So far I've only seen very rough estimates when it comes to ordering custom top hats. Such as: if you're running Bilstein's at a street/trackish height, then 1"F/1.5"R are the way to go. But, what if you're 12" hub to fender, or 12.25" hub to fender? I'm currently in the process of tearing my NB hats out to modify them (welding 2.5" tubes in to extend the mounting, and retain factory rubber isolators, etc). For reference, I have a '99 with MSM Bilsteins, ravalved, eBay sleeves with 550/400 6" springs, and 36mm FCM bumpstops.

The height of the top hat determines the bump/droop ratio, which depending on the amount of damper travel, obviously also determines the amount of bump and droop. The goal of selecting the top hat height is to provide enough bump travel for the worst bumps, and having an adequate bump/droop ratio. But, what bump/droop ratio should we be shooting for? Several forum posts mention that 2:1 bump:droop is "ideal", but ideal is not a real word, and every car/track/driver is different.

My main question, does anyone have experience testing different top hat heights, with other variables staying constant? Is 4.5" of damper travel enough for a mid-stiff Miata to just get the piston somewhere near the center of the damper and not worry about maxing out bump or droop travel? With a monotube shock, are there any advantages to having the piston closer or further away from the floating piston? I've been out of the suspension game for a few years, but I'm going to pull up some old FSAE spreadsheets to see how much the wheel actually travels at max lateral G, bumps, etc, for different tire and spring choices.

I took measurements from the center of the shock bolt to the mating surface of the NB top hat at various ride heights. Then using that, subtract the bump stop height, the rubber isolator thickness, and the distance from damper mount hole to the end of the body. This gives bump travel with stock NB hats. Then using the damper travel (minus bump stop thickness) and bump/droop ratio as an input, my spreadsheet outputs a suggested modified top hat height. I will post this spreadsheet when I get the front dampers out and can get a more accurate measurement. The end goal is to collect damper body lengths and travels of the common Miata dampers, such that top hats can be selected before they've ever been installed on the vehicle.

 

I'm running 1 inch front and rear. I found that at 12.5-13 inches the 1.5 rear tophats reduce droop too much for my liking.

 

It'd be best to take measurements yourself at this point. Bump is usually limited by the tire/chassis anyway no matter what your spring rates are. I have 1.75" front droop because I'm using a non-flat (don't know exact measurement) front top hat. If it wasn't, I'd have more droop but would have to look into assist springs (springs would be loose). 2.25" droop in the rear if I recall, but droop is limited by sway bar use anyway.

With 115mm shock stroke up front, with 14kg/mm springs, the shock compresses 80mm MAX and never hits my (too short) 20mm bumpstop. I have some extra stops to slip on but haven't done so. With stickier tires maybe it would.

The longer your shock body is, the larger the top hat you can/should use. Otherwise you'd end up eating bump travel anyway.

HTH.