Hoernle's Belt Tensioner - Detailed Build Instructions

[Hoernle]

Introduction
What is a belt tensioner you might ask?A belt tensioner converts telemetry data from the game into pulling movements of your harness. Simply speaking, you brake and the belts tighten thereby simulating the sustained G-forces that occur in a real race car under braking. Several designs for a belt tensioner already exist since many years, however, I wanted to combine all the important learnings from others with my own ideas and create a new system with the following key properties:

1) 100% free
2) Off the shelf components and 3d prints
3) Maximum compactness
4) High pulling force (24kg/belt)
5) Rigid construction

Importantly, with this project I wanted to give something back to the lovely DIY community in the spirit of Saxxon's original SFX100 project.
Hoernle's Belt Tensioner already exists since June 2022 and several community members have built it.

Check out the forum posts about this project:
English: This thread
German: https://forum.virtualracing.org/threads/hoernles-belt-tensioner-detailierte-bauanleitung.132799/

New version 1.4
The belt tensioner now got another update (version 1.4) that improves several aspects of the previous design. The belt tensioner will also be available as a dual axis system next to the original single axis setup. Since the build manual for version 1.4 is still work in progress, please check out the attached documents in the meantime.

Building instructions for old version 1.3. Follow only the assembly guide: https://drive.google.com/file/d/1zObA9j1YTS1SuNJGmorgw5Okn7C-JL-q/view?usp=drive_link
Shopping list for v1.4 single BT: Attached
Shopping list for v1.4 dual BT: Attached

Want to see how the new belt tensioner looks like? 3d model of the systems are right here:
Single axis belt tensioner: https://a360.co/4d14Ypr
Dual axis belt tensioner: https://a360.co/3R370fW

Costs
Assuming you have already a controller and a harness it will set you back roughly 400€ for the single BT and 700€ for the dual BT. Obviously, this depends where you buy the parts and in which country you live.

Software
Similar to the SFX100, you can run the belt tensioner with all common motion simulation softwares e.g. SimFeedback, SimHub, SRS, FlyPT and more. The system can be fine-tuned to everyones liking.

Do you need help with the different steps during the build? Post your questions here.

Best,
Hoernle

 

[Deleted member 1066209]

Thanks for sharing a well-detailed set of instructions.

I am curious as to why you've opted to drive the harnesses using a larger gear, as opposed to directly driving from the servo's shaft. Is it for better details?

 

[Hoernle]

Thanks for sharing a well-detailed set of instructions.

I am curious as to why you've opted to drive the harnesses using a larger gear, as opposed to directly driving from the servo's shaft. Is it for better details?

With 4 tensioned belts the servo gets hot if you use the servo shaft to drive the system. A 90ST has 2.4NM which is fine for 2 belts but struggles with 4. The gear ratio results in sacrificing rpm (reduction from 3000 to 750rpm) for more torque (increase from 2.4 to 9.6NM). More torque allows you to run the belt tensioner at a higher intensity without putting too much stress on the servo.

 

[Deleted member 1066209]

Oh, I see, thanks! I'm surprised that 4 belts could stress a 90ST like that. I've always assumed that they could handle anything simracing-related.

Maybe you've got a steel abdomen.

 

[stigs2cousin]

Thank you very much for this excellent guide.
I´ll probably give it a try in the near future, as soon as I have better understanding of the electronic side. I really am "mechanically gifted"

MFG Carsten


(and, damn, I hoped I´d gotten over idea of an active harness system )

 

[Hoernle]

Here a short video of the belt tensioner in action:

As you can see the belt tensioner is silent and only the noises from the sfx100 can be heard.

 

[Alvaro de la Orden]

brilliant stuff, thanks for sharing

 

[Falconsurfer]

Hi Hoernle

Just find your excellent description to diy a belt tensioner here at racedepartment. I must build it was my first thought! Looks superfun and so pro! I'm already sourcing parts. About the belt? From were and what sort/brand do you bought?
About the servo, there is so many different ones to choose from. I'm almost 100% about this but this cost me more than 400€ so don't want this to fail. This must be the right one?
90ST-M02430 220V 750W
WH version A.
The A 90 or the mod versions is those to avoid i"ll guess?
/Patrik

 

[Hoernle]

Hi Hoernle

Just find your exellent description to diy a belt tensioner here at racedepartment. I must build it was my first thought! Looks superfun and so pro! I'm already sourcing parts. About the belt? From were and what sort/brand do you bought?
About the servo, there is so many different ones to choose from. I'm almost 100% about this but this cost me more than 400€ so don't want this to fail. This must be the right one?
90ST-M02430 220V 750W
WH version A.
The A 90 or the mod versions is those to avoid i"ll guess?
/Patrik

Hi Patrik,

Great that you like my belt tensioner solution .

A few points to consider before buying a harness:

1) I would recommend a 5-point harness since the 5th belt between your legs prevents the harness from being pulled upwards under tension.
2) A harness where the shoulder belt come down in parallel instead of merging in one central belt buckle could be also an advantage since it can prevent the shoulder belts from scratching your neck. However, this is dependent on your seat. If the seat openings are close to each other, a parallel harness is certainly better.
3) Make up your mind if you want a 2 or 3 inch harness. 2inch offers the advantage that you can use a 3d printed insert in the shoulder seat opening that raises the exit point of the belts above your shoulder level. This is important otherwise your shoulders are pulled down. 3inch belts provide more contact area to your body which could enhance the tensioning effect. I cannot comment on that since I am using a 2inch harness.
4) Check local homepages for used/expired harnesses. Sometimes people get lucky and find a nice deal there.
5) One affordable harness that ticks all those boxes and is available in 2 and 3inches is this one: https://50caliberracing.com/4-5-poi...oint-harness-seat-belt-50-caliber-racing.html. Credit goes to FlagGhost who initially made me aware of it. Of note, I am using another cheap Tenzo R harness that I cannot really recommend.

 

[Hoernle]

Hi Hoernle

Just find your exellent description to diy a belt tensioner here at racedepartment. I must build it was my first thought! Looks superfun and so pro! I'm already sourcing parts. About the belt? From were and what sort/brand do you bought?
About the servo, there is so many different ones to choose from. I'm almost 100% about this but this cost me more than 400€ so don't want this to fail. This must be the right one?
90ST-M02430 220V 750W
WH version A.
The A 90 or the mod versions is those to avoid i"ll guess?
/Patrik

Concerning the 90St servo motor. Theoretically all the different variants would work for the project. Important is that it is a 90ST M02430 with 3000rpm, 750W and 2.4NM.

The different variants then describe what controller is used, either the black one (BK) or the white one (WH). Please don't use the black one since it emits a high pitched noise. The 8€ more for the white version are 100% worth it.

The difference between versions A and B is that with version B you can unplug the cable on the motor side whereas with versions A is is fixed. I use version A since it is a bit more compact.

Mod describes if the mod bus function is included which allows you to program the motor settings with a software instead of typing the parameters in by hand with the motor controller. In my opinion, mod function is not needed.

The A90 and B90 describes versions where the cable exiting the motor is angled in a 90 degree to make it more compact. I actually don't know exactly why there is a A90 since the cables are fixed on the motor side with variant A.

For your reference, I use the 90ST M02430 in the WH version A.

 

[Falconsurfer]

Thank's Hoernle for your very fast and superdetailed answer to my questions

 

[Falconsurfer]

As a newbie in simracing a new question popped up. Maybe a few more will appear? I now know that I can use a few different softwares to set up the belt tensioner. Unfortunately i don't have the SFX100 motion and not using simfeedback. As it is now i use Simhub for dash, shakers and wind. The belt tensioner add-on in Simhub maybe work with this belt tensioner as well? Have anyone tried? Or maybe it is possible to use both Simhub and other softwares in parallel?

 

[Hoernle]

As a newbie in simracing a new question popped up. Maybe a few more will appear? I now know that I can use a few different softwares to set up the belt tensioner. Unfortunately i don't have the SFX100 motion and not using simfeedback. As it is now i use Simhub for dash, shakers and wind. The belt tensioner add-on in Simhub maybe work with this belt tensioner as well? Have anyone tried? Or maybe it is possible to use both Simhub and other softwares in parallel?

This belt tensioner solution requires either an Arduino Leonardo or the Thanos controller to run it. With the Arduino Leonardo you have to use Simfeedback as a software, with Thanos you can use several softwares e.g. SRS, FlyPT or Simcommander. In the guide I describe how to build it with the Arduino.
Simhub will not work to run this belt tensioner but you can have the software running in parallel for other things. I do that as well. Of note, there is also a belt tensioner solution out there that works with Simhub.

 

[Falconsurfer]

The progress proceeds slowly. The motor and driver showed up yesterday. About the settings, should I follow the settings in the guide sfx100? Is that "insert coins" settings you refer to in the guide? Except the setting p98 that should be 8? Or is that only if you have the sfx100 system?

 

[Hoernle]

@Falconsurfer these are the settings that I am using.

P8 = 75
P9 = -75
P51 = 500 (rpm)
P97 = 0 or 1 (0 = counter clockwise rotation, 1 = clockwise rotation)
P98 = 8 (corrects for the 1:4 gear ratio)
P109 = 1
P110 = 30
P113 = 20
P114 = 10
P115 = 100

See attachment for the modified small pulley prints with only two jaws. I dont have a two-jaw coupling here so I had to design it with some models I found online. Please try it and report back if they fit or not.

 

[Falconsurfer]

Unfortunately these 2 did not fit, thank's anyway for the try to help me. Have a new one on the way.

 

[Hoernle]

@Falconsurfer just to confirm, you used the link from the guide to buy the small coupling and received a 2-jaw version?

 

[Falconsurfer]

Unfortunately not! Didn't realized that these D20L25 couplings had different jaws. Found a shop that had much faster shipping for these 2025 but now i know wrong jaws for the 3d print. My bad, but it's ok. A new one is soon here

 

[daz_bike]

This is very innovative and thanks for making it public.

How does having 4 belt tensioner (shoulder + waist) compare to the 2 belt (shoulder) tensioners? Is there a noticeable improvement?

 

[Insert Coin]

This is very innovative and thanks for making it public.

How does having 4 belt tensioner (shoulder + waist) compare to the 2 belt (shoulder) tensioners? Is there a noticeable improvement?

I started with 2 belts and added waist belts later. Immersion wise I like 4 belts more, it's much better.