire? Longer tread life is a consideration.

**Blueberry** · 05-25-2011, 09:34 AM

Motorcycle tires are made to run on the edge when turning. Car tires are not. Two highly useful items but IMHO not interchangeable.

**brach77** · 05-25-2011, 09:40 AM

going dark side is done on touring bikes that tend to run lots of miles on the interstate. It can and has been done many times and if i remember right Flitecontrol just did this change to one of his bikes.

**Twister** · 05-25-2011, 12:45 PM

Yep it's been done before on heavier bikes. You would probably have to run your air at 28 psi tops, perhaps way lower if you want to keep more tread on the asphalt. On the other hand the less pressure you have the softer the sidewalls will be, radial tires are way softer on the walls compared to bias. So it would be a matter of looking for that sweet spot of air pressure to have the perfect balance between traction in curves and stability

I just did it on my GL, 2 up plus gear without a trailer is over 1000 lbs. 165-80-15; I'm playing with the pressure to see how it handles with more and less air. And also waiting for clutch friction plates to arrive on the mail from bike bandit, so the GL is grounded for right now.

I was looking at mounting a car tire on my rebel when the dunlop 404 140-90R15 that is currently on, gives up, several thousand miles down the road. My bike is stock, so I was looking at 135-90-15.The 140 that I currently have barely (1/16th") misses the fender nuts. If you are going to use a wider fender, and remove your chain guard, I don't see a problem of it fitting there. Please keep us posted, specially the handling changes while cornering at lower speeds. Oh also it would be great if you post before and after pics.

A lot of people will tell you that your bike will go down in flames and stuff. The rear car tire does NOT ride on the sidewall and even if you lean it to scrape the pegs, it will still have at least twice as much contact area vs. a MC tire. Wet traction increased, longevity increased. It does handle different, but it's pretty much like the handling on a "different bike" but you get used to it by the 2nd or 3rd ride.

Do your research and plan ahead, you don't want to be stuck with a tire you can't use and you can't return. Good luck and keep us posted.

**Buickguy** · 05-25-2011, 01:15 PM

The thing with car tires is not the handling. It can be done on bikes but it does have a cost. The main reason people go with a car rear tire is the increased mileage longevity.

Why does a car tire give more miles than a motorcycle tire? The compound (rubber).
The compound for motorcycle tires is softer. Being softer it will wear quicker.
Harder compounds wear longer. Softer compounds have a greater coefficient of traction.
You do sacrifice using a car tire on the rear. Everything in life is a trade off and there is no such thing as a free lunch.

Motorcycles generally stop faster (less distance) than cars in any given situation (same road conditions, temps and pavement type). Ever wonder why? It isn't weight.
there is a complex physics equation that shows the relationship of weight versus speed. I won't go in to all that but say that the commonly held belief that weight difference between a car and a bike isn't a factor in emergency stopping distance.
It isn't the braking systems. Both vehicles are capable of locking the wheels in a panic stop (without ABS).
What makes the difference is coefficient of traction. The softer compound (motorcycle) tires will hold their grip under greater braking forces.
In short, it takes more force to get a motorcycle tire to skid.
What you sacrifice in using a car tire on the rear is emergency braking distance.
As long as you are aware of that, run what you want.

**Twister** · 05-25-2011, 05:14 PM

Not trying to start a spitting contest; that's not my style. BC I do agree 100% that the softer compound makes for a better grip or traction coefficient. Now you have to multiply that coefficient, whatever that is by the contact patch or contact area (the part of the tire that touches the asphalt) A MC tire contact patch is a tiny oval half the size of a credit card (per tire) yes when you measure it, it's amazing how small it is. A car tire being more cilyndrical and less spherical, when standing up will give you 5 or more times the contact area. Therefore even if it has half the traction coefficient you still come out ahead. I use to be able to stop the GL on a dime, now with the car tire, I only need half a dime, you can NOT lock that rear tire, and believe me I've tried. That's the reason a track mounted vehicle has so much traction.

The same applies to aerodinamic coefficients, you have to multiply it by the frontal area of a vehicle (outline when lookin at it from the front or rear) doesn't matter if it's shape like a bullet, the bigger it gets, the more drag it will develop.

Again, not trying to start an argument, I don't sell car -or motorcycle- tires. I'm not even recommending or endorsing it, I'm just sharing my opinion.

**Buickguy** · 05-25-2011, 06:09 PM

Larger contact patch is less coefficient of traction assuming the same compound, et al. The same weight gets distributed over a larger area. Less psi.
Since snow offers less traction that dry pavement the coefficient of traction versus contact patch can be more easily observed since the example vehicle can easily over drive the coefficient of traction no matter what tire is used. Take a car on snow. Put on 190/80-R15 tires on 5 inch width wheels. It will go, stop and corner better than having the same car with 7 inch wide wheels and 220/60-R15 tires over the same snow.

I'm not saying to not do the automotive tire, just be aware that there are downsides to the increase mileage the tire will go.

Remember what Dark Helmet said. There are two sides to every Schwartz!

**Badger** · 05-25-2011, 06:16 PM

Watching a few motorcycle races, I doubt very seriously that they would corner like that on passenger car tires. I guess bottom line is I would not want to give up any edge I might need in an emergency.

regards
Badger