Do I Need a Visa to Go to Thailand?

U.S. Citizens, Canadian Citizens, most European citizens, and citizens from a total of 40 countries will get an automatic 30-day visa on arrival.

According to the Interior Ministerial Announcements dated 1 October
B.E. 2545 (2002), 20 December B.E. 2545 (2002), 18 October B.E. 2547
(2004) and 6 May B.E. 2548 (2005), passport holders from 40 countries
and 1 special administrative region – Hong Kong SAR – are not required
to obtain a visa when entering Thailand for tourism purposes and will
be permitted to stay in the Kingdom for a period of not exceeding 30
days on each visit.

Thailand's Ministry of Foreign Affairs has a complete list of countries who are eligible for a 30 day visa-on-arrival on their website.

If people are joining the entire 4 weeks of the TdeT it is likely they will want a 60-day visa that they can obtain from a Thai Visa or consulate  before coming.

Riders will also have the option to extend their 30-day visa in Chiang Mai, Bangkok, and Phuket.  If participants are coming for extended periods before and after the Tour they can apply for longer a tourist visa or plan a few trips to neighboring countries (Laos, Cambodia, Malaysia, Myanmar, and others) and they will be granted a new 30-day visa each time they re-enter Thailand (up to 90 days total in the Kingdom out of 180 days consecutive calendar days).