Builder Interview: Dave Kirk, Part II
There are lots of designs out there that claim to offer compliance. You’ve done some research on the subject, haven’t you? Do any of those swept seatstay designs really off any sort of suspension effect?
I may be one of the only people on the planet that feels this way but I think road bike suspension is the Holy Grail of road bike design. I’ve done years of work on this and was awarded a patent for the Serotta DKS design. It’s patent number 6,109,637 and was awarded on August 29, 2000. It’s fun to look up and you can easily Google it.
Road bicycles are the only high-speed device raced in the world that I can think of that doesn’t employ some sort of device to improve traction. Everything from skis to cars to skateboards all employ suspension to great effect. Please note that I didn’t list comfort as being the primary reason for suspension even though one could make a very good argument for how added comfort will reduce rider fatigue and make them more competitive in longer events. I see road bike suspension as being a means to keep the rear tire pressed against the road with the most constant force possible, full stop. I think the rear wheel travel need be no more than 10mm max and that as little as 5mm can be extremely effective.
A traditional road bike has near perfect front suspension in the form of a cantilevered beam. The fork is allowed to flex a lot. Just clamp your front brake on firmly and rock the bike back and forth and watch how much the fork moves. Even super stiff forks move a good bit and this acts as a form of trailing arm suspension. The front wheel encounters a bump and slows as it tries to go over the bump yet the rider’s mass keeps the whole thing moving. The fork flexes rearward (and in the case of a properly designed curved blade fork compresses vertically) and then it rolls over the bump with very little interruption of the rider’s momentum or with the tire’s contact with the road.
The rear wheel is another matter entirely. With a traditional double diamond road frame the rear end of the frame forms a triangle and this triangle cannot move or at least cannot move in any meaningful way. So when the rear wheel encounters the same bump that the front wheel just sailed over it loads up the frame and that in turn loads up the rider and the rider then bounces off the saddle. This little bump redirected the entire mass and momentum of the bike and rider upward for just a moment and that has two effects. The first one is that there is a loss of forward momentum or speed and this obviously slows the rider down. Not a lot but we are constantly hitting small bumps in the road that do this and the cumulative effect is large. The second thing it does is lessen the pressure of the rear tire on the road. In many cases it loses contact altogether. Either way traction is compromised. If you are just rolling straight down the road without a need to turn or brake or accelerate traction is not a real issue. The rider doesn’t need the to use the full limit of the traction of the tire. But if the rider is cornering, braking, or accelerating then it’s a different matter. We’ve all gone around a fast downhill corner and had the bike all loaded up with the force of the turn and then hit a bump mid-turn and had to do some serious correcting to keep it all in line and on the road. Similarly we’ve all been sprinting at our limit for a townline sign at our local Tuesday night World Championships and had the rear wheel skip and bounce causing us to back off and/or correct to hold our line.
Well it takes very little rear wheel suspension travel to minimize or even completely eliminate the issue and I’ve spent more time working on this issue than I care to admit. The amount of time I’ve spent lying on the road with my face pressed against the cold hard ground so that I could see the rear wheel of the racers going by bounce is embarrassing. But that said it’s a real eye opener when you do this. There is daylight under the rear wheel all the time. The front is stuck like glue and the rear spends a surprising amount of time skipping and bouncing along.
Tell us about your involvement in the Serotta DKS suspension system.
Way back in the day when I was the R&D department at Serotta, Ben was very cool and gave me lots of leeway to work on what I wanted to work on. I’d seen some of the bikes like Ritcheys and Litespeeds with a long graceful bend in the seat stays and wondered if it could be a benefit. Ben and I were walking around the Interbike show one year and every other bike had this same curved stay and they all claimed it made the bike more comfortable. I didn’t give a damn about the comfort thing at all but I did care about the traction issue. Standing outside the hotel that night I suddenly had an idea of how to do it better.
So we went back to New York and I was excited to work on the idea. It turns out I was the only one that was excited but Ben let me do my thing as long as other stuff didn’t slide too much. So the first thing I did was to make a frame like everyone else was doing (simple long radius curve from end to end) and put it on my testing table and load it up (like the rider was hitting a bump) to see where and how much it moved. Most of the current designs had no more wheel movement than a traditional straight stayed frame, or less than 1 mm. So I started playing with different radii and duration of bends and while I could do better than what was being offered it still wasn’t worth the trouble in my opinion. I knew something else needed to be done to free up some movement. At the same time I was worried about fatigue issues where the stay attached to the seat tube and the dropouts. It was then that I flashed on the idea of putting a pivot at the bottom of the stay where is attaches to the dropout and then have most of the bend of the stay low so that most of the flex would take place there and not where the stay was welded to the seat tube. This not only took care of the fatigue issue but also allowed the stay to compress more allowing more rear wheel travel.
A this point I built a frame that had bolt on seat stays so I could make any configuration of stay I wanted and lab and road test them. Some stuff worked pretty well and some stuff really sucked. I ended up with the “J” curve design and it worked very well but I was concerned with it having too much travel and with it being bouncy. The stay was now acting as a spring but it had no damper to control its movement.
The next task was to figure out a way to damp the movement. What I originally wanted was rebound damping only and it proved very difficult to do in a simple and super light way. I then realized that if I gave it compression damping that it would have nearly the same effect because it would just interrupt the bouncing cycle. It was at this point that I developed the “strap on” which was a stainless steel strap with some special silicone made by GE to be an ‘ultra damper’, bonded to it. It was then bolted to the stay and acted as both a travel limiter and a damper. I ended up picking three different hardness’s to give more or less travel based on rider preference and/or weight. The funny thing was that I gave this damper part the nickname “strap-on” knowing it’s other meaning and we used the term inhouse and snickered about it the way boys do…… especially when one of the girls from the office would come out and ask if we had a strap-on or how a strap-on worked. Good fun. At some point the product was released and I couldn’t believe that the Serotta catalog listed that part as a “strap-on.” Somehow it got through editing.
In the end I think the design was successful. I wanted to continue to develop and refine it but at some point one needs to draw a line in the sand and call it good and sell some of the things. The design allowed for about 12mm of rear wheel travel for most riders, which I now think, was more than we needed. But it was a good first step and I would have liked to make the design more race oriented, more aero and lighter. But I had worked on the design for about 14 months and other stuff needed to be done so I moved on. I left Serotta shortly thereafter to move to Montana and to be in the big mountains and in the snow.
Serotta continued to produce and sell the bike for a few years after I left but it was never a big seller. I think that the sales and marketing folks there didn’t like the time it took to explain what it did and how it did it when they could just push the normal offerings and make the same money. The DKS (Dave Kirk Suspension) now has a cult following of sorts and I get a few emails a week about it with questions about how it works and about finding a used one somewhere. They seem to command a hefty price on eBay at this point. I think over the years I’ve had all of the big three bike companies contact me as ask about the design. One engineer even pretended to be a customer interested in buying one when in reality they were looking for a way around the patent. I think the Specialized Zerts inserts deal is a good example of the design being tweaked and using different materials to get around the patent. I’ve never ridden one but hear some folks like them.
Do you ever build with it today?
No I don’t. Even though I am listed as the inventor on the patent Serotta is the holder of the patent and it is their design. Some have told me they think this is unfair and I firmly disagree. Ben Serotta gave me a place to work and paid me well to develop the idea in the first place and without his backing it would have never been more than a napkin sketch in a bar at a tradeshow. He paid for it and he owns it. It was Ben that decided it should be called the DKS. I only found out it was named after me when the decals showed up and I was given one. I was honored then as I still am. Ben is a good man and treated me very well.
When I started my company I knew I’d revisit the idea at some point but also knew that there were changes I wanted to make if I could. The fact that I chose to work only in steel also required a major design change since the original DKS was titanium. I knew I wanted it to be firmer and to have less travel. I knew I wanted it to be less complex and cheaper to make and I knew I wanted it to look cooler. It was then that I developed the “Terraplane” design (Terraplane meaning “flies over the land”). I experimented with different tubing, bend radii as well and bend duration and then did a lot of road miles on prototypes to get the final design nailed down. Most riders will see 5mm or less of wheel movement with a Terraplane and one can’t not feel the difference from a straight stayed bike while climbing or sprinting. It takes a sudden and large load to get the wheel to move and the rider cannot move that fast so it will not react to rider movement. So there is no mushy or ‘MTB on the road’ feeling that some expect. The Terraplane just gives a more hunkered down and calm feeling than a traditional bike. Some folks will get their new Terraplane and ride it for a few weeks and then get back on the bike they rode before and only then feel the marked difference in cornering and descending. It can be a real eye opener for some.
I’ve extremely proud of the Terraplane and how it performs. Some love the look of it and some hate it and I know I can’t please everyone that way but I’ve never put someone on a Terraplane and had them not like the performance.
In your view, what are the pros, cons and challenges with regard to the development of suspension for road bikes? Do you think it would help that much?
I think that there are large gains to be had with a proper road bike suspension for the reasons I’ve listed above. I think the down side could be complexity and cost if the design isn’t properly elegant. There were some suspension road bikes years ago that were really short travel versions of MTB designs and they sucked for road racing use. It has just too much travel, weight and complexity to work as it should for the road.
I think the big thing that will prevent a good design from being adopted by the masses, and therefore be used in the pro race ranks, is that the marketplace is just too traditional. I think the marketplace pats itself on the back a bit too much for how innovative and forward thinking it is when in reality it hates anything truly new. A change in material from steel to aluminum to titanium to carbon to whatever is just fine but to do something truly different and better has historically not been rewarded in the performance road bike market. Look at all the crap being thrown at the new Shimano electric stuff. It work and works well and my hat is off to them for even going there but it’s not like it’s gotten a very warm reception. I’ll bet if they stick by their guns the marketplace will adapt and we will see the other two major players scrambling to catch up and we’ll see little kids riding around our neighborhoods pushing buttons to shift.
It’s going to take a bit of letting go of the traditional fashion of this industry to allow it to make any real jumps forward. Hell there are still interweb forums full of people arguing about which is better – sloping top tubes vs, horizontal tube tubes. It’s all fashion and that is just the way it works. I am for the most part OK with that but it can be frustrating at times. What did that Billy Crystal character on SNL say years ago? “It’s better to look good than it is to feel good”?
Thanks for the opportunity to address some of this stuff and thanks for reading.