Both of the frames noted below are 4130 chrome moly steel frames per the Dahon Speed model. I had the first bike, the frame cracked as noted, and replaced with the second frame.

That first frame, used for about 6 years with frequent 14-16 mile rides in dry months, but I only recall using 2 or 3 sets of road tires (I use up tires from other folders with little use that I've bought, before buying new), so I think perhaps 10k miles, I rotate the tires and get good life from them. The second frame doesn't have the miles of the old one yet, I'm nearing the end of life on the second set of tires , I haven't been riding as much as I should, swapped the frame in spring 2022, so guessing perhaps 4-5k miles. I added an improvised deltech cable to the new frame which has been fantastic in reducing stress on the frame hinge, eliminating need for adjustment. If the seat tube holds up, I expect longer life out of the frame.

To compare, my '89 Cannondale road race bike, 35 miles daily when dry for about 15 years, I estimate I put 75k miles on it when it went into storage for a townie. It was on its 3rd wheel set due to fatigue cracks at the spoke holes, on smooth roads. Frame was still going strong.

I didn't take pics of this recent repair, as I suddenly had access to a drill for a brief period, so did the repair quick, as I didn't want to ride the bike until that was done. Here are pics of my previous (first) frame after that crack was first discovered:


Note the earlier Dahon plastic bushing, and the resulting taper in the slot, due to plastic elastic compression under the tightness needed to keep the seatpost from slipping. At that time I was a more svelte 170 lbs. I continued to ride the bike I think for another five years before the cracks on both sides spread aft to along the seatstay welds. Had I drilled holes, and replaced the bushing with aluminum, it might have held off further cracking, don't know. I finally drilled a hole on the left side, turned the bike around to do the right, and saw the cracking was worse and going in two directions, back past the seatstay welds, and vertically down the seat tube, the frame was toast at that point. But in the pic above, that's only about 2 years of 14 mile rides in dry weather, that's only about 5k miles max, perhaps less because most of it was on that one set of tires at that point. That's not much frame life.

I do frequently carry big loads on the bike, but still well within rated limits of me plus grocery load, and just my normal "empty" is on the heavy side, with 4 panniers, trunk bag full of tools and spares, and accessories I have added, fleeces and raingear, puts the weight at about 55 lbs, with no groceries or other cargo. However, ALL of that weight is on racks right over the axles, so the load has a very short path into the axles, it doesn't even stress the hinge more. The seatpost and seat tube load is all due to rider weight, and seatpost extension. I'm pushing 180 lbs these days and riding in hiking boots, but that's still way below the max rider weight of 231 lbs, and I'm 7 inches shorter than their maximum rated rider height of 6'3" if I recall correctly, so the seatpost is inserted well below maximum.

A later Speed frame that I left rackless and rode at my ex-GF's, that one has the seat tube slot in back, plus aluminum bushing, should be better life. But I sure would like a steel bushing there, I don't have lathe access but I may be able to fabricate one from steel sheet, but at high strength, it'll be a bear to get it to roll into a cylinder without it springing back. Most stainless steels will easily air-harden after heating red, so I could anneal, bend, heat, if I had an oxy-acetylene torch...