The key to the development of a suspension with the "ultimate street specifications" is through the voice of a rally driver... HKS

The key to the “ultimate street-tuned suspension” comes from voice of a rally driver… HKS

HKS is set to launch a new product, the HIPERMAX MAXIV GT SPEC A, by the end of July 2015. Their automobile development department head, Tatsuya Sakazume explains: “It is a very ambitious product for HKS.”

HKS develops and markets mufflers and suspension parts for various automobiles. Their popular products include the HIPERMAX MAXIV GT series of street suspension kits. Within that series, they recently developed the HIPERMAX MAXIV GT SPEC A for Subaru’s all-new WRX. It is a coilover suspension kit with full height adjustability; it also includes a spring and upper mount. It has a 30-step adjustable damper. The front upper mount is equipped with an adjustable camber feature. Its estimated price is 190,000 yen (excluding tax).

â—†Which part of this new product is ambitious?

Its main point is its spring rate. Its front part is 5kgf/mm, and its back part is 4kgf/mm. These numerical figures are almost equal to a pure spring. Moreover, the vehicle height span of its adjustable range is less than those of our previous products.

In other words, the height of the car does not lower to that extent, and its spring does not become tighter than stock. In this case, it is normal to think that there is no reason to pay and exchange suspensions. In short, developing suspension with such specifications was the “ambitious” part mentioned in the first part of this article.

“Perhaps, in the aftermarket style, we have been valuing the specifications in the catalog for business. We had been highly regarding the enhancement of specifications indicated in the catalog such as a higher spring rate from the standard model and a wider car height adjustment,” said Sakazume. Certainly, if the specifications are enhanced, the drivability increases during sharp cornering. It’s the riding feel of the slow speed that you use everyday. They truly strived for oustanding performance at high speeds because the riding feel in the standard model wasn’t good.

You’ll end up changing your mind about it. Thus, when we proceeded to attempt the creation of a very good product, we went down the path of eliminating uncomfortable riding feel.”, said Sakazume

At that time, HKS also met the 2005-2007 PWRC champion rally driver, Toshihiro Arai. In the field of rally driving, it is common to lower the spring rate of the suspension and to maintain its road stability. Arai and the people at HKS thought the same thing. Taking the advice of Arai, they manufactured the suspension for the WRX model.

â—† Making use of a rally driver’s opinions and the coexistence of drivability and comfort

They aimed for the “ultimate street specification.” They aimed to achieve a “good riding feel,” something “not tiresome”, and “comfortable” maneuverability on public roads, from towns to highways. Their major priority was, of course, its drivability.

They aimed for a harmonious balance of good drivability and comfortable regular riding. They used several schemes to implement that system. First is the normal and identical low spring rate. However, the persistence of its high G specification at that rate would not work. Its bump rubber, which is 1.5 times bigger than before, supplements the low spring rate. If you make its stroke big, the bump rubber catches the stroke midway. To not make it jarringly hard, they designed its form by making it narrow. The spring rate is proportionate by 2kgf/mm. In other words, the identical springs have a normal rate at the first stage of rolling, and if the rolling becomes deeper, it will turn over a little bit to the hard bump rubber. The spring rate will elevate more when rolling, and it can do the same thing.

Then its shaft was made 10mm longer than those in the previous models. With this, its fully adjustable strokes can be expanded. Its compliance to the road surface when the car feels like it’s floating will increase. Instead, we reduced its car adjustment width by 12 to 38mm. However, the front portion was reduced to 34mm, and rear portion was reduced to 35mm. It wouldn’t have been able to pass vehicle inspection standards if we had reduced it any further. Practically, there is no need to have the car height adjustment width to that extent.

Arai said, “I think it does have a better riding feel compared to the normal one. It does not also feel stiff when cornering, and you can roll it well. If you speed up, the rolling will stop when it hits the bump rubber, so you can turn the car with without worries.” The back-and-forth movement was made bigger, and they had a final testing of the suspension product at a closed course with bad road conditions, but he looked satisfied at its finished structure.

“The standard WRX model makes you feel like it’s being pushed from behind. Therefore, there are times when you have to push its understeering in small corners. However, the setting of this product at the time of shipping makes it seem like the front portion is pulling you. If you touch its attenuation, it makes you feel like you’re in an RWD car. It might not be enough to drive it all-out on a circuit like the Fuji Speedway, but it’s already enough for mini-circuits. Isn’t the best suspension the one which lets you enjoy driving safely?” said Arai.

Sakazume also said, “if the market response for this product is good, we are planning to develop it for various models as well. This is the first product that HKS has made towards a new direction”. He mentioned their plans of broad development from here on out.

(Translated by Claire Marie Sausora)