How to determine your belt lenght
When changing your Harley Davidson final belt drive you probably have little difficulties finding a replacement belt. Most reproduction belts can be identified by the Harley Davidson replacement numbers.
The problem starts when you have changed the setup of your bike or you are building a new one from scratch and there is no way you can relate to the stock belts that are used.
To get you started there are some little tools that can help you determine the correct belt length for your bike. Also don’t forget to measure the width of your belt. Harley Davidson uses 1 ½”, 1 1/8”, 1” 20mm and 24 mm wide belts. The correct determines on your model/production year and/or if you modified the pulleys.
When using a stock bike but you only changed the front/rear pulley you can use this conversion table to determine the correct belt length.
Number of tooth required on belt with specific drive ratio on stock frames with stock swingarms. Note: 00-07 Softail models add 3 tooth to the belt.
61 Tooth Rear
| Front pulley | Softail | FXR | DYNA |
| 29t | 127 | 130 | 127 |
| 30t | 128 | 132 | 128 |
| 31t | 128 | 132 | 128 |
| 32t | 128 | 132 | 128 |
| 33t | 128 | 132 | 128 |
| 34t | 128 | 132 | 128 |
65 Tooth Rear
| Front pulley | Softail | FXR | DYNA |
| 29t | 128 | 132 | 128 |
| 30t | 128 | 132 | 128 |
| 31t | 128 | 133 | 128 |
| 32t | 130 | 133 | 130 |
| 33t | 130 | 133 | 130 |
| 34t | 130 | 135 | 130 |
70 Tooth Rear
| Front pulley | Softail | FXR | DYNA |
| 29t | 130 | 135 | 130 |
| 30t | 130 | 135 | 130 |
| 31t | 132 | 135 | 132 |
| 32t | 132 | 136 | 132 |
| 33t | 132 | 136 | 132 |
| 34t | 133 | 136 | 133 |
Note: If you also changed the length of your swing arm this table is useless and you will have to start calculating. Luckily the formula to do this isn’t that hard.
3 step formula:
A= number of tooth rear wheel pulley
B= number of tooth transmission pulley
C= center to center distance between both pulleys in mm
Step 1.
( C * 2 ) / 13.8
Step 2.
( A + B ) / 2
Step 3.
Step 1 + Step 2 = number of tooth required on belt.
Example:
A = 70 tooth
B = 34 tooth
C = 584mm
Step 1.
( 584*2 = 1168 ) / 13.8 = 84.6
Step 2.
(34 + 70) / 2 = 52
Step 3.
84.6 + 52 = 136.6 teeth => The recommended belt would be 137 tooth.
Information used from the MCS website.
Hello if I change the rear pulley on my sportster from 61 to 55 teeth and the front pulley stay stock which belt I need to buy?
Hey Shay, Sorry for the late reply. Your comment was ranked as spam so i can only see it once a week in a report.
If you have a stock front sprocket you probably run a 128 teeth belt with a 61 pulley. Changing to a 55 teeth in the back will require a 125 teeth belt.
regards Rick
Hello, can you tell me if it makes any difference if it is a trike or not?
If your using Harley Pulleys and Belts and you can determine the center length between pulleys than you can use this formula.
Is the order of operations incorrect in the example? 37+70/2=52
Shouldn’t it be (37+70)/2=53.5 ?
Hey Daniel,
There was a slight error in the example. It should be 34 instead of 37. But if you’re looking for grammatical errors you’re also right, it should be (34+70)/2=52 🙂
i have a 92 fxr evo and have BDL system but i need to know how to determine belt for primary drive i have 48 tooth engine sproket and 72 clutch basket sproket what is the formula to get my belt length
My first guess would be that if you can determine the center distance between the two sprockets this formula would apply.
On the second hand I think BDL can give you a even quicker solution. If you run a BDL sprocket and clutch than they can tell you which belt you will need on your FXR.
I have a custom build motorcycle with a Kraft Tech rigid frame 180 tire with a 34 tooth 6″ pulley up front and a 70 tooth 12.5″ pulley on the back wheel. The center distance is 20″. I took a string and measured around the pulleys to imatate the belt. I put the string to a tape measure and got 68.5 ” with the wheel all the way forward and 69″ with the wheel all the way back. I got a 133″ belt and it is to long with the wheel all the way back. Can you help me find a belt or do you make custom belts
Hey Colin,
If you use the formula than you will see that you need a 126 tooth belt for your current setup.
that is with the centerwidt of 20″ ( 508mm )
Cheers Rick
The belt formula is awesome. Thanks for listing it