No, you don't.
I did -- the OEM rack and topcase -- because my wife, riding pillion during two five-day Edelweiss tours we took in 2022 on a C 400 X, liked the ergonomics so much that she suggested that I buy one (of course, the C 400 GT here in the USA). I'm talking about the seat width and padding, the foot and leg positions, and -- this is the relevant part here -- the topcase position and the flat pad that goes on it.
So I didn't want to mess with success, but there are other ways to accomplish this.
If I were starting from scratch, I'd probably go with the BMW luggage rack and the Shad 58X (
https://www.shadusa.com/collections/sh58x-expandable), along with its back pads.
- The BMW luggage rack replaces the passenger grab rails, and is one(1) solid mother... You could lift the bike by it, if you were strong enough. That's what I like about it.
- The BMW topcase is small -- 30L -- and expensive. Nothing particular to recommend that (aside from my wife's liking it).
- That Shad is size-adjustable, a nice feature, so you could have it low-slung for around-town riding and full-size for big trips. And Shad has a nice reputation for quality. (And if you
almost complete the process of buying one, leave it in your on-line shopping cart for a day or two, and Shad will email you a discount coupon.)
- Shad seems to have given some actual thought to the backrest pads, in terms of function and design. This is unlike, say, Givi*, whose hard lump of a so-called pad are worse then nothing, IMO.
Shad sells its own rack structure for mounting their topcase on a C 400 GT, by the way. There's at least one good YT video on that, which I can dig up, given a few minutes. So there's that. But it's not as strong, IMO, as the BMW version.
Or you can attach the 58X to the BMW rack. There are also at least a couple of YT videos describing that process, including one by Scooteria (a German vlogger who specializes in Vespas, and now a Royal Enfield Classic 350, and who used to own two C 400 GTs). I can dig those up for you, too, if you like. This looks pretty simple, and I'm actually still thinking about the Shad.
OTOH, small as the BMW topcase is, I'm also fairly satisfied with it. I'm going on a multi-day trip next month, and I'll be riding solo and putting a luggage bag across the passenger seat, anyway; see this particular pic and caption in my C 400 GT gallery if you're interested:
https://billanddot.com/C400GT/#S-67
But, as I say, starting from scratch, and without my wife's demonstrated comfort in mind, I'd probably do things a little differently.
We'll see what some other folks have to say, although there seem to be only a few of us C 400 GT** owners who are active here.
* (I had two Burgman 650 Execs over the years -- I've been on BurmanUSA.com since 2007 -- and had Givi topcases on both of them. On the first one, I eventually had a local upholsterer construct a one-off backrest for my wife; see
https://www.billanddot.com/burgman/#55 .)
** (BTW, the nomenclature seems to be Cspace400spaceGT. I discussed this with some folks when I joined the forum two years ago; you can also look at the USA site
https://www.bmwmotorcycles.com/en/models/urban_mobility/c400gt.html for confirmation.)
