If you're browsing through mountain bike frame listings looking to upgrade your current bike, here's everything you should know before hitting that 'buy' button.
When you’re looking for a new frame there are a series of questions and choices that you have to go through. Choosing a new frame is a big decision and is not something you want to get wrong. After all, it’s pretty much like choosing a whole new bike. The frame will dictate how the bike rides. The frame will dominate your riding experience.
Even with the sheer variety and high quality if complete bikes available in the modern era, there is still something to be said for cherry picking a frame to build up into the best mountain bike for you and your particular sets of requirements and preferences.
Mountain bike frame choices
There are some questions that are easier to answer than others.
There are questions about compatibility that have definite yes and no answers. These are relatively simple things to get right.
But then there are questions that are less black and white. Questions about suitability. Questions about value for money. Questions that have “well, it depends” as the answer.
Let’s deal with the easier compatibility questions first.
Frame compatibility with your existing components
Starting at the front of the bike and working backwards, here are the vital standards to get right.
You’ll need to make sure any new frame accepts the wheel size of your existing wheels. There are three wheel sizes out there. 26in, 27.5″ and 29″.
Even if some wheels will technically fit into the wrong wheel size-d frame (26″ wheels will fit into 27.5″ wheel size-d frames for example) you shouldn’t do this as it will almost certainly mess up the handling.
Assuming your existing fork is a suspension fork, it’s important to match any new frame to the amount of travel that your suspension fork has. In other words, if you have a 140mm travel fork then you need to get a frame designed around a 140mm travel fork.
Having said that, there is a bit of leeway here. It’s generally okay to running a fork that’s slightly longer in travel than the frame is technically designed for ie. running a 160mm fork in a frame designed for a 140mm fork. We’d advise that you don’t want to run more than 20mm over the intended amount. If the fork gets too tall it raises the front end, shortens the reach, slackens the head angle (not always a good thing) and jacks up the BB height.
But we wouldn’t recommend AT ALL getting a frame that’s designed around a longer fork than your existing fork ie. getting a 140mm travel frame and running a 120mm fork in it. Running a shorter fork in a frame will make the bike really sketchy to ride.
This is again related to your fork. Frame head tubes technically only accept certain types of fork steerer tube (1-1/8th, tapered, 1.5in etc) so you need to check whether your existing fork’s steerer tube can fit in any new frame you’re contemplating.
There are however, ‘conversion’ headsets available that allow you to run incorrect steerer tubes in certain frames. The most common conversion headset being used to run older 1-1/8th straight steerer tube forks in new tapered head tube frames.
If you get a frame with a tapered head tube chances are it’ll be able to accept your existing fork. You’ll ‘just’ need the correct headset.
Speaking of headsets. This is the worst part of any bike build project due to the number of different standards out there!
Even sticking with the same general design – say, tapered – there are loads of non-interchangeable standards. Internal, external, integrated, integral.
Unless you’re really strapped for cash, we’d always recommend being prepared to buy a new headset with any new frame. Research the new frame’s headset type and/or contact your chosen retailer to find out what headset to get.
We certainly wouldn’t base any frame choice around what your existing headset is. It’s not worth it. If you have a really expensive existing headset – bung it on eBay and flog it.
This is a bit like the headset problem above but not quite as bad. There are two main types of bottom bracket: threaded or press-fit.
Check your new potential frame’s spec to see what bottom bracket standard it has. Hopefully it’ll be the same as your existing bike and you can just swap it over. If it’s not the same, don’t worry. You can still probably get that new frame. There are very few chainsets that won’t fit in a variety of bottom bracket designs.
For example, it is possible to run your existing press-fit chainset in a new frame that has a threaded bottom bracket shell. You just need to buy a ‘conversion’ bottom bracket. Similarly, you can have a chainset that used to plumb into a threaded BB and you can run it in a press-fit BB frame. Again, just get the required ‘conversion’ bottom bracket.
Some new frames aren’t designed to have a front mech. These frames are specifically designed with modern 1x single-chainring drivetrains.
If you still like having a front mech, or you don’t want to shell out for a new 1x drivetrain just yet, make sure any new frame will accept a front mech.
Seatposts come in three main diameter sizes: 27.2mm, 30.9mm and 31.6mm. If you have a look at your existing seatpost shaft, somewhere on it (usually near the bottom) it will say one of the numbers.
27.2mm isn’t very common on new frames – although it is still out there so watch out as 27.2mm seat tube frames won’t be able to run very many types of dropper posts.
If your existing seatpost is 30.9mm then it will fit any new frame with a 30.9mm seat tube (obviously!) but it will also fit into any new frame that has a 31.6mm seat tube too. You just need to buy a 30.9-31.6mm seatpost shim to convert it.
If your existing seatpost is 31.6mm then it will fit any new frame with a 31.6mm seat tube but it won’t fit any new frame that has a 30.9mm seat tube. Sorry, it’s new seatpost buying time.
This is a biggie. There are various types of rear axle design out there and your existing rear wheel may well have a strong influence in what new frames end up on your shortlist. Having to buy a whole new rear wheel is not a small amount of money.
You may be able to convert your existing rear wheel’s hub to a variety of different axle standards ie. you can convert it from 135mm QR to 12 x 142mm bolt-thru by swapping the end caps of the hub. It’s well worth checking this early on in your search for a new frame.
Your existing rear hub will either be 135mm QR, 12 x 142mm bolt-thru or (if it’s fairly new) it may be 12 x 148mm Boost bolt-thru.
If the new frame you’re contemplating has a Boost back end and your existing rear hub is not Boost, don’t worry. You can get converters to space out 12 x 142mm bolt-thru hubs to fit into Boost back end-ed frames.
Will your desired new frame accept the volume of tyre you want it to? Does it have enough space at the back to run 2.35″ tyres covered in a layer of your local sticky mud?
Some frames – especially 29ers – can be compromised for wide-tyre and/or heavy-mud clearance at the back. Check the new frame’s description to see what tyre volume it can accept up to. In the UK you really want to be running clearance up to a 2.5″ tyre (AKA a 2.35″ tyre covered in muck!)
We see far too many people out there on bikes that are wrong size for them. People who’ve been seduced by a ‘bargain’ only to end up with an ill-fitting bike that harms their riding and limits their fun. Don’t be one of those people.
It can be really, really easy to convince yourself into buying a frame that you know isn’t the right size for you. “I’ll just run a different length stem” or “I can re-adjust my saddle position” or “small bikes handle better” etc etc. All of this is wrong. Don’t do it.
The problem here is that frame sizing – and frame sizing theory – is a bit all over the place. You’ll hear conflicting advice. You’ll read about experienced riders ‘upsizing’ to a larger frame size. You’ll have mates who ride bikes that are too small for them extolling the virtues of small bikes.
Frame sizings based on seat tube length don’t help things either. Seat tube length doesn’t really matter. It’s top tube length (and ‘reach’) and standover that are what you need to look out for.
So don’t browse for a new frame based on its stated frame size. It’ll potentially be misleading. A lot of riders think they’re a Medium/18″ size when in actual fact they’re a Large/20″, for example. You should be browsing for a type of frame first, drawing up a shortlist and THEN inspecting its sizing dimensions.
Standover: just make sure any new frame has enough standover for your leg inseam length. Simple.
Length/reach: this is a bit more complicated. We’re generally fans of longer reach bikes paired with short (35-50mm) stems. If the standover is still acceptable, go for a large a frame as you can. Smaller is not better. Smaller is sketchier and slower. Larger is faster, more stable, more controllable, and actually more nimble as it turns out.
Read more: The complete guide to mountain bike geometry
Questions of suitability and value for money
Now we’re going to discuss the more tricky aspects of choosing a new frame. The questions where there is no obvious right or wrong answer. Questions that are all about “well, it depends”.
Full suspension or hardtail?
A common thing is to go from a hardtail frame to a full suspension frame.
In our opinion the main reason not to go to full suspension is money. Although this is only a single issue, money is a big thing for everyone so it’s a big issue nonetheless.
To buy a decent full suspension frame costs significantly more than buying a decent hardtail frame. Cheap full suspension frames will weigh a lot, may have durability issues and will usually sport outdated geometry.
Full suspension frames can often have more compatibility conflicts (than a hardtail) with your existing stuff, which necessitates you having to buy a significant amount of new bits (forks and wheels mainly).
The debate around full-sus versus hardtail has been around for decades. To put it bluntly, if you have more than £1,000 to spend on a frame, get a full susser. If you have less than £1,000, get a hardtail frame.
Carbon versus aluminium
Often you can find the same frame available in either aluminium or carbon fibre versions. The carbon fibre version will be significantly more expensive.
The general idea is that the carbon frame will be lighter than the aluminium. And it will be. But often the weight difference isn’t actually very much. Often there’s less than a pound between them.
What is often misleading about comparing carbon v alu bikes is that the rest of the stuff on the bike isn’t the same. Alu bikes often get built up with lower spec, cheaper, heavier build kits. Carbon bikes inevitably get build up with top end, expensive, lighter build kits. This gives the illusion that carbon is significantly lighter than aluminium. It often isn’t really.
An aluminium frame built up with high end parts will typically end up being lighter than a carbon frame built up with low end parts. Remember this.
Perhaps the more obtuse debate here is ‘ride feel’. Carbon bikes feel different to aluminium bikes. They’re usually stiffer than aluminium. Carbon bikes can feel more damped. They can also feel more harsh. It depends. A bit of flex in frame chassis can lead to a faster and more involving ride.
A significant development of late has been the introduction of steel into full suspension frames. These steel full sussers can be almost as expensive as carbon frames and yet they weigh more than even aluminium frames. So why make a steel full suspension frame then? Because of how they feel to ride. Chassis feel. That particular blend of give-and-go that makes some bikes more fun to rid then others.
Mail order versus bike shop
We’re not going to debate the whole ‘mail order direct sales versus high street local bike shop’ thing here. But we are going to highlight something practical and financially relevant.
Namely, if you buy a frame via your local bike shop, chances are the whole process will go through without hitch or any stress on your part. They’ll make sure your existing parts fit, or will sort out the correct parts to make it all work. They will also have all the right tools – and decent quality tools too – suitable for the latest standards. Whereas you might be thinking of doing it all with your ‘trusty’ multi-tool and a hammer.
And if you’re going to be approaching your local bike shop anyway to help with building up your new frame, then the grand total of what it will cost won’t be as more expensive as you might think. Any decent independent local bike shop will always help you out and leave you happy.
But we do live in the 21st century. Online mail order bargains are a fact of life. If you’ve got the tools and know-how to build up a new frame on your own, go for it.
Even if you’re going to end up taking your new frame (and your old bike) to your local bike shop and paying the subsequent workshop fee, it will probably still be cheaper overall if the new frame was a significant bargain deal. Local bike shops know this more than anyone and you needn’t worry about offending them by presenting them with a frame bought elsewhere. They’ll be used to it and will welcome the workshop job.
New frame versus whole new bike
Although this whole guide is about how to choose a new frame, we should also point out that it may not be the best option for some people.
Basically, if you’re going to end up shelling out a load of money because you need to buy loads of bits to successfully build up a new frame, you should think long and hard about just getting a whole new bike.
If you’re still riding on old standards – 26in wheels, QR wheels and forks, 1-1/8th head tubes and so on – it’s time to cut your losses and get a whole new bike. Keep your existing bike a spare bike or a ‘winter bike’ or a ‘bike for the kids’ or a ‘loaner bike’. Don’t try and make a 2010 bike into a 2017 bike, it doesn’t really work like that.
But if you’re not having to shell out for loads of extra stuff – or if you want a frame that isn’t available as a complete bike or as a complete bike in the spec you’d like – we hope you’ve found this buyer’s guide helpful.