Modern mountain bikes have thin frames. Even clamping one in a work stand can dent it. So it shouldn’t come as a surprise when even your slow motion couldn’t-unclip-from-the-pedals-in-time sideways fall onto a log leaves its mark.
Of course, there are dents and there are dents. A small, contained dent in the middle of a bigger tube may never cause you problems. A deeper or wider dent – especially in a thinner tube or near a weld – will need frequent checks. A bulge (especially on the down tube near a weld) suggests that the bike has been crashed into something and that the frame absorbed the shock.
What you are checking for in all these cases is a crack that radiates from the dent, or a crack that appears in the welds close to the dent.
It is possible that the dent changed the shape of the tube. That means that the bike geometry may have changed, or that your rear wheel or cranks are now skewed. Check this before riding any further – any rubbing sounds need to be investigated.
A bulge in the frame will definitely signal a changed geometry – check for instance that the front tire doesn’t now rub on the down tube. Even if it doesn’t, the wheel may have moved far enough back that your toes now hit the tire when you turn. Your steering may feel “quicker” than it was before, probably because the steer tube angle is now steeper.
While frame material does have some bearing here, don’t get complacent. Steel and Titanium frames will typically survive dents the best. Aluminum frame tubes are typically larger but thinner, so the dent will be more damaging. Carbon frames seldom dent – they just snap.
So, if you dented the frame, finish the ride, but do so carefully. Inspect the area every time you stop. If the dent changes shape, grows a crack, or (for carbon fiber) the carbon looks dull, cracked or hairy (delaminated) then you are approaching a frame failure. Decide whether the trail is smooth enough to ride, or whether you should walk. Any jarring action is likely to compound the damage.
Tip: There is little you can do to repair an aluminum or carbon frame at home. Even with good welding skills, the aluminum used in bike frames needs to be heat treated afterwards to anneal it, which is beyond the reach of most welding shops.
You could stick carbon frames back together with epoxy and more layers of carbon fiber cloth, but the bike will never be as responsive or strong as before the crash. Calfeedesign.com will fix carbon frames for a reasonable price.