2557 Swerve design

I am currently designing swerve drive in the offseason for my team and have hit a roadblock. I have been trying to get the belt. that drives the pully that turns the wheel tensioned but all of the belt tensioners online have not been able to do the calculations as the pulleys need to be as close together as possible. Is there any way I can do the calculations by hand or is there a swerve specific calculator? Thanks for any help you can offer.

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If you’re using a VEX-brand belt, you can use this calculator here: WCP - How To: Belts

If your CAD software supports path length dimensions, create a path made up of tangent lines between the pitch circles of the two pulleys, and dimension the path length of the overall path to be the number of teeth multiplied by the pitch of the belt.

Above is what I sketched out in SolidWorks to connect an 18T HTD 5mm pulley with a 36T HTD 5mm pulley using a 50t belt.


Thank you for the response. I guess i should have been a little clearer on what my dilemma is.When i use the wcp belt calculator and i set the minimum distance possible it gives me a distance that is totally unreasonable to fit in a swerve module

When i try to use the calculator and i type in say 3" it automatically adjusts the distance to about 4.5" and there is nothing i can do to get it to give me the dimensions that i need. I know that it is possible looking at some other swerve modules such as the swerve drive specialties one but i have gone through the entire internet looking for a calculator that will give me what i want and have come up short

While the WCP belt calculator is good, it only allows you to use their own belt sizes (3mm and 5mm) and it only allows you to use their own belt groove counts. I prefer this one because you can put in any pitch and any groove number. Timing Pulley Distance Between Centres Calculator


Thank you so much I have no idea how i missed this calculator. I can continue working thanks to you

Be very careful to type in the pitch length as well as selecting your tooth profile. I left it at GT2 and it automatically used a pitch length of 2mm instead of the FRC common 3mm. Ended up with the wrong belt and it cost us a few days

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Yeah, I just ignore the belt profile. I only ever look at the pitch length

Yah I got a good lesson in belt nomenclature and dimensioning after that screw up lol

Hey, look at the upside, you’ll never make that mistake again.


I just wanted to add that there is a reason why many belt calculators enforce minimum distances between pulleys.

A rule of thumb when designing synchronous (toothed) belt drives is that you want at least 60 degrees of wrap, and at least 6 teeth of engagement, on each non-idler pulley.

Further, for every tooth under 6, the drive can carry 20% less torque before the belt slips.

Source: Page #s 12 and 68 of the Gates “Light Power and Precision Manual” (4.0 MB)

When you use a small pulley with a large pulley to obtain a large reduction in a single stage, you need to move the pulleys farther apart to maintain the minimum wrap around the small pulley. You can get around this by adding an idler pulley. The idler can be made adjustable, or you can use the belt drive generator in your CAD program to find the right location for it.

By having a small pulley “too close” to a large pulley, you risk having the belt slip. Depending on how much torque it takes to do this, and… a lot of other stuff, this can either be a fail-safe feature, or a crippling flaw.

For people designing FRC belt drives, I recommend reading

  • “Drive Selection Procedure” (page # 12),
  • “Belt Pitch Selection Guide” (page # 17)
    (that chart is for GT belts, but there are charts in the manual for HTD, XL, and other belts), and
  • “Synchronous Belt Drives – Engineering” (page #s 60-71)

of that manual. There is a lot of information in those 14 pages that is very helpful in designing belt drives, including the center-center equation @Buildboy was asking for.


Where is the center distance formula? I’ve never found it in that manual.

In case it’s not actually in the manual and someone reading this thread wants the formula,

b = 4 * ( belt_teeth * pitch ) - 2 * PI * ( pulley_1_pd + pulley_2_pd)

center_distance = ( b + ( b ^ 2 - 32  * ( ( pulley_1_pd - pulley_2_pd ) ^ 2 ) ) ^ 0.5 ) / 16

On page # 12 (“Drive Selection Procedure – single sided”), and 103 (“Useful Formulas and Calculations”)


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