Seems like folks prefer 15 inch and smaller but I'm not sure why?
Seems like folks prefer 15 inch and smaller but I'm not sure why?
You want enough sidewall to keep rocks/stumps/railroad tracks/etc from destroying the wheel. Currently, 13", 14", and 15" are the only sizes with good rally tire availability, and 15" is by far the most popular- I run that size because it's what I can get easily.
For a while there were 16" gravel tires as well, but they seem to have fallen out of favor.
Taller sidewalls also make it harder to debead a tire when you smack it sideways into a rut or something.
You can't find 16" rally tires, they are like diamonds and not the easy to find ones either.
Yep it's not that they like small wheels, it's that they like meaty tires. Same reason offroad 4x4s mostly run 15" or 17" wheels...or maybe 20" if it's a 40"+ tire.
You can see how debeading is a factor in GRC. Teams try to go to lower-profile tires for more grip and better handling because they run on smooth surfaces (yes there are jumps, but the surfaces are smooth). Then they have debeads, then some teams run bead-lockers to solve it!
I see, I would think a taller sidewall would lead to more debeads because of the leverage.
NickyDaFish wrote: I see, I would think a taller sidewall would lead to more debeads because of the leverage.
The extra flex from the impact being further from the bead helps reduce the load on the bead. In addition, the increased sidewall surface area leads to more force holding the bead in place for a given tire pressure.
That second bit is why lower profile tires are often harder to mount (high pressure is needed before the beads will pop into place).
rslifkin wrote: Taller sidewalls also make it harder to debead a tire when you smack it sideways into a rut or something.
This is literally the opposite of how it actually works.
Mister Fister wrote:rslifkin wrote: Taller sidewalls also make it harder to debead a tire when you smack it sideways into a rut or something.This is literally the opposite of how it actually works.
Please explain your thoughts on the matter?
I'm seeing it as this:
A tire at 40 psi with 50 sq inches of surface area on the inside of each sidewall has 2000 lbs of force pushing the sidewalls outwards. A good bit of that is helping to hold the beads in place.
Make that tire taller so it now has 100 sq inches of sidewall surface and that force goes up to 4000 lbs, requiring a bigger impact to overcome it and pull the bead out of place.
With the taller sidewall, assuming the sideways impact occurs at approximately tread level, the sidewall will flex more than with a shorter sidewall. This flex dissipates some of the impact energy, so less energy is transmitted to the bead.
If shorter sidewalls were harder to de-bead, wouldn't most rally cars (and especially rallycross cars) be running 18" wheels with rubber band tires? Especially the rallycross cars that are running snow tires and not limited to 15" wheels by tire availability...
Don't forget the wheel aspect of the equation. A smaller wheel is easier to make rigid, light and strong. Which is all very important on bumpy roads.
I always thought it was for higher ground pressure. Or is that just for the goofy looking snows they use?
rally gravel tires have extremely stiff sidewalls - I can put mine flat on the ground loose off the wheel and stand on the inner bead, and it hardly even flexes under my 175lbs. Tire mounters hate rally tires because they often take 100+ psi to seat the bead. I have never debeaded a rally/gravel tire in 5+ years of rally/rallycross (though it does happen).
The wheels are small so we can have tall sidewalls. Going sideways into a corner full of 6" rocks, if you have a low profile tire you will end up with a bunch of broken wheels. Taller sidewall takes the impacts. I'd like a taller sidewall, personally (we run 185/60/14), but our little 1.8L and gearing limits us if we actually want acceleration. Rally/gravel tires are HEAVY.
NickyDaFish wrote: Seems like folks prefer 15 inch and smaller but I'm not sure why?
Gravel rally tires have a maximum diameter of 15", that's why.
Tarmac tires IIRC have a maximum diameter of 18".
Mister Fister wrote:rslifkin wrote: Taller sidewalls also make it harder to debead a tire when you smack it sideways into a rut or something.This is literally the opposite of how it actually works.
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irish44j wrote: rally gravel tires have extremely stiff sidewalls - I can put mine flat on the ground loose off the wheel and stand on the inner bead, and it hardly even flexes under my 175lbs. Tire mounters hate rally tires because they often take 100+ psi to seat the bead. I have never debeaded a rally/gravel tire in 5+ years of rally/rallycross (though it does happen). The wheels are small so we can have tall sidewalls. Going sideways into a corner full of 6" rocks, if you have a low profile tire you will end up with a bunch of broken wheels. Taller sidewall takes the impacts. I'd like a taller sidewall, personally (we run 185/60/14), but our little 1.8L and gearing limits us if we actually want acceleration. Rally/gravel tires are HEAVY.
All this too. I got a lot faster when I ditched the 65cm tires for some nice 62cm tires. Heavy tires suck! And the new car is going to be running on 58cm because it's going to have NO power. Like maybe 165hp on a good day.
Debeading is easy on a front driver or AWD car since the driving style that gets you the most speed tends to have you nosing into ruts with a slip angle at the front. I've debeaded a rear tire in the RX-7 (heck, you were there that day) but I think it was low at the beginning of the run, it felt funny rolling up to the start line.
And then there was the time I almost debeaded (you can see when it happened...)
And the time I thought I debeaded but didn't...
Oh! And the time I really did debead (in a significantly heavier car)
Also, this is why you want sidewall. And this is the kind of abuse rally tires can take.
Direct cross-hit on train tracks at about 50mph with both right-side wheels. Wheels were heavily bent/broken at the rim as you can see. We only had one spare, and we still limped another 3-4 miles on the stage to the end on one of the flats. That's how stiff rally tire sidewalls are.
irish44j wrote: rally gravel tires have extremely stiff sidewalls - I can put mine flat on the ground loose off the wheel and stand on the inner bead, and it hardly even flexes under my 175lbs. Tire mounters hate rally tires because they often take 100+ psi to seat the bead. I have never debeaded a rally/gravel tire in 5+ years of rally/rallycross (though it does happen). The wheels are small so we can have tall sidewalls. Going sideways into a corner full of 6" rocks, if you have a low profile tire you will end up with a bunch of broken wheels. Taller sidewall takes the impacts. I'd like a taller sidewall, personally (we run 185/60/14), but our little 1.8L and gearing limits us if we actually want acceleration. Rally/gravel tires are HEAVY.
The tire/wheel/drivetrain interplay is starting to make sense now. Any good reccomendations for vendors that sell wheels and tires? Thanks to everyone who has chimed in.
In reply to NickyDaFish:
Hit up Paul E. with Braid wheels, they aren't cheap, but it's unlikely that they'll ever need replacing. For tires, hit up JX2, ask for Eric Jackson and tell him you need gravel pirellis and that Dave Z from Porsche sent you. Every vendor in North America that sells Pirelli rally rubber gets them from JX2 now the Bob Woodman no longer doesn't pirelli.
In reply to NickyDaFish:
What are you trying to do? RallyX vs. Stage Rally vs. just for looks?
Irish44j and I run stock wheels because they're cheap and therefore disposable, and sometimes stock wheels from other cars can give you a lot of good options depending on your bolt pattern- I use stock Saab wheels on my Merkur for example.
¯\_(ツ)_/¯ wrote: In reply to NickyDaFish: What are you trying to do? RallyX vs. Stage Rally vs. just for looks? Irish44j and I run stock wheels because they're cheap and therefore disposable, and sometimes stock wheels from other cars can give you a lot of good options depending on your bolt pattern- I use stock Saab wheels on my Merkur for example.
My Honda has a mix. I have wheels from an AWD Isuzu Impulse, but mostly VW. Just picked up a set of VW Bottlecaps to put the tractionized winters on. Only the wheels that have the street tires are Honda!
This car always seemed to have quite large wheels for a rally car and it's one of the most successful ones.
In reply to kanaric:
The 80s was the era of 16" wheels I think- the only reason I know that is from reading the Ford Sierra prep manual!
If 15 inch gravel tires are so standard and 16s are so hard to find how are people rallying WRX and EVOs that OEM need a minimum of 17s to clear brakes?
Downsized brakes.
In reply to kanaric:
the subaru guys retrofit on 4 pot calipers to run 15" wheel and tire packages
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