Short Summary-
Today involved more welding practice, finishing the lecture on frame geometry and creating our full scale frame drawing. From this frame drawing, dimensions were taken, and machining dimensions for tomorrow were taken.
I'm building a cyclocross frame, which look most similar to road bikes, but have several key differences from road bikes.
Longer chainstays to account for Tires- Cyclocross frames require clearance for slightly fatter tires and the mud accumulation from the courses. Road tires are typically 21-28mm in width (with the most widely accepted size 23mm), while cyclocross tires run from 30-35mm and have some type of tread. Unlike road tires that are typically pumped up to 120 psi, cyclocross tires are run at much lower pressures for greater traction, typically 35psi for clincher tires. In cyclocross, tire selection and tire pressure are extremely important and largely determined by course conditions which differ greatly in cross.
Traditional geometries with top tube cable routing for carrying Cross Bike- Cyclocross involves riding over all types of course conditions, including (but not limited to), road, gravel, sand, grass, and mud pits. Cross races are filled with technical sections that are either faster to run with the bike, or made impossible to ride over and force dismounting and carrying the bike, such as barriers. Compact frame geometries with sloping top tubes are less commonly seen in cyclocross than they in road bicycles due to the need to easily carry the bicycle on the shoulder. Top tube routed derailleur cables are also used to help combat mud and make carrying the bike easier.
Higher Bottom Brackets and a more upright geometry-
Cyclocross bikes typically have a more upright geometry than road bikes that are more concerned with the aerodynamic benefit of being as low as possible. A more upright geometry is more beneficial for slower speeds and for the technical handling that cyclocross demands. Bottom brackets are sometimes raised off the ground to aid in clearance over rough ground.
Cyclocross Forks
Cyclocross Forks typically have more rake (fork offset) than traditional road frames. (see longer section for explanation of rake and trail). More rake causes slower steering, longer wheelbase, less stability at high speeds but very stable at lower speeds, more shock absorption. Cyclocross forks are also mounted with cantilever brake bosses with cantilever or V-brakes as discussed in Day 1's post. Unfortunately, the UCI (the governing body of cycling) has banned the use of disc brakes in cyclocross, which are superior to the more traditional brake options available. While disc brakes are now allowed for non-UCI events (which is basically all racing most people would be doing, bike companies emulate the professionals instead of catering to the public, which is a shame.
Frame geometries typically vary very little, as over 100 years of frame building has established a range of values that work for almost everyone. Commercially available bikes will fit almost everyone except for really short and really tall riders, with some small adjustments made. The other exception I will make is for women, who proportionately have longer legs and shorter torso's than men, for whom frame geometry is based on.
However, small changes in frame geometry make a difference. Switching the seat tube angle 2 degrees from 73 to 75 degrees will have an enormous difference on the feel and handling of the bike, and will alter the rest of the frame dimensions and angles.
What handles well for one person may handle horribly for an identically sized individual. Changes that make a bike more responsive may be twitchier to others just as changes that make the bike more stable for some may make the bike sluggish for others. More aggressive handling is usually made at the expense of rider comfort. While essential to professionals, such geometry is usually not as important as a bike that rides well to the average rider/ racer. Some bike companies acknowledge this difference by offering frames that are generally sold to the public and frames used by the professionals.
Ultimately, designing a frame is about tradeoffs and finding the desired balance of several attributes. While I love cyclocross and cross bikes in general, I was never floored by my Scott cross bike. The bike has a slight compact geometry, routes shifting cables along the downtube, runs compact (34/48) instead of true cross gearing (36/48) and for me has toe overlap, which is annoying when constantly remounting in a cross race. I've also always been between a 54 and 56cm bike (or between M and M/L), so I look forward to designing a bike with a desired 550mm top tube.
Addendum 1- For those joining Penn Cycling and thinking about using a cross bike to race Cross and road cycling....
Having a cross bike and a road bike, I can definitely say there is a difference in the frame geometry and how the bikes handles. Other than changing tires, the biggest difference between road bikes and cross bikes are geometry, gearing and braking. You will typically want more difficult and more diverse gearing for road racing (although plenty of people successfully race on compact gearing all the time.) The more important difference is braking, as cyclocross brakes just don't work as well as dual caliper brakes found on road bikes, which becomes a problem in a group of riders who are all braking at one rate as you brake at another. Easy solution is to use V brakes with travel agents, which I've seen done very successfully.
If your going to only buy one bike and want to focus on road, buy a road bike, as while a cross bike will allow you to do both road and mountain biking, it wont allow you to do them as well as the bikes designed for that specific purpose. If you already own a mountain bike, you can use that for cross, and for some courses, mountain bikes are probably better equipped for the course conditions than cross bikes.
However if you want to in fact focus on cyclocross, or are more cross focused with the interest of riding with road riders and the occasional road race, then a cross bike could serve that purpose.
L o n g Technical Section - Commonly used frame dimensions and their implications of ride quality (Tomorrow I show you how to use these frame dimensions in designing your own custom geometry).
1. Rim Bead Seat Diameter
26" wheel= 559 mm
24" wheel= 507 mm
20" wheel= 406 mm
29" wheel= 406 mm
650B wheel (between 26" and 29" wheel= 584 mm
27" wheel (old road bike size)= 630 mm
700c wheel (road tire)= 622 mm (not 700, 700cc has no numerical importance)
650c wheel (tiny frame wheels)= 571 mm
Rim beat seat diameter is the diameter of the wheel- the rim bead depth where clinchers sit.
2. Tire profile (larger range values used)
MTB= 50mm
Road= 25mm
Cross/ Touring= 35mm
This is the distance from the outside edge of the tire to the bead seat diameter line (usually the center of the rim).
3. Wheel diameter
Wheel diameter= (Tire profile x 2) + Rim Bead Seat Diameter
Wheel diameter is an outside measurement of the wheel in mm with a fully inflated tired.
4. Bottom Bracket Drop- y distance from center of wheel axis to center of bottom bracket
Mtn Bike:
Range: 0-45 mm, Most Common: 25-35 mm, Most used: 30 mm
Road Bike:
Range: 50-75 mm, Most common: 65-75 mm, Most used: 70 mm
Cross:
Most used= 60 mm
Height of tires will already bring BB height up further than road bikes.
Less drop means a higher and stiffer bottom bracket and shorter chain stays, with more cornering clearance but less stability, good for racing bikes. More drop means a softer ride but less efficient torque transmission and longer chain stays , more stability, but less cornering and tire clearance, good for touring bikes.
5. Seat tube angle- acute angle from seat tube to horizontal line of wheel axis
26" MTB:
Range: 72-74, Most Used: 73
700C Road Bike:
Range: 72-76, Most Used: 73
A shallow angle (72) means a softer ride, with more weight over the rear wheel, but more difficult to spin with a high cadence, suitable for long femured riders and touring bikes. A steeper angle (74-75) produces a stiffer ride, with more weight distribution over the front wheel. It is easier to spin with a high cadence, which is why its used for racing or tri bikes.
Notes:
1) 1 degree equals approximately 20 mm horizontally
2) Steepening the seat tube angle is used to minimize toe clip overlap
3) Seat Tube angle assumes bike with setback. You need to use fewer degrees for Thompson seatposts that have No setback.
6. Seat Tube Length
Seat tube length is derived from your inseam measurement taken from the pelvic arch in your groin to the ground.
Seat tube length for road bikes= insteam x .65
Seat tube length for MTN bikes= inseam x .56
Seat tube length can also be calculated by
Seat tube= (Inseam x 1.09) - (Seat Height + Crank Length - Pedal Offset)
7. Top Tube Length
Top tube length is determined by adding arm length (point at which your shoulder rotates to base of wrist) and torso length (shoulder height- inseam measurement)
8. Head Tube Angle
26" Mountain Bike:
Range: 70-72, Most Used: 71
700 C Road Bike:
Range: 72-74
Most Used: 73-74
As the head tube gets steeper, steering quickens but at the expense of shock absorption. Most MTB's use 70-71 with more aggressive MTB's using 71 to 72. Touring and cross bikes generally use a slacker 72 to 73 angle while road racing and sport bikes use 73-74 degree head tube angle.
9. Rake (Fork Offset)= ranges from 35-50 for both MTN and road bikes)
Fork offset is the distance from centerline of fork to the intersection of the imaginary line of the head tube angle extended to the intersection of the horizontal line of the center wheel axis.
Less rake causes quicker steering, shorter wheelbase, less stability at low speeds but more stability at high speeds, less shock absorption, suitable for racing bikes. More rake causes slower steering, longer wheelbase, less stability at high speeds but very stable at lower speeds, more shock absorption, suitable for touring bikes.
10. Trail
MTB: Range 60-80 mm, Most Used: 65-75 mm
Road Bike: Range: 50-70 mm, Most Used: 55-60 mm
More Trail causes a tendency toward more stability at higher speeds. Less trail causes a tendency toward more stability at lower speeds. Trail is affected by both head tube angle and rake.
11. Chainstay Length
26" MTB: Range 420-435mm
700C Road Bike: 405- 425mm
Shorter chainstays mean stiffer bottom bracket, less tire and fender clearance, less shock absorption but more torsionally efficient. Longer chainstays mean more shock absorption, more tire and fender clearance, more heel/ pannier clearance but less torsionally efficient, suitable for touring bikes.
12. Front tire clearance- amount of clearance between tire and bridge and/or fork crown. Higher numbers represent room for fender clearance.
26" MTB: Range: 20-30 mm
700C Road Bike: 10-20mm
13. Bridge/ Crown Brake Reach- Applicable to road bikes only whose caliper brake mounts are centered over the wheel
Road/Short: Range 39-49mm, Default: 44mm
Standard/ Long: Range 47-57mm, Default: 53mm
14. Lower Headset Stack
Both Road and MTB: Range: 12-14 mm, Default: 13 mm
Chris King Headsets are 13.7mm
15. Fork Height
Use specific fork specs from manufacturer, or use commonly used values
Default mountain: 40 mm rake, 450 mm height
Default road: 43 mm, 370 mm
Default cyclocross: 45 mm, 400 mm
Default touring: 50 mm, 400 mm
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