Here I will make various observations and comments on how the chassis, swing-arm, forks, brakes &c. function, and in some cases how to diagnose minor faults and make improvements. New material will be added regularly, and eventually better organized. Click below to jump directly to an individual topic.

Frame

Tube stiffness

Forks

Shocks

Steering damper

Engine plates

Wheels

Tires

Chain

Transmission ratio

AHRMA rules

Flywheel inertia

Swing-arm

Brake torque link

Tools, mtl.

    The Tables below are ranked by the stressed dimension in order of OD; select the correct Table first. The values appear below the heading.
    A round tube’s wall thickness in either fractional or U. S. gauge size (Columns 1 & 2), ID (Column 3), stiffness (resistance to bending; Column 4) is calculated by (Π/64) ×((D^4)-(d^4)) or .049087 ×((D^4)-(d^4)), the cross-sectional area (Column 5). The weight is in lbs. per linear foot, with steel at .2836 lbs. per cubic inch (Column 6), and stiffness to weight ratio (× 10,000 to make an easy comparison) (Column 7). Compare tubes of the same wall thickness but different OD for a huge change in stiffness but almost no weight penalty.     For oval or elliptical tubing*, which Table to use is based on the orientation of the tube in the swing-arm. The long (major) axis is normally the height (vertical); if so, use this Table. If not, use the short (minor) axis.     For rectangular tubing*, the same rule applies; if the long axis is vertical use this, &c.     For square tubing* positioned parallel to the ground, use the length of one side.     For square tubing* rotated 45° (with its corners vertical & horizontal), use the length of one side × 1.4142 (the sine of 45°). Simply turning the tube so that its greatest dimension becomes its stressed axis greatly increases its bending resistance. Since the relative size of this axis (proportionate to the original dimension) is known, the increase in stiffness can be calculated from the relationship without knowing the specific dimension. The math: 1.4142^4 = 4. The tube’s bending resistance in its new orientation is 400% of the “normal” position, or an increase of 300%. * With these shapes the cross-sectional area, weight, and stiffness/weight values shown in Columns 5-7 are not applicable.

Round tube outside diameter: .875” (7/8”, 22.2mm)

U.S. Gauge

Wall

ID

Stiffness

X-Area

Weight, lbs./ft.

Stiff/Wt. * 10³

1/8”

.125

.6250

.021284

.294524

1.002324

212

11

.120

.6350

.020793

.284628

.968646

215

12

.109

.6570

.019628

.262304

.892673

220

3/32”

.094

.6875

.017808

.230097

.783066

227

14

.083

.7090

.016370

.206516

.702815

233

16

.065

.7450

.013653

.165405

.562906

243

18

.049

.7770

.010882

.127153

.432727

251

20

.035

.8050

.008161

.092363

.314330

260

Round tube outside diameter: .9375” (15/16”, 23.8mm)

U.S. Gauge

Wall

ID

Stiffness

X-Area

Weight, lbs./ft.

Stiff/Wt. * 10³

1/8”

.125

.6875

.026953

.319068

1.085852

248

11

.120

.6975

.026300

.308190

1.048832

251

12

.109

.7195

.024764

.283706

.965508

256

3/32”

.094

.7500

.022387

.248505

.845712

265

14

.083

.7715

.020528

.222813

.758277

271

16

.065

.8075

.017048

.178168

.606341

281

18

.049

.8395

.013538

.136774

.465469

291

20

.035

.8675

.010119

.099235

.337717

300

Round tube outside diameter: 1.000” (1”, 25.4mm)

U.S. Gauge

Wall

ID

Stiffness

X-Area

Weight, lbs./ft.

Stiff/Wt. * 10³

1/8”

.125

.7500

.033556

.343612

1.169380

287

11

.120

.7600

.032711

.331752

1.129018

290

12

.109

.7820

.030731

.305108

1.038344

296

3/32”

.094

.8120

.027747

.267551

.910530

305

14

.083

.8340

.025339

.239110

.813739

311

16

.065

.8700

.020965

.190930

.649773

323

18

.049

.9020

.016594

.146395

.498211

333

20

.035

.9300

.012367

.106107

.361103

342

Round tube outside diameter: 1.125” (1-1/8”, 28.6mm)

U.S. Gauge

Wall

ID

Stiffness

X-Area

Weight, lbs./ft.

Stiff/Wt. * 10³

1/8”

.125

.8750

.049854

.392699

1.336433

373

11

.120

.8850

.048516

.378876

1.289391

376

12

.109

.9070

.045409

.347912

1.184014

384

3/32”

.094

.9375

.04071

.303728

1.033647

394

14

.083

.9590

.037110

.271704

.924663

401

16

.065

.9950

.030516

.216456

.736643

414

18

.049

1.0270

.024021

.165637

.563696

426

20

.035

1.0550

.017818

.119852

.407880

437

Round tube outside diameter: 1.250” (1-1/4”, 31.8mm)

U.S. Gauge

Wall

ID

Stiffness

X-Area

Weight, lbs./ft.

Stiff/Wt. * 103

1/8”

.125

1.0000

.070755

.441786

1.503486

471

11

.120

1.0100

.068762

.426000

1.449763

474

12

.109

1.0320

.064164

.390717

1.329688

483

3/32”

.094

1.0625

.057284

.340544

1.158939

494

14

.083

1.0840

.052065

.304298

1.035587

503

16

.065

1.1200

.042602

.241981

.823510

517

18

.049

1.1520

.033389

.184880

.629184

531

20

.035

1.1800

.024673

.133596

.454654

543

Round tube outside diameter: 1.375” (1-3/8”, 34.9mm)

U.S. Gauge

Wall

ID

Stiffness

X-Area

Weight, lbs./ft.

Stiff/Wt. * 10³

1/8”

.125

1.1250

.096833

.490874

1.670542

580

11

.120

1.1350

.093999

.473124

1.610136

584

12

.109

1.1570

.087497

.433521

1.475359

593

3/32”

.094

1.1875

.077849

.377359

1.284228

606

14

.083

1.2090

.070585

.336892

1.146511

616

16

.065

1.2450

.057525

.267507

.910380

632

18

.049

1.2770

.044924

.204122

.694668

647

20

.035

1.3050

.033093

.147341

.501431

660

Round tube outside diameter: 1.500” (1-1/2”, 38.1mm)

U.S. Gauge

Wall

ID

Stiffness

X-Area

Weight, lbs./ft.

Stiff/Wt. * 10³

1/8”

.125

1.2500

.128663

.539961

1.837595

700

11

.120

1.2600

.124781

.520248

1.770508

705

12

.109

1.2820

.115911

.476325

1.621029

715

3/32”

.094

1.3125

.102836

.414175

1.409520

730

14

.083

1.3340

.093054

.369486

1.257435

740

16

.065

1.3700

.075582

.293032

.997247

758

18

.049

1.4020

.058851

.223364

.760152

774

20

.035

1.4300

.043240

.161085

.548204

789

Round tube outside diameter: 2.000” (2”, 50.8mm)

U.S. Gauge

Wall

ID

Stiffness

X-Area

Weight, lbs./ft.

Stiff/Wt. * 10³

1/8”

.125

1.7500

.325012

.736311

2.505814

1297

11

.120

1.7600

.314399

.708743

2.411994

1303

12

.109

1.7820

.290403

.647542

2.203715

1318

3/32”

.094

1.8125

.255635

.561437

1.910682

1338

14

.083

1.8340

.230048

.499862

1.701130

1352

16

.065

1.8700

.185142

.395134

1.344720

1377

18

.049

1.9020

.142989

.300333

1.022093

1399

20

.035

1.9300

.104317

.216063

.735306

1419

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