SkinnyG said:
Before building my Locost (and still now, 12 years later), I waffled back and forth between a stick axle and IRS.
My stick-axle Locost has been on the road, travelled 500km on the Trans Canada to autocross events and back, commuted as a daily for a time, and I've driven it everywhere, rain, shine, snow, even dirt roads (-that- sucked).
I've come to a couple conclusions:
1) If you want comfort, you really wouldn't pick a Lotus 7 or replica thereof. Comfort is not their forte.
2) If, on the street, the stick axle is a limited factor, you are likely traveling faster than you legally should be going. If it got you in trouble, it's going to be a big "off."
3) I'm not totally convinced an IRS is going to net a "lighter overall" weight of vehicle, and maximum lightness is what I want.
I don't believe that swapping TO an IRS is going to be worthwhile for -me-. If I was building a project and I had to source an axle, I'd probably go IRS, but the stick works well enough, and if it came with your donor vehicle, or it's already under your project vehicle, it will work fine.
"Comfort" is a very subjective term and not something everyone's going to agree on. Your experience with it only being an issue at high speed is probably safety-related, but I want to stick to what's comfortable. I have real-world back-to-back experience with both as well and can say that a heavy rear axle and light car will always make for uncomfortable driving, regardless of speed.
For comparison, my brother has a Seven clone, front LS3 engine and straight rear axle. I have a mid-engine "Seven", with IRS. Both weigh the same, have the same power, and are nearly the same size. We each drove both cars down the same street at about 30 mph. At one point, my brother said I needed to slow down for a bump - a 1"-high step in pavement height. I saw it and didn't slow, and my brother was dumbfounded how he could hardly feel the bump. Later when I drove his car, I understood why he spoke up, as hitting the same bump at the same speed was like driving over a curb - it hurt.
Two things were at work, obviously the straight axle versus IRS, but also sprung-to-unsprung weight. His car is front engine while mine is mid, so my car has a lot more weight over the tires that most affect ride comfort, so the weight ratio is a lot higher. Also, because he's using an LS3, he needs a heavy duty rear axle assembly, and this one weighs 150 lbs just for the axle alone, without brakes, wheels, or tires. As a result, the weight riding directly on the ground is a fairly large fraction of the sprung mass riding on it. The result is that a far greater fraction of a given bump is fed straight through into the chassis - and occupants.
It's interesting that this thread came up now, as just last week we were discussing him converting to IRS. I asked why, because the reason matters. If it's for a faster lap time, I doubt the conversion would help, and might even hurt. On the street though is a completely different story and it absolutely will improve the ride.
So at the end of the day, what's "better" and more "comfortable" are however you define them to be.