The products mentioned in this article are selected or reviewed independently by our journalists. When you buy through links on our site we may earn an affiliate commission, but this never influences our opinion.

Bird Forge Stainless SLX custom build review

Traditional tubing meets modern geometry

Our rating 
4.0 out of 5 star rating 4.0
GBP £3,877.00 RRP
inc shipping
Pack shot of a Bird Forge Stainless SLX custom build hardtail mountain bike

Our review

A hugely capable trail hardtail, but stainless steel tubes hike the price considerably
Pros: Excellent geometry gives good handling; fully customisable parts spec; stainless steel frame
Cons: Frame material adds significant cost
Skip to view product specifications

This review has been published as part of our Headline Bikes test, where we put eight trend-setting mountain bikes for 2023 through their paces. Read more about the bikes setting the trend for the year ahead. 

Advertisement MPU reviews

The Bird Forge was first launched in 2021, with the stainless steel version landing this autumn.

Bird has always designed its bikes towards the forefront of modern geometry, with longer reaches, steeper seat tube angles and slack-ish head angles. This is no exception.

Bird Forge Stainless SLX custom build frame

Swinley-based Bird has gone to Californian manufacturers KVA for its stainless steel MS3 tubes.

Bird says the frame should feel pretty much the same on the trail as the Reynolds 853 Forge, but be more impact- and corrosion-resistant, especially with the clearcoat it applies.

The downside is this steel is expensive and hard to work with, reflected in the price.

The reach on my large frame is very long at 496mm, while the bottom bracket is almost scraping the ground at 302mm.

Frame details include ISCG-05 mounts, external cable routing, a nicely forged yoke to give impressive tyre clearance, plus two bottle cage bosses.

Bird Forge Stainless SLX custom build specification

The Shimano SLX drivetrain is paired with an XT shifter.
Andy Lloyd / Our Media

Bird allows you to customise the spec of your bike at the point of purchase. My build may not be cheap, but that’s because the frame is dripping with components that leave little to complain about.

The Shimano SLX drivetrain on my test bike is improved by an XT shifter, the Hope-hubbed wheels come with torque caps pre-installed, and the 200mm dropper gets the saddle right out of the way. These are all neat touches that show Bird really considers its builds.

The dual-piston Formula Cura brakes were hugely impressive during testing.
Andy Lloyd / Our Media

RockShox’s new Pike Ultimate is one of the leading trail forks, and I love Formula’s Cura brakes.

The only element of the spec that I wasn’t sold on was the thin EXO-casing Maxxis tyres – I’d spec some burlier mountain bike tyres to make the most of the geometry.

Builds on this frame start at around £2,300.

Bird Forge Stainless SLX custom build geometry

Chainstay (mm)430430435435
Seat tube (mm)395420445470
Top tube (mm)605630655680
Head tube (mm)100120130140
Wheelbase (mm)1,1831,2101,2421,269
Stack (mm)654663672681
Reach (mm)605473496518
140mm fork150mm fork160mm fork
Axel to crown (mm)551509561516571523
Seat angle – virtual (degrees)7576.874.676.574.276.2
Seat angle – actual (degrees)73.27572.974.772.474.4
Head angle (degrees)6465.863.665.563.265.2
Bottom bracket drop (mm)748770.98567.983
Bottom bracket height (mm)304290307293310295

Bird Forge Stainless SLX custom build ride impressions

The steel tubes and long wheelbase help smooth things over when the going get rough.
Andy Lloyd / Our Media

No frame without rear suspension and 2.4in rubber at the back (rather than a currently-out-of-favour ‘plus-size’ tyre) is ever going to feel ‘smooth’. However, the classically skinny steel tubes on the Forge S give a slight respite compared to a stiff carbon or alloy frame.

Add in a long 1,200mm wheelbase, and the Bird pitches forward and back less over bumps than a shorter bike would, calming the ride in rough terrain. These two elements enable the Forge S to hold good speed off-road, without feeling like it’s rattling your fillings.

In turns, the moderately slack 65.5-degree head angle (with a 140mm fork) combines well with the slammed BB. Your weight is dropped low below the wheel axles, encouraging you to lean the bike to carve corners.

The RockShox Pike Ultimate is a fork that takes some beating.
Andy Lloyd / Our Media

The long front centre, mid-length chainstays (435mm) and low BB make the Forge sound as though it could be a handful in tighter terrain, but I never found it held me back. It’s easy to pivot from side to side, and even encourages the odd cheeky Scandi flick.

Its biggest nemesis is thick, moderately spaced roots, which rob speed more than closely packed equivalents. Here, and over similarly spaced rocks, I felt the EXO-casing tyres were at risk.

Given the speed the Forge S inspires, I’d want to run an EXO+ casing as a minimum, or perhaps a tyre insert.

The EXO-casing Maxxis tyres could be upgraded.
Andy Lloyd / Our Media

Bird’s spec options change with availability; at the time of going to press, an EXO+ Assegai would be a (slower-rolling) option. At least DT Swiss’s broad EX 511 rims offer decent support to the tyres.

Down steep descents, the new Pike’s spring and damping give great support as your weight is pitched forwards. While the front wheel doesn’t look miles ahead of the bar, the Bird never felt nervous as I tipped it into more precipitous terrain.

The Forge has no problem getting up hills.
Andy Lloyd / Our Media

The firm bite and ample power of the dual-piston Cura brakes boost confidence further.

On paper, the 75.2-degree seat tube angle, measured at my pedalling height, doesn’t sound steep, but with the fork sagged, the seat tube becomes more upright, rather than slackening as it would tend to on a full-suspension mountain bike.

As such, it puts you in a good position over the BB. Along with the lack of suspension bob, this helps the bike ascend decently.

Bird Forge Stainless SLX custom build bottom line

Bird’s new Forge is a very capable hardtail mountain bike, with well-considered modern geometry that delivers stellar handling.

Advertisement MPU reviews

The Forge is lavished with high-quality components, but the tyres specced with my test bike struggle to keep up with the pace and there’s no getting away from the high price that accompanies that stainless steel KVA MS3 frame.

Product Specifications


Price br_price, 5, 3, Price, GBP £3877.00
Weight br_weight, 5, 6, Weight, 14kg (L), Array, kg
Brand br_brand, 5, 10, Brand, Bird


Brakes br_brakes, 11, 0, Brakes, Formula Cura 2, 200/180mm rotors
Cranks br_cranks, 11, 0, Cranks, Shimano SLX
Fork br_fork, 11, 0, Fork, RockShox Pike Ultimate, 140mm travel
Frame br_frame, 11, 0, Frame, KVA MS3 stainless steel
Handlebar br_handlebar, 11, 0, Handlebar, Race Face Next R carbon, 800mm
Rear derailleur br_rearDerailleur, 11, 0, Rear derailleur, Shimano SLX (1x12)
Saddle br_saddle, 11, 0, Saddle, Bird Blank
Seatpost br_seatpost, 11, 0, Seatpost, Bird Down Dropper V2.1
Shifter br_shifter, 11, 0, Shifter, Shimano XT
Stem br_stem, 11, 0, Stem, Race Face Turbine R 35, 40mm
Tyres br_tyres, 11, 0, Tyres, Maxxis Minion DHF EXO 3C MaxxTerra 29x2.5in WT (f) and Dissector EXO 3C Maxx-Terra 29x2.4in WT (r)
Wheels br_wheels, 11, 0, Wheels, DT Swiss EX 511 rims on Hope Pro 4 hubs