Design A Frame

OK let's get one thing straight: I don't design frames. Other people design them and I make them according to that design. I am an expert at building to spec. I am not an expert at fitting and sizing.

There are many websites that are dedicated to bicycle design. I would recommend 'the bicycle forest'. They have a free applet that you can download and draw a frame. It's a useful resource to see the relationship between various design elements

Some people know what they want in every detail, some people only have a vague idea. For those who need a little help, I have summarized below my thoughts on frame design in three parts: Geometry, Materials and Other Details.

GEOMETRY

A full scale drawing is best, but not practical for most people. I can build a frame if I'm given a few critical specs:

Head tube length : pretty straight forward, a small person rides a small bike with a shorter head tube ( minimum 3 1/2" ) A large frame may have a head tube as  long as 7". Personal preference comes into play here too. A taller head tube results in a more upright position.

Seat tube length : Measured from the centre of the bottom bracket to the top of the seat tube. This is directly related to the inseam measurement of the rider. ( I'm 6' 1"  and have a 35" inseam, I ride a 20" mountain bike  or a 58 cm road bike. ) The smallest frames I have made are 13.5" and the largest : 23"

Effective top tube length : This is a critical spec because it determines the length of the bikes cockpit. Obviously a smaller rider will need   a shorter length, and a taller rider will need more space.  However personal preference is  also a factor. Do you want to be stretched out  or do you like a compact, upright position?

This can lead to confusion because the ACTUAL top tubes on different bikes may be on different angles and attatched at different places. So for the sake of comparison we need to measure a  horizontal line from the top-centre of the head tube to where it would intersect the seat post of the bike . ( please refer to the diagram spec #5)

Head tube angle : 90% of all hardtail mountain bikes are 70 to 71 degrees, very small frames are slacker to push out the front wheel, very large frames are steeper to limit the overall wheelbase. Cyclcross range from 70 -72 degrees; road bikes can be as steep as 73 degrees. As a general rule: steeper means tighter , quicker steering with less stability. Slacker angles would achieve the opposite.

Seat tube angle : A steeper angle puts the rider more directly above the bottom bracket. A slacker angle can lengthen the top tube without stretching the wheelbase too much. Almost all mountain bikes are around 73 degrees. There is more variation in road/ cyclocross. Generally a very tall rider with a big frame would be slacker and a very short rider with a small frame would be steeper.

Chain Stay length : This is mostly related to wheel size/ type. Most 26" wheel MTbikes are 425mm (16.75")  A 29" wheel MTbike would be more like 445mm ( 17.5")  a road bike : 400mm (16"); cyclocross: 420mm (16.5"). As a general rule, the longer the chainstay, the more stable the ride. However, shorter chainstays provide a stiffer, faster handling frame

Bottom bracket height : Since the height of the bottom bracket off the ground is affected by the choice of tire, it is more useful to the framebuilder to know the bottom bracket drop. This is the difference between the height of the wheel axle and the height of the bottom bracket. Most 26" wheel MTbikes have a 35 -37 mm drop;  29er Mtbikes and cyclocross would be around 62 mm; road bikes around 70 mm.

Fork length : Different forks will have a different length from the axle to fork crown. A rigid road fork is about 380 mm, cyclocross - 400 mm, 4" Mtb fork is about 460mm, 5" travel fork 485 , 29er fork - 465mm. The important thing is that I need to know what kind of fork you intend to use.

MATERIALS

There are only a few high quality , bicycle tube manufacturers. I have used Tange, True Temper, Reynolds and Dedacciai. They all carry a range of high quality, seamless tubing.

What I have found in the course of making thousands of frames is that the dimensions of the tube are more important than the brand name. Different parts of the frame are subject to varying amounts of stress and my goal is to make a frame as light and as durable as possible. For a steel , hardtail, mountain bike, these are my recommendations for tube diameter/ thickness:

Head tube - must be at least 1.2 mm thick

Seat tube - can be 28.6 mm or 32.8 mm in diameter but should be at least .8 mm thick at the bottom end and 1.2 mm thick at the top end.

Down tube - possibly the most stressed tube on a bike, should be at least 34 mm in diameter and .9 mm thick at the ends. Some of the newer alloys that claim a higher tensile strength have a thickness of .8mm. This makes for a lighter frame but may not be strong enough for rough use

Top tube - can be 28.6 - 34 mm in diameter, must be at least .8mm thick at the ends

Seat stays - usally tapered, 16 or 19 mm in diameter at the upper end and should be at least .7mm thick

Chainstays -  are a high stress area and require clearance for chainrings and tires. I like to use  stays that are ovalized, bent, and tapered and at least .8mm thick

OTHER DETAILS

Frames will require braze-ons for cable guides. hydraulic brake tubing, and water bottle cages. I have standard configurations but can do something different if the customer requires it.

Slot for seat tube - the frames are made to fit a 27.2 mm seat post. The seat tube extends 60 mm above the top tube and a slot is cut in the front of the tube. This is my standard set-up but can be changed upon request

Rear axle spacing - the standard is 135 mm for MTB and cyclocross ; 130 mm for road. This can be changed upon request

BB shell width - I almost always build frames using a 68 mm BB shell. However 73 mm shells are also available

Choice of brakes - most MTbikes these days use disc brakes. This requires a disc brake mount on the seat stay and a reinforcement strut. I also make 'zip guides' which hydraulic lines can be secured to with a 'zip tie'. However some people prefer V-brakes. Road bikes and cyclocross bikes  use caliper, and cantilever brakes respectively. I need to know what kind of brakes the customer will use.