Got to be a contrarian. While I prefer Billy Allen curbs, I’ve used Tom Thumb bits without any issues. They do not poke a horse’s mouth any more than any other curb with a single joint mouthpiece, which means it is rare. As a rule, the folding action depresses the tongue more than it lifts the center, although it can be a problem in a smaller mouth. The Rashid article is nonsense. For one thing, it would apply to a lot more than just Tom Thumb bits. It would apply to ANY curb bit with independent sides, including Billy Allens, Argentine bits, Jr Cowhorse bits, etc! His theory that horses break down the different actions of a bit and then compare them to determine what is wanted is fundamentally flawed. Horses do not ANALYZE bits. They MEMORIZE bits - the total feel of the bit and what gives them relief from the bit.
I’m not a big fan of Tom Thumbs. Most are balanced for a horse who carries his head vertically. Since most western horses do not, the bit is badly balanced for western riding. It will rotate in the mouth under just the weight of the reins until the curb strap engages. This means the horse will lose the "signal" of the mouthpiece and sides rotating prior to the curb strap engaging, giving him a chance to obey before pressure is applied to the mouth. If you want to play with that style of bit, at least get one with curved sides. That will eliminate one of the biggest flaws in the Tom Thumb design.
Most Tom Thumbs are cheap bits, poorly made. As a rule, they have a large knuckle sitting in the horse’s mouth and tend to pick up nicks easier than a better quality bit. Reinsman makes a Tom Thumb bit that is well made, at least, but the majority are not.
And Tom Thumbs DO have a knuckle in the middle of the mouthpiece. Lots of other curbs bits do too, and I dislike that design because it can poke - rare or not, it CAN so why use it?
But I have used it with a total of 4 horses just to see how they would respond. All of them did fine. None of them tossed their heads, acted confused, etc. Just moved it a little in their mouth and quickly figured out what to do based on their memory of similar feeling bits. And rode fine. If you ride with slack much of the time and don’t yank hard on the bit - which no rider should do except in rare emergencies - then a Tom Thumb bit is…OK. Not horrible. Certainly NOT the cause of most of the problems associated with it. Those problems are rooted in bad riders applying pressure and not giving relief, or in yanking the reins - both serious sins! Ride a Tom Thumb with a light hand on a horse who has been taught how to get relief and it will work fine.
There are better designs. If you have a horse who already likes a Tom Thumb - and I know a couple - then nothing wrong with continuing to ride him in it. But if you are looking for a good bit, try something better. I like both solid low port curbs and Billy Allen curbs. If you want a broken mouthpiece in a curb, look for one with TWO joints instead of one. Look at the balance - the bend in the sides should match the angle the horse normally carries his head.
It doesn’t help that some riders buy a Tom Thumb in hopes of teaching their horse a lesson via intimidation and force. But that is part of bad riding, and most of the issues seen with Tom Thumb bits are caused by the rider’s bad riding.
Also: The length of the sides has nothing to do with the mechanical advantage of a curb bit. The mechanical advantage is based on the TOTAL length of the side (cheek) divided by the purchase (upper length).