Spend enough time reviewing other people's laps and a pattern shows up. Half the inconsistency I see in data is not driver error. It is the driver moving in the seat. The seat is the one part of a sim rig that touches almost all of your body, and it is the part most people spend the least on. This is how I choose one, and why I think it comes before the wheelbase.
What the seat does to your driving
Your braking reference is a pressure, not a position. Under a hard stop with a load cell pedal you push against the pedal with your leg and the seat pushes back through your hips and your back. If the seat gives, even a little, some of that force goes into compressing foam or flexing a backrest instead of into the pedal, and the pressure you feel does not match the pressure the game sees. Your brain learns the wrong number. On the next lap it is a slightly different wrong number. That is why people on soft seats trail off the brake early and never quite know why.
The same thing happens laterally. A seat without real side bolsters lets your torso lean in long corners, so you brace against the wheel, and a braced arm has no feel for what the front tyres are doing. A proper shell holds your hips and shoulders still and your arms stay light. That is the single biggest change in feel most drivers notice when they move from a gaming chair to a motorsport seat, and it is the change they describe as the wheelbase suddenly feeling better. The wheelbase did not change.
Gaming chair, generic bucket or motorsport seat
A gaming chair is an office chair with a racing stripe. It reclines, it swivels, it rolls, and every one of those movements is a reference point you lose. It is fine for playing. It is wrong for training.
A generic sim bucket, the kind bundled with entry level cockpits, is shaped like a racing seat and is a big improvement over a chair. The shells are usually thin, the bolsters are usually low, and the foam is usually soft, because the seat is priced to be included, not to be chosen. Most people replace it within a year. If you are on a tight budget, it is still better than the chair.
A motorsport seat is built to hold a driver still in a car pulling real g, and everything about it follows from that. A rigid fibreglass or carbon shell, high dense foam, deep bolsters, and mounting through the sides of the shell rather than a swivel base. On a sim you do not need the crash protection, but you get the rigidity and the support for free, and those are exactly the two things the sim needs. Sabelt build seats for professional race teams, and the seats I sell are the same construction with sim specific mounting or, for the motorsport range, the exact seat that goes in the car.
Choosing the driving position
This is the first decision and the one you cannot undo. A GT seat like the SRS-1 is reclined with the side support built for a car that brakes hard and corners long. It suits GT, touring car and prototype racing, and it is the shell I run at home. A formula seat like the SRP-1 lays you down close to horizontal with your legs up, and nothing else replicates the single seater position. If you race open wheel, this is not optional. An upright seat like the SRX-1 is the road car position, which is correct for rally, drift, oval and anyone who drives a bit of everything. If you cannot decide, the upright seat is the least wrong across disciplines.
Size
Motorsport seats come in sizes because bolsters only work if they touch you. Too big and you slide inside the seat, which is the gaming chair problem again. Too small and the bolsters dig into your hips and you stop wanting to drive after forty minutes. The measurement that matters is your hip width sitting down, not your waist standing up, and your shoulder width for seats with shoulder support. The Sabelt sim seats come in one size that fits most adults. The motorsport seats come in Medium, Large and X-Large on models like the Taurus and the GT-PRO, in Large only on the GT-3, and in XL only on the GT-PAD. If you are between sizes, email me your height, weight and hip width and I will tell you which shell.
Shell material
Fibreglass is the standard. It is rigid enough for the job, it is what the SRS-1, SRP-1, SRX-1 and GT-3 are made from, and on a sim the extra stiffness of carbon buys you nothing you can feel. Carbon fibre is lighter and stronger, which matters in a race car where weight is lap time and strength is safety. On a sim it matters if the rig moves, if you lift the seat often, or if you simply want the seat you race in to be the seat you train in. The X-PAD Carbon at 19 lb shipping weight against 30 lb for the fibreglass X-PAD shows the difference.
Homologation
FIA 8855-1999 and FIA 8862-2009 are the two homologation standards you will see on motorsport seats. They matter if the seat is going in a race car, because the series rules will require one or the other. They do not matter for a sim. What they tell you on a sim seat is that the shell was built and tested to hold a human in a crash, which means it will hold you under braking without complaint. The Sabelt sim range, the SRS-1, SRP-1 and SRX-1, is not homologated because it does not need to be, and that is part of why it costs $699 to $899 rather than $929 and up.
Mounting
Motorsport seats are side mounted. Brackets bolt to the sides of the shell and then to the rig or the car floor. Sim seats from Sabelt are also side mounted, which is why they fit any cockpit that takes side brackets. The bracket is sold separately, because it depends on which rig it is going on, and sliders are optional if you share the rig or switch between disciplines. I wrote a separate guide on fitting a Sabelt seat to a Trak Racer or Sim-Lab because that question comes up every week.
What I would buy
For most sim racers, the SRS-1 at $699. It is the shell my coaching drivers run, it fits every Sabelt cockpit and any side mount rig, and it is the biggest single improvement in consistency I know of for the money. For open wheel, the SRP-1. For everything else, the SRX-1. If you race a real car and want to train in the exact seat, the GT-3 is the accessible motorsport shell and the carbon seats are there when the car requires them. And if the answer for your situation is a cheaper seat from someone else, ask me and I will say so.