I recently fell over while riding slowly on slippery pavement. Bruised my hip a bit and messed up my derailleur. I don't think I hit my head, but it all happened very quickly. I'm aware that helmets are only designed to protect you from one crash, and have to be replaced afterwards, but I assume that only applies if the helmet actually hit the pavement (and prevented my head from doing so). I've inspected the helmet and don't see any obvious damage, or even any markings on it from the road. Is there a way I can tell if the helmet is still safe to wear, or if I should replace it?
If your head didn't hit the road, the helmet absorbed no impact so it will be as good as before the event.
You're not sure whether the helmet did take an impact - which is reasonable. A lot happens in a short time while crashing. So check:
- the outside of the helmet - you know roughly what part of the helmet would have hit the ground. If there are more scratches than elsewhere on the outside, or a dent or damage to the outer plastic, then "yes"
- your own head - if it's sore at all, or if your chin straps have bruised your chin, or your ears got yanked, then you did hit the ground, so "yes"
- the inside of the helmet, specifically the foam. If it seems different in any area, or if you find any cracks or separation of the foam from the plastic outer then the helmet is compromised. Perhaps the fall, or perhaps it's getting old.
- witnesses: if anyone observed the accident, a co-rider or a passer-by, it is worth asking "did my head hit the road?" which gives another viewpoint.
Separately, if your helmet is over 10 years old it's probably due for replacement. There's plenty of debate whether a helmet has a lifespan of 5 or 10 years, but over 10 and it's probably due no matter what happened in your fall.
Ultimately it's up to you - no-one here can categorically answer your question. If the helmet was damaged, suffering the same fall again will cause a higher transmission of shock, right up to 100% and possibly even beyond.
Your description sounds exactly like my experience of an ice fall. It is over amazingly quickly - you don't get time to get a foot anywhere useful, so your hip hits the deck and takes most of the impact.
The good news is that your shoulder also takes some impact, and to a lesser extent the arm/elbow on that side. In my fall, I bruised my hip badly and mild bruising on the shoulder. The elbow was scratched, but the helmet was undamaged in my fall. Yours could have been different, so check helmet closely and make your choice as a responsible adult who is liable for the consequences of that choice.
Although it is definitely a good idea to err on the side of caution when it comes to head protection, and a seriously crashed helmet will not give full protection anymore, there's no need to replace your helmet after each and every crash. Even if you did hit the head lightly – at slow speed and if your hips took most of the impact, then the helmet should still be good. Only perhaps a low quality helmet would crack.
Generally speaking, the “don't use after crash” rule does apply to all helmets, whether cheap or premium quality, because the squishing of the foam filling is a design feature (a fully rigid helmet would harm the head inside). So don't assume even an expensive full-face helmet will still be safe after a crash with an actual head collision. But the foam is specifically designed to squish at high-but-still-safe pressure levels. If it were too easily squished, then it wouldn't be able to properly feather against really hard impacts. So an impact that you're not even sure happened at all should not have squished the foam. Of course, it's still good to check carefully.
If the helmet is scratched a little bit on the outside, that's probably no problem. A scratch from rubbing over the pavement, without a hard impact, doesn't affect the buffer zone thickness. Only a deep scratch would compromise the shell's rigidity or lead to cracks.
This answer covers an assessing and visually undamaged helmet, the answer by @Craggie that covers how to inspect for damage.
This study indicates that the helmet is almost certainly safe to use.
MEA and collaborator Collision Analysis collected 1,500 used helmets from consumers and eliminated any that showed damage or did not have date of manufacture stickers. The helmets studied had dates that ranged from 1987 to 2013.
They crash tested them ...
There were only four that exceeded the 300g maximum threshold: three of the oldest models made to meet only the old ANSI standard, and one newer model that had been recalled. So 671 of the helmets passed the current CPSC impact performance standard.
So based on the results of this study, if your helmet passes a visual inspection, it will be as safe to use as before the crash.