Torsion Bar vs. Puna Coil: ʻO ia ka mea kūpono no kāu ʻōnaehana hoʻomaha?
You're designing a vehicle's suspension and face a fundamental choice. Ke hoʻohana nei ʻoe i kahi pūnāwai coil maʻamau a i ʻole kahi pahu torsion hoʻopakele wahi? ʻO ke koho hewa ʻana hiki ke hōʻino maikaʻi holo[^1] a hoʻololi i kāu hoʻolālā chassis holoʻokoʻa.
A puna wili[^ 2] a e hana ana ka pahu torsion ma ka loina hookahi: kū'ē kūpilikiʻi. A puna wili[^ 2] ʻo ia ke kumu a pā ʻūhā[^ 3] ʻeha i kahi helix. Aia ka ʻokoʻa nui i ko lākou ʻano a me ka hoʻopili, e kuhikuhi ana i kā lākou hihia hoʻohana maikaʻi loa i nā ʻōnaehana hoʻokuʻu kaʻa.
E like me ka mea e hana ana i na uwea puna no na wili a me na kaola kila ikaika no na nenoai torsion., ʻIke wau i kēia nīnau ma ke ʻano o ka ʻeke, ʻaʻole physics. He ala maikaʻi nā mea ʻelua e mālama a hoʻokuʻu i ka ikehu. Hoʻopili ka pūnāwai coil i kahi uwea pūnāwai lōʻihi loa i loko o kahi ākea kū pololei. A pā ʻūhā[^ 3] hoʻohana i ke koʻokoʻo pololei e hoʻokō i ka pahuhopu hoʻokahi, akā, mālama ia i ke ākea kū i ka huila no ka hoʻohana ʻana i kahi ākea longitudinal ma ke kiʻi. ʻO ka koho ʻenekinia e iho i lalo i kahi wahi i ʻoi aku ka waiwai i kahi hoʻolālā i hāʻawi ʻia.
Pehea e hana maoli ai ka hoʻokuʻu ʻia ʻana o Torsion Bar?
Ke nānā nei ʻoe i kahi pahu torsion, and it's just a simple steel rod. Pehea e paʻa ai i kahi kaʻa kaumaha? It doesn't compress or stretch, making its function seem mysterious.
He pūnāwai ka lāʻau torsion ma ka wili ʻana i kona lōʻihi. One end of the bar is fixed to the vehicle's frame, ʻoiai ua hoʻopili ʻia kekahi i kahi lima hoʻomalu. I ke ku ana o ka huila i ka puu a nee iluna, hoʻoikaika ia i ka lima hoʻomalu e wili i ka pā, which resists the motion.
The magic of a pā ʻūhā[^ 3] aia i loko o kāna mea a me ka mālama wela. It has to be made from incredibly strong spring steel that can twist repeatedly without deforming or breaking. Ma kā mākou hana, producing a bar with a perfectly consistent grain structure along its entire length is the main challenge. Any weak spot could lead to a sudden failure. The bar's resistance to this twisting motion is what provides the spring force. It’s an elegant and incredibly robust solution, which is why it became so popular on heavy-duty trucks and SUVs that needed high ground clearance[^4] and a durable suspension design.
Twisting for Support
The entire system is based on controlled resistance to rotation.
- The Anchor and The Lever: The bar is anchored in the middle of the chassis. The control arm acts as a lever. When the wheel goes up, the lever twists the bar. The bar's internal structure wants to return to its untwisted state, e hoʻokuke ana i ka lima hoʻomalu a me ka huila i lalo.
- Hiki ke hoʻololi i ke kiʻekiʻe holo: ʻO ka hapa nui pā ʻūhā[^ 3] Loaʻa i nā ʻōnaehana kahi bolt hoʻoponopono a me kahi "kī." Hoʻopili kēia kī i ka pā i ka lima hoʻomalu. ʻO ka hoʻopaʻa ʻana i ka bolt e hoʻohuli i ke kī, e hoʻohui i kahi wili mua i ka pahu. This pre-load raises the vehicle's resting ride height without changing the spring rate.
- Nā Pūnaewele Kūʻai: He mea ʻole ka pā me ka ʻole o nā mea pono. Pono e hana ʻia mai ka hui kiʻekiʻe puna kila[^5] (like 4140 a i ʻole 5160 kila) i mālama pono ʻia i ka wela e mālama i ke koʻikoʻi torsional nui ma luna o nā miliona o nā pōʻai.
| ʻO nā mea maikaʻi o nā Bars Torsion | ʻO nā pōʻino o nā Bars Torsion |
|---|---|
| Paipai maikaʻi loa | Hiki ke hana i kahi paʻa, kaʻa hoʻokō emi. |
| High Ground Clearance | Hiki ke paʻakikī ka hoʻoponopono ʻana i ka ʻaoʻao. |
| Hiki ke hoʻololi i ke kiʻekiʻe holo | Generally heavier than a coil for the same spring rate. |
| Piha & Simple Design | Performance is highly dependent on bushing quality. |
No laila, Is a Coil Spring Just a Coiled Torsion Bar?
ʻIke ʻoe i kahi puna wili[^ 2] compress under load. It seems to be bending. How can that be the same as the twisting force in a straight pā ʻūhā[^ 3]?
ʻAe, from a physics standpoint, a puna wili[^ 2] is a torsion bar wrapped into a helix. When you compress or extend a puna wili[^ 2], the round wire it's made from is actually twisting under a torsional load. The coil shape cleverly converts a linear force (kaulike) into a torsional force in the wire.
This is one of my favorite concepts to explain. When we draw spring wire and then form it on our CNC coilers, we are creating a very, very long torsion bar and packaging it efficiently. If you were to unwind a typical automotive coil spring, the wire could be over 10 feet long! By coiling it, we allow that entire length to twist and contribute to the spring force in a very small amount of vertical space. This is why coil springs are perfect for modern MacPherson strut suspensions, where the entire spring and shock absorber assembly needs to fit inside the wheel well.
The Genius of the Helix
The coil shape is what makes this spring so versatile.
- Converting Force: The helical shape is a simple machine that translates the up-and-down force from the suspension into a rotational stress on the wire. Every millimeter the spring compresses, the wire along its entire length twists a tiny amount.
- Progressive Rates: Unlike a pā ʻūhā[^ 3], which has a single, linear spring rate, puna wili[^ 2]s can be designed with variable rates. By changing the distance between the coils (the pitch) or using a conical shape, a spring can be made to be soft during initial travel and become stiffer as it compresses further. This provides both comfort and performance.
- Packaging Efficiency: The main reason puna wili[^ 2]s dominate the passenger car market is their packaging. They can be mounted directly over the shock absorber (creating a "coilover[^6]"), which makes for a very compact and easy-to-install suspension module.
| Pros of Coil Springs | Cons of Coil Springs |
|---|---|
| Excellent Ride Comfort | Takes up more vertical space in the wheel well. |
| Can Be Made Progressive-Rate | Ride height is not adjustable without new parts. |
| Compact Coilover Designs | Cannot be easily "re-indexed" like a torsion bar. |
| Lighter Than Torsion Bars | More susceptible to bowing under heavy load. |
Which is Better for My Application: Torsion or Coil?
You have a choice for your project. One system offers adjustability, and the other offers a smoother ride. How do you make the final decision on which is truly superior?
Neither technology is "better," but one will be better suited to your specific goals. The choice depends entirely on your design priorities: packaging constraints, ride height requirements, desired handling characteristics, and the vehicle's intended use.
I always tell engineers that the spring is a component of a larger system. You must choose the spring that best serves the system's goals. If you are building a 4x4 truck where maximum ground clearance and the ability to easily adjust the ride height to compensate for a heavy winch are top priorities, ka pā ʻūhā[^ 3] is a fantastic choice. If you are designing a passenger car where ride comfort, hana mālie, and a compact suspension that doesn't intrude into the engine bay are the goals, the coil spring is the obvious winner. The right choice is the one that aligns with your primary engineering objectives.
A Decision Based on Priorities
Let's break down the final choice based on key performance indicators.
| Pili | Torsion Bar | Puna Coil |
|---|---|---|
| Hana Kumu | Twists a straight rod along its axis. | Twists a coiled rod via compression. |
| Space Savings | Winner. Low vertical profile, ideal for ground clearance. | Self-contained, but requires more vertical height. |
| Adjustability | Winner. Ride height can be easily adjusted with stock parts. | Requires aftermarket coilover[^6]s for height adjustment. |
| Ride Comfort | Generally firmer and less compliant. | Winner. Generally provides a smoother, more isolated ride. |
| Performance Tuning | Limited to changing the bar itself. | Winner. Progressive rates offer advanced tuning options. |
| Hoʻohana maʻamau | Heavy-duty trucks, kekahi mau kaʻa hana maʻamau. | ʻO ka hapa nui o nā kaʻa kaʻa o kēia wā. |
Hopena
ʻO nā kaola torsion a me nā pūnāwai coil he mau hoʻohana maikaʻi o ka loina kino like. Your final choice should be guided by your application's specific needs for packaging, maikaʻi holo, a me ka adjustability.
[^1]: E ʻike i ka hopena o nā ʻano hoʻomaha ʻokoʻa i ka ʻoluʻolu a me ka lawelawe ʻana o kahi kaʻa.
[^ 2]: E aʻo e pili ana i ka hana a me nā pono o nā puna coil i nā kaʻa hou.
[^ 3]: E ʻimi i kēia loulou no ka hoʻomaopopo ʻana i ka mīkini a me nā pōmaikaʻi o nā kaola torsion ma nā ʻōnaehana hoʻokuʻu kaʻa.
[^4]: E ʻimi i ke koʻikoʻi o ka hoʻomaʻemaʻe ʻāina i ka hoʻolālā kaʻa a me kona hopena i ka hana.
[^5]: Discover the properties of spring steel and why it's crucial for suspension components.
[^6]: E aʻo e pili ana i nā ʻōnaehana coilover a me kā lākou pono i ka hoʻolālā hoʻokuʻu kaʻa hou.