Convert 29er to 27.5?

[sam.spoons]

I have a Orbea Urrun 30, size small as I am 5' 6"/168cm. Despite being close to the top end of the hight range for this frame, as recommended by Orbea, I find the standover hight is pretty marginal. I'm thinking of upgrading to some new 27.5" wheels, partly to lose a little weight, partly to gain a few mm standover clearance and partly in the hopes of gaining a little agility on the trail (my other, analogue, bikes are old school and run 26" wheels). The main disadvantage I can see is a reduction of BB height, from 322mm to around 295mm depending on which tyres I choose. Has anybody done something similar? Have I missed anything obvious that makes this a stupid idea?

 

[Bummers]

That's quite a drop in the bb. What length cranks are you using?

 

[KnollyBro]

I have a Orbea Urrun 30, size small as I am 5' 6"/168cm. Despite being close to the top end of the hight range for this frame, as recommended by Orbea, I find the standover hight is pretty marginal. I'm thinking of upgrading to some new 27.5" wheels, partly to lose a little weight, partly to gain a few mm standover clearance and partly in the hopes of gaining a little agility on the trail (my other, analogue, bikes are old school and run 26" wheels). The main disadvantage I can see is a reduction of BB height, from 322mm to around 295mm depending on which tyres I choose. Has anybody done something similar? Have I missed anything obvious that makes this a stupid idea?

Other than it being an expensive experiment, I am wondering why stand over height matters on bike sizing? If your reach is fine and your saddle is at the right height for pedalling, the only reason I would do it would be see if the bike rides better on the trails you like to ride. I would try to borrow a set of wheels (even rent a wheelset or a bike with 27.5 wheels on it) and try them on your bike to see if make a big difference.

 

[sam.spoons]

They are 170's, my Orange (17" G3) has 293.5mm BB hight and came with 175's.

Standover matters from a confidence PoV, I've been mountain biking since the late '80s and, back then, a few inches 'ballroom' was considered essential. I have none on the Orbea, despite buying the smallest frame they make.

The rent or borrow idea makes sense but I may well be up for a new set of wheels even if I stick with 29".

 

[B1rdie]

Maybe you will feel the bike good when stopped and bad while riding? I would rather ask the manufacturer what they have to say about ...

 

[Shjay]

FS generally have a lower stand over than their HT counterparts. Am guessing you need your feet flat on the floor at all times whilst on the bike

 

[sam.spoons]

Fair comment, though it's not so much confidence when stoped as confidence when riding a nadgery trail where dabbing is a likelyhood. I'm too old for fast downhills to hold any attraction these days and the consequences of a big crash become more onerous as you age so I prefer technical to 'balls out' rides where agility and control are important. The Urrun has a ridiculously slack head angle and short reach compared to my Orange and the Saracen race frame (nearly 8º slacker) but I haven't seen an eMTB with anywhere near as steep a head angle as my old school bikes.

The smaller wheels shouldn't alter the overall geometry just the ride height. I'll ask Orbea what they think though.

 

[sam.spoons]

FS generally have a lower stand over than their HT counterparts. Am guessing you need your feet flat on the floor at all times whilst on the bike

Not at all, I usually have them on the pedals I ride SPD's and don't lack confidence as such but the ability to dab when things get hairy is, undoubtedly, useful. The high top tube does limit that ability unnecessarily. FWIW I have a 30" inside leg measurement and the standover on the Urrun is 31½'.

 

[B1rdie]

Fair comment, though it's not so much confidence when stoped as confidence when riding a nadgery trail where dabbing is a likelyhood. I'm too old for fast downhills to hold any attraction these days and the consequences of a big crash become more onerous as you age so I prefer technical to 'balls out' rides where agility and control are important. The Urrun has a ridiculously slack head angle and short reach compared to my Orange and the Saracen race frame (nearly 8º slacker) but I haven't seen an eMTB with anywhere near as steep a head angle as my old school bikes.

The smaller wheels shouldn't alter the overall geometry just the ride height. I'll ask Orbea what they think though.

Its not about geometry, but I believe that the rear suspension might end working out of the design scope and need some adjustment.

 

[Shjay]

Not at all, I usually have them on the pedals I ride SPD's and don't lack confidence as such but the ability to dab when things get hairy is, undoubtedly, useful. The high top tube does limit that ability unnecessarily. FWIW I have a 30" inside leg measurement and the standover on the Urrun is 31½'.

Can I ask why a hard tail over a full suss apart from price? A full suss has a significantly lower standover to a hard tail

 

[sam.spoons]

I've always ridden hardtail (I did own a full suss way back but didn't get on with it) and that, plus the weight/build/cost equation meant I was looking for a hardtail. If I was doing it again I might consider a full suss but I'm pretty happy that the Urrun is a good compromise for me at the price.

TBH I hadn't even looked at the geometry of the Rise but you're right the standover is 3 ½" lower. I can't understand why they designed the Urrun frame to be so tall.

 

[Shjay]

Unfortunately most people want to ride full suss & because of that there’s less choice of hard tail bike out there & almost like they can’t be arsed

 

[Paulquattro]

I done just this
But only because i like the feel and handling of 27.5" wheels better than 29"
It wont be as much as you think in BB drop Generally tyre dependent around 17mm give or take
As already said shorter cranks like a 155mm and a 10mm bump with an air shaft in the forks all helps
I personally haven't regretted it one bit as the bike is much more agile and fun which for me is more important than a second here or there in speed .

 

[sam.spoons]

Thanks FR. The standover height is only one of the reasons I'm considering the change. It's good to hear that there is a noticeable improvement in agility as that's probably the biggest negative about the eMTB. I never had/have a problem with pedal strikes on the Orange which has a BB hight of 293.5 even when riding the original 175 cranks (which I rode for years before changing to, IIRC, 165s). If the actual drop in BB height is under 20mm I'll still have more pedal clearance than on the Orange even with 165s on it (though the longer wheelbase will reduce the effective clearance a little over rocks/bumps). Either way the reduced weight/rotational inertia of the 27.5"s can only improve the ride, especially if I go for lighter wheels and tyres.

 

[B1rdie]

Its not about geometry, but I believe that the rear suspension might end working out of the design scope and need some adjustment.

Oops, sorry. I had not noticed that this bike is a hardtail, so, absolutely a change in wheelsize is not going to disturb its rear suspension.
The top tube is higher on hardtails because it makes the frame stronger, since there are no moving parts and a shock to help absorb/dissipate the impacts from the terrain.

 

[sam.spoons]

That makes sense

 

[1oldfart]

Where i ride i enjoy a high bike/Ebike.
This is looking for troubles lower most Emountain bikes.
Maybe you can switch to a longuer fork to compensate
but switching Ebike is worth considering.

 

[sam.spoons]

Thanks for the reply, I'm not quite sure what you mean by 'high bike' here? Also what you mean by 'looking for troubles lower most Emountain bikes'?
I suspect switching for a longer fork would be counterproductive if my understanding of bike geometry is good, it will slacken the already slack head angle even further and also increase the ride height/standover clearance back to where I started. But the bike may be a little more agile with the smaller/lighter wheels?
By 'switching ebike' do you mean sell the Urrun and buy something different? That's probably not an option right now and looking at the geometry of most bikes they are all much slacker than I'm used to so I think the only benefit would come from finding one with a smaller frame and 27'5" wheels and there ain't many of those about.

 

[1oldfart]

You might be far from average. Your legs are probably on the short side.
Like me you can probably only select a bike by trying it. How tall we are
is the wrong way to select frame size.
If you are short LIV might be an option. Their 29 wheels are 27.5 for
their smallest frame. This is just an option

Embolden E+ 2 (2022) | Women bike | Liv Cycling Canada

www.liv-cycling.com

This is an old geometry so switching from 130mm to a 140 or 150 fork is no problem.
Also going with wider tires also helps. It can take 3.0
I use dog trails some times so a high BB helps me to clear bad rocks.
Ebikes are heavier, they are designed with less ground clearance to help
us deal with the extra weight.

 

[sam.spoons]

I'm not particularly short, 5' 6"/168cm and my inseam measurement is at the lower end of typical. but not unusually short (my wife is at the other end and her jeans are 2" too long on me... don't ask me how I know that ).

WRT geometry that Liv is 2º steeper than my Orbea but It has a longer wheelbase and chain stays which reduce manoeuvrability so maybe that's the reason? And it's 3½º slacker than my old Orange, which was pretty relaxed back in the day. If I'm looking for increasing agility surely I'd need to go for shorter forks not longer though (I could try by dumping some air from the forks on the Orbea to find out as long as I don't ride anything to gnarly)?

BTW Urrun has about 27mm more ground clearance than my old Orange so I'm not sure where you got the idea that eMTB have lower BB hight but it's certainly not true in the case of my bikes.

The Orange has an ETT length of 593 plus an 80mm stem, the Urrun 565/35 so it's 73mm shorter, my ageing back quite likes being less stretched out but it's a bit too much shorter, and wheelbase and chain stay length are, understandably, much shorter, so I might try to source a longer stem as I'm used to having my weight a bit more over the front and see if that helps (and it's cheaper than new wheels). Any thoughts?

 

[1oldfart]

I'm not particularly short, 5' 6"/168cm and my inseam measurement is at the lower end of typical. but not unusually short (my wife is at the other end and her jeans are 2" too long on me... don't ask me how I know that ).

WRT geometry that Liv is 2º steeper than my Orbea but It has a longer wheelbase and chain stays which reduce manoeuvrability so maybe that's the reason? And it's 3½º slacker than my old Orange, which was pretty relaxed back in the day. If I'm looking for increasing agility surely I'd need to go for shorter forks not longer though (I could try by dumping some air from the forks on the Orbea to find out as long as I don't ride anything to gnarly)?

BTW Urrun has about 27mm more ground clearance than my old Orange so I'm not sure where you got the idea that eMTB have lower BB hight but it's certainly not true in the case of my bikes.

The Orange has an ETT length of 593 plus an 80mm stem, the Urrun 565/35 so it's 73mm shorter, my ageing back quite likes being less stretched out but it's a bit too much shorter, and wheelbase and chain stay length are, understandably, much shorter, so I might try to source a longer stem as I'm used to having my weight a bit more over the front and see if that helps (and it's cheaper than new wheels). Any thoughts?

What we read and listen to is mainly general stuff but we are unique.
If we are average others might help us but we might have to be curious
and try different options. I am 5feet11 and i love small frame bikes/Ebikes.
I simply modify them raising the saddle and the bar. It is cheap and i
can enjoy hours of riding everyday without pain. My bars are cut at 600mm.
Try riding for 2 hours an Ebike with a size smaller frame than your usual.

 

[sam.spoons]

Changing the bike is not an option but I'm basically happy with the Urrun*, I don't want a full suss or a heavier bike. What I'm trying to do is improve the ride I have without spending another £2k on changing the bike. My analogue bikes are all small frames and, like you, I like them. The two MTBs have much shorter bars than the Urrun and I may shorten the bars on that further (down from 780 to 760 so far) but I doubt I'll go as short as 600mm, that seems excessively short?

* Even small frame eMTBs have much longer wheelbase and chain stays than my analogue bikes so I don't think any eMTB is going to match the agility of my analogue MTBs but I can live with that. If not I may have to consider a custom 26er

 

[1oldfart]

I kind of enjoy having 2 bikes.
A HT 20 pounds, 120mm on 29, it is short.
My Ebike is also 29, FS 130/120 obviously not as short.
It is a Giant/Yamaha and the rear is kind of long.
I simply went from 740 to 720, 700, 680, 620, 600.
Every few months i did cut after testing with my brakes
a little inside.

 

[sam.spoons]

None of mine are that light (20lb=9kg) not even my roadie which is close to 10kg. The Killi Flier bitsa is 11kg in 9 speed form and the Orange is about 13kg.

How did the narrower bars affect the handling?

 

[1oldfart]

I guess if you are into going down very fast it might feel
like you have less control but it does not attrack me.
It allows me to pass others much easier and i avoid trees
in some snowshoes trails where trees are closer than 760.