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The hidden dangers of buying a second-hand bike

Posted: 1 November 2024 Inside the Workshop

A young man rides a bike along a city street, the word 'NO' graffitied behind him on the wall.

Read this before you put any money down on a used bike

"Reluctant sale."

"Barely ridden."

"Meticulously serviced."

That bike you've spotted online looks great, and what a steal.

... Or is it?

Look, we don’t want to tell you to not buy a second-hand bike.

Maybe you want to test out a different riding discipline without spending too much money. Perhaps you'd like to avoid adding another bike to landfill. Or possibly you’ve just got a very limited budget.

All of these reasons are good ones to look into the used bike market.

The problem is that this market is full of bikes which contain some nasty—and expensive—surprises.

A customer recently came in with one he’d purchased from a reseller who said it was in good condition. Once we started working on it, however, we discovered that it wasn’t. The shifters weren’t fully functioning, the rear hub was about to fall apart…and that was just the beginning. That bargain bike ended up not being such a bargain after all.

That bike is, however, the reason why this blog post exists.

A disassembled rear bicycle hub, extremely dirty and muddy.
Just one of the many Easter Eggs we found on it.

Over the past 20 years, we’ve seen too many customers purchase second-hand bikes in good faith which have turned out to have everything from hopelessly seized seat posts and shredded drivetrains to un-serviceable suspension, cracked rims, and even broken frames.

Forget checking tyre treads or looking for rusty cables. We’re going to show you the far more critical things you need to pay attention to—namely suspension, seat posts, service history, frames and wheels—so you don’t waste your hard-earned cash.

Ready to buy? Here’s what to look for in the used bike market, and what to avoid.

Close-up on the dropout of a white bicycle frame, highlighting a crack in the material.
Surprise!

Problem area one: suspension

(Roadies and gravel peeps, don’t tune out; there’s stuff here that’s relevant to you, too.)

You probably know that suspension needs maintenance, but it can be more complex than anticipated, and can turn a seemingly good buy into a nightmare.

Check compatibility

Firstly, ensure the potential bike's suspension is actually still serviceable. Get clear photos of the forks and shock and do some research. Ask your local suspension service centre if they rebuild these models, and how much it typically costs. If they say it’s too old and parts are no longer available, walk away from the sale.

If the suspension turns out to be a proprietary system, proceed with extreme caution. Once a proprietary rear shock is damaged or worn out, you can’t just replace it with another brand or generation – they usually won’t fit the frame. You’re then stuck with a bike with dead suspension that can’t be repaired or replaced.

We go into a lot more detail on proprietary stuff here, but the TLDR is to avoid it in most cases if you can.

A Fox Dyad rear shock on a mountain bike.
If you see this particular setup, seriously rethink your purchase.

Check the condition

Once you’ve confirmed that the suspension can still be serviced, check it for wear.

Examine the stanchions and damper body, especially around the seals. If you see silver aluminium where the anodising has worn off, or if there’s oil residue or a sticky, grimy buildup, that fork or shock probably needs immediate attention, no matter how much the seller assures you that it "rides fine".

You’ll also want to push down on the suspension to test its responsiveness. If it feels rough or sounds squelchy, that also indicates servicing is required, and you’ll need to allow at least $300 per end to cover this, assuming there are no broken internals.

Don’t forget to check the dropper post as well. If it’s springy, it can be a few hundred dollars to service.

Bicycle fork stanchions, the silver sections showing the damage caused by neglect.
What's that sound? That's cash falling out of your wallet!

On road and gravel frames

Check to see if the frame has suspension built into it.

Several major brands incorporate this technology, such as Specialized’s Future Shock, BMC’s Micro Travel Technology, Wilier’s Actiflex system and Trek’s IsoSpeed. Sometimes the design isn't directly referred to as 'suspension', so be aware of alternative terms like the ones above.

If the bike you're looking at has this kind of suspension (no matter what name they're using for it), test it out—wiggle the area and try to get it to move sideways. If there’s lateral movement, don’t buy it.

A man wearing glasses and a trenchcoat, sitting and steepling his fingers as he looks at the ground, deep in concern.
"Can you... weld Actiflex?"

Problem area two: seat posts

A seat post may not seem like a big deal, but can be a huge problem if it’s seized, cut too short or a proprietary system.

Is the seat post stuck in the frame?

Start by trying to adjust the seat height.

If the post won’t move, there’s a good chance it’s corroded inside the frame.

While some posts can be removed via penetrating oils and heat (and, let’s face it, brute force), others can’t come out without damaging the frame. In either case, it can be an expensive, and sometimes un-solvable, problem.

Close up on a mountain bike dropper seat post, the body badly corroded.
Death of a dropper post

Has the seat post been cut?

Certain road, triathlon and time-trial bikes have seat posts which are part of the frame, rather than a separate component.

If the post has been cut to fit the seller’s leg length and you don’t have exactly the same proportions (what are the chances, really?), that saddle height will be wrong for you.

A young man poses next to a road bicycle, wearing full lycra, his foot up on the rear wheel for some reason.
You can still pose with it, even if you can't ride it.

Is it a proprietary seat post design?

Many modern bike designs now use D-shaped or aero-designed seat posts.

This is fine until you realise that each post is designed around a specific frame.

If the post cracks or breaks, replacing it with a non-original part won’t work – you’re reliant on that bike brand for a replacement, and some of them are a lot easier to find than others.

Proprietary posts are more and more common now; they aren’t necessarily a dealbreaker, but they are a potential headache—especially if that model is discontinued.

A bike rider sitting on the edge of a cliff, looking towards the far horizon where a snow-topped mountain looms.
Googling the nearest Ridley dealer.

Problem area three: service history

We've all seen the claims of a bike being regularly looked after, freshly serviced or fastidiously maintained.

The question you should be asking is: by whom?

Without this info, these claims are essentially meaningless.

Ask for service documentation

A reputable bike shop keeps service records. Good ones will detail exactly what has been done to the bike, including what parts have been replaced. Ask the seller to provide this information.

You can also ask the shop directly. We’ve seen listings claiming that a particular bike has ‘just been serviced at BMCR’, when in reality we haven’t seen it in years.

There’s also the possibility that the bike has recently been looked at by a bike shop, but was deemed to need some costly servicing and repairs… and that’s why this bike is now up for sale.

A couple seated at a desk, both looking at a piece of paper and reacting in shock.
"Screw that, this baby's going on Marketplace."

Beware of DIY service work

If the service work hasn’t been done by a reputable shop, be wary.

Many people can competently change cables and wax their own chains, but the vast majority of home-grown service work we’ve seen has been problematic at best.

Overall, if the seller refuses to give you any service history, don’t buy that bike.

A man wearing glasses and business attire, examining the rear wheel on a bicycle.
"I'm gonna need a bigger drill."

Problem area four: frame condition

While frame damage isn’t as common as other issues, it’s a pretty serious problem. Frame repairs are complex and costly (assuming they're actually possible, of course).

How do I check if a carbon frame is OK?

On a carbon frame, look for points of impact.

Inspect the top tube carefully; if the bike’s been in an accident, it’s common for the handlebars to swing around and smash into the frame.

An impact point will often show up in a star shape. If you find one, press on it, hard. If that area is more flexible than the surrounding carbon, it’s cracked and will need to be sent off for repairs.

A white carbon bicycle frame, a crack in the frame faintly visible.
Sometimes cracks aren't that obvious.

How do I check if a metal frame is OK?

Look for any dark marks or lines at the welds.

Check for any bent and dented tubes.

On old steel frames, make sure that the forks haven’t been bent backwards from an impact. There will usually be a matching bend in the top tube and down tube where the frame has collapsed, so take a good look from side-on to see if you can spot this issue.

Corrosion is the other big one. While you can sometimes see it coming through the paintwork, it’s most commonly hidden inside the frame. Check by removing the post (assuming you can) and looking inside with a torch.

Close up on a crack in a titanium bicycle, shot in a workshop.
Sometimes cracks are extremely obvious (if you're looking in the right spot).

Problem area five: wheels

Wheels can take a lot of punishment, especially on downhill and enduro mountain bikes.

Check the spokes and bearings

Are the spokes rusty? If so, mentally add the cost of a new wheelset to your potential purchase. Rusty spokes are very likely to break, and it’s more cost-effective to replace a wheel rather than one single spoke at a time.

Do the bearings feel smooth? Take the wheels out and turn the axle to see if anything feels crunchy or rumbly, indicating wear or damage.

A young man in cycling gear crouches down on the side of the road, looking at the front wheel which he has removed from his bicycle.
"Damn it."

Check for cracks

Alloy rims can develop cracks, particularly around the spoke holes. Often they’re hairline cracks, so inspect the rims carefully.

Be especially vigilant if you’re looking at a downhill bike, a discipline made for smashing wheels (among other things).

Close up on a bicycle wheel showing multiple cracks radiating out from the nipple seat.
This one is less 'hairline' and more 'explode-y', but you get the idea.

Check for rim wear

If you’re looking at a rim brake bike, feel the braking surface of the wheel.

If it’s concave or you feel grooves and uneven patches, it’s worn and needs to be replaced.

Close up of a bicycle wheel, showing the extreme wear on the rim's braking surface.
Less than 1mm between brakes and no brakes

Be wary of carbon wheels

Cheap carbon wheels are bad at the best of times, and second-hand ones are usually worse.

Firstly, if they’re cracked, they’re cactus. That wheel is going in the bin.

Secondly, delamination is a huge problem, usually caused by excessive heat from poor braking technique. It’s also hard to spot, especially if you're going by photographs.

Overall, think twice—or thrice, really—about buying used carbon wheels, especially if they’re rim brake.

Close up on a carbon bicycle wheel, showing damage to the rim.
Yeah, that's not fixable.

What areas shouldn’t I worry about when buying a second-hand bike?

Worn tyres and rusty cables are cheap and easy to fix.

A saddle can be simply changed out, as can pedals.

Same goes for worn bar tape – unless it’s encrusted with salt.

(How do you tell? Smell it. No, really. Salty tape smells like the ninth circle of hell.)

Basically, if a worn tread pattern is the biggest problem with the bike you’re looking at, you’re golden.

Close up on the handlebar tape on a road bicycle, partially unwrapped to show the deposits of salt and sweat.
Bleargh

What if I find the perfect bike?

If you’ve done your initial checks and it’s all looking good, see if the seller is willing to let you take it to a bike shop for an independent assessment. You may have to come to some sort of joint arrangement – e.g. if the shop says there’s less than $X worth of repairs, you’ll buy the bike – but this way you’ve got an expert’s opinion.

A bike shop can also check for things that you may not be able to, such as:

  • Bearing wear
  • Crushed handlebars
  • Worn or damaged fork steerers
Close up on a set of carbon bicycle handlebars, showing a crack in the material.
Things like this.
Close up on the carbon fork steerer from a bicycle, showing a deep groove where the material has been worn away.
And this.
Close up on a bicycle frame's bottom bracket section, showing how the material has broken up through neglect and lack of maintenance.
And this.

The bike shop will need to be paid for their time, but their assessment could save you thousands. It also gives you leverage to negotiate with the seller if issues are found.

If the seller flatly refuses to have an independent assessment done, walk away.

Any other red flags?

Bail on the sale if:

  • It’s a dual-suspension mountain bike and there’s excess play in the back end. (Wiggle the rear wheel to check.) This means that there will be multiple, costly problems with that bike.
  • It's been owned by a pro rider. That bike will be completely trashed. Even if they swear that it was just driven around on top of a car, don't believe them.
  • The seller insists it's been meticulously looked after but the pads/rotors are worn or you can see daylight between the chain and chain ring. No-one maintains a bike well but ignores the chain or pads.
  • The whole bike is spray-painted matt black. It’s probably stolen.
A young man wearing a moustache and beanie, looking wistful. He is sitting on a road bicycle, a 'no bicycles' emblem sign behind him.
"If only there'd been some kind of sign."

Overall, a good rule of thumb is to assume around $1000 in potential repairs and added costs, even if the bike is in reasonable condition.

It may end up being far less than this (or you may decide that your money can be better spent on a new bike with the safeguard of a warranty), but this way you've got a buffer of expectation.

A bearded young man in a forest, his bicycle leaning against a tree. He is examining something near the front wheel's axle.
"Wait, where's the other half of the caliper?"

Summing up

You should never worry about pulling the plug on a second-hand bike sale. There are many, many used bikes out there on the market, and there always will be. By taking your time and making sure to check the areas we've detailed—suspension, seat posts, service history, frames and wheels—, you can save yourself a lot of time and money in the long run.

Even better, get an independent bike shop to assess your potential purchase for extra peace of mind.

Happy bike shopping!

Want us to check out a bike you've got your eye on? We can take care of it.


What You Say

Gave my mountain bike a good sesh on the weekend -- first time since the service. For a four-year-old bike, amazingly it feels better than new! Thanks. Will more than likely get my road bike in for some lovin' in the near future too!Andrew Hawkes
A custom-built Surly Midnight Special bicycle, Perry Winkle's Sparkle blue, viewed against a graffitied wall
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