HALsprings: Japanese Coilover Springs and Why Rebound Speed Matters
If you know coilover springs, you're probably going to read this and think, Steven, what are you talking about? Two springs with the exact same rate that feel different? That was my reaction too, the first time HALsprings walked us through it.
HAL is one of the brands we import from Japan, and honestly it's one of the more interesting ones to explain. So let's walk through who they are, what "rebound speed" means, and why it matters for the way you actually drive.
Who is HALsprings?
HALsprings was developed by Aki Hatano-san. They design and manufacture high performance springs for cars and motorcycles out of Saitama Prefecture in Japan, and they've been growing in a lot of directions at once.
A few things that tell you they're the real deal:
- They've developed springs for the vehicles shown in the Toyota booth at Tokyo Auto Salon.
- They worked with Super Taikyu driver Masahiro Sasaki of Rookie Racing on lowering springs for GROW Design.
- They run their own racing and development program with a 2.0 liter A90 Supra in the NLS endurance series at the Nürburgring.
The Mod in Japan team also got to visit the HAL factory in Japan. Seeing how they think about springs in person is a big part of why we wanted to bring them to the U.S.
The biggest takeaway: same rate, different feel
Quick refresher. A spring rate like 8KG means it takes 8 kilograms of force to compress the spring one millimeter. Most people stop there. If two springs are both 8KG, they assume they're the same.
HAL's whole approach says otherwise. They make springs at the same rate in different rebound speeds:
- Low Rebound Speed
- Medium Rebound Speed
- High Rebound Speed
- Racing, which is very specific and usually custom
Same rate. Same support in a corner. But how fast the spring gives that energy back after a bump is different.
How can that even work?
So before you start asking engineering questions, even I don't fully understand every detail of how they pull it off. But here's the basic principle they explained to us.
When you hit a bump, that energy travels along the spring, following the winding path of the coil, before it reaches the chassis. When the spring decompresses, the energy has to travel back. A longer path, more coils, takes longer. Fewer coils, shorter path, faster return.
Then add the other variables. The Low Rebound Speed spring has a slight barrel shape, bulging a little in the middle, while the others are straight. The coil count is slightly different. The wire thickness varies a bit. Combine all of that with the material and the manufacturing process, and you get springs with the same rate but different personalities.
HAL sent us a couple of oversized demo springs to show this. Both were the same 8KG rate with the same inner and outer diameter. The low speed one had a massive number of coils and weighed a ton. The high speed one had far fewer coils. Once you see them side by side, it clicks.
What it feels like from the driver's seat
The HAL product pages describe the Low Rebound spring well. A conventional spring snaps back fast after every bump, and on rough pavement that shows up as a sharp, choppy feel. The low rebound design works like a built-in pre-damper. Through hysteresis, the spring dissipates a little energy within itself, so small, quick inputs are smoothed out before your damper has to deal with them. Big inputs like cornering, braking and weight transfer are still supported at the full rate.
In plain English: same body control, less jarring. That's what most street-driven cars need.
Is low rebound just a street spring?
Partially yes, but it's not that simple. If you use your coilovers for both street and track, Low Rebound Speed is a great fit. But it doesn't mean it's not track oriented.
Think about a bumpy road course. If your spring rebounds super fast, the tire can come off the ground over every hit and you get skipping. A slower spring keeps it planted. HAL's own A90 Supra endurance car runs low speed springs at the Nürburgring, which is about as bumpy and demanding as it gets. Rally and off-road are other places where low speed shines, because the surface is always changing.
So speed doesn't tell you which one is "better." It depends on the roads and tracks you're actually driving.
How we'd pick a rebound speed
To be honest, it depends on your application. Here's how we usually walk customers through it:
- Street and track dual purpose: Low Rebound Speed. You keep the performance of the right spring rate with a less jarring return.
- Coming from a typical U.S. spring brand: most of what's sold here would be considered medium rebound speed, so Medium is a familiar starting point.
- Smooth, well maintained high speed track: High Rebound Speed may suit you.
- Dedicated race car: Racing springs are very specific. Reach out and we can talk about a custom spring.
For context, most OEM replacement springs, the kind where you keep your factory dampers, land somewhere between low and medium rebound speed.
Where you'll see HAL on our site
Most HAL springs we carry are metric coilover springs sold individually, listed by inner diameter, free length and rate. For example, the ID65-203 8KG Low Rebound and ID65-203 10KG Low Rebound. There are also Medium and High versions, like the 8KG Medium and 8KG High.
HAL also makes complete spring sets with GROW Design. Mod in Japan is the exclusive distributor for GROW Design, so this one is close to home. There's the GROW Design x HALsprings sports springs for the GR86 and BRZ and the GROW Design x HALsprings sports springs for the GR Corolla, both developed to work with the factory shock absorbers and manufactured by HALsprings.
Some honest concerns
- Springs are sold individually. Most HAL coilover springs are one piece, not a pair. Order the quantity your setup needs.
- Match your coilover. Inner diameter and free length need to match your coilover. Check before ordering.
- Not every size is listed yet. We're still adding HAL sizes to the site. If you don't see yours, ask.
We're also planning some real world testing with a friend of mine on a road course, so we can talk about how each speed translates to steering feel. Stay tuned for that.
Fitment: reading a HALsprings part number
The metric HAL springs we list use a standard coilover format. The listing gives you the inner diameter (ID65), the free length (203mm), the rate (like 8KG or 10KG) and the rebound speed (Low, Mid or High). Before you order, measure what's on your coilover now or check your coilover maker's spec sheet. The inner diameter has to match your coilover, and the length affects how much ride height adjustment you'll have left.
If you have a GR86 or GR Corolla, there's an easier path. GROW Design and HALsprings make car specific sports springs for both, so you don't have to work out the sizing yourself.
Install notes: medium difficulty
Swapping springs on a coilover means taking the coilover off the car and compressing the spring safely. That's shop work unless you have the right spring compressor and experience. After the swap, reset ride height, let the springs settle, then align the car.
Maintenance
Springs themselves are low maintenance. Keep an eye on the coilover threads and perches, clean off road grime, and recheck ride height after the first few hundred miles because new springs can settle a little.
What to pair HALsprings with
Springs work with your dampers, not alone. If your coilover dampers are worn out, a nicer spring won't fix that. And if you're also adding aero, set the ride height first so the aero fits the stance you'll actually run. We explain that order in our RC F build guide, and the same logic applies to any car.
FAQ
Does a Low Rebound spring mean a softer spring?
No. The rate stays the same, for example 8KG is still 8KG. What changes is how quickly the spring returns energy after a bump.
Are HAL springs sold as a pair?
Most HAL metric coilover springs are sold as one spring. Check the quantity on the product page.
Which rebound speed should I start with?
For a street car that sees some track time, Low Rebound Speed is a great starting point. If you're unsure, tell us how you use the car.
Will HAL springs fit my coilovers?
Match the inner diameter and free length to your coilover. Tania or Kenia can help confirm.
What is GROW x HAL?
Complete spring sets for the GR86/BRZ and GR Corolla, developed with GROW Design for the factory shocks and manufactured by HALsprings.
Do HALsprings fit my coilovers?
Check the inner diameter and length on the listing against your coilover. The metric springs we list are ID65 and 203mm long.
Does rebound speed change the spring rate?
No. The rate is the rate. Rebound speed is about how the spring behaves as it extends, which is why two springs with the same rate can feel different.
Next step
If you're putting a plan together, the easiest thing is to add the parts you're looking at to your cart and hit Add to Quote. We'll look it over, confirm fitment and availability, and get back to you. If you'd rather talk it through first, reach out and Tania or Kenia can help with fitment and stock questions. No pressure at all. We'd rather you buy the right part once than the wrong part twice.
