Is it allowed in Tour De France for cyclists to use triple chainrings on their bikes or they use only double?

Good question. The rules allow it, but common sense does not. As you can see by watching the Tour, a strong rider can ride the steepest roads in Europe with a double.

I'm glad you asked it because I'm often frustrated by beginners buying (and being pressured to buy) road bikes with triples. Triples certainly have their place. They are for loaded touring. If you're going to tour carrying 40 pounds or more then get a triple. Otherwise, get a double. If you live in a very mountainous area then get a compact double.

Triples on road bikes are unnecessary, and harder (especially for beginners) to shift and maintain.

That's my 2 cents worth.

Ben

I've heard that some pros used triples in Italy, not sure what race though.

It was on a section of (supposedly) 30% grade road...

Besides extra weight, etc, of a triple, I think pros don't use them because it would make them slower (uphill) due to the tiny gear inches involved with a triple's smallest gear.

At that point you're barely moving, and they can't afford that.

Many times pros will climb in the big chain ring anyway, only dropping to the small one for the really steep stuff. (On hills that most cyclists would be doing 8 mph, they do 15-20!)

They are allowed to use any combination they feel they need. They can change the whole thing out depending on the terrain for that stage. Wide gear range for MTNS and close range for flatter territory. Yes they are very strong and I doubt if they chose a triple since the bike needs more chain and that can cause more chances of a mechanical problem.

To add to Ben's excellent answer, Tour riders dont use them because the add weight also. When building a very lightweight bike, you don't want a heavier, more complex triple crank on it when you can simply swap out the rear cassette to accomodate the type of riding you will be doing..

I don't actually know for certain. I have never heard of a triple being used so it has never come up. With so many, many rules it's possible triples aren't allowed, but I think they likely are.

They are probably allowed, but all the riders use doubles. They change out the rear cassette as needed if there are tough climbs during the day's stage. They may use a compact crank (36/53) if the climbs are extra difficult.

they are fit enough that they donot need a triple up front anyway