When testing the 2013 KTM 350 xc-f and exc models, I stumbled into a very interesting topic: chassis flex. It is always one of those vague areas of dirt bikes that gets talked about all the time but remains incredibly hard to understand. It usually manifests itself in some seemingly simple question like “should I buy machined triple clamps for my bike”.
Any single part is just one piece of the flex puzzle. It is a challenge for me because I certainly don’t feel like I have any particularly special knowledge or skill to pass conclusions on many of these parts. But I guess it is some comfort to see that even the best minds are constantly changing their ideas of how to deal with chassis flex, it is an evolving science.
It was only by having the chance to test two versions of the same bike side-by-side that I was able to see the real handling differences and get a tiny perspective into the engineering differences between them. If it was a comparative test of two different brands, it would be hard to make anything but a large sweeping choice of one over the other. How would you ever begin to break down a comparison of all the individual parts involved. In the case of the KTM’s we can see the exact parts that are different and understand some of the impacts on performance.
It is not just the frame, but the entire array of components that contribute to the flex characteristics of a motorcycle. The list is long: rims, spokes, hubs, axle, forks, triple clamps, steering stem, bearings, head stay, frame, swingarm pivot, motor mounts, swingarm & bearings, and so forth back to the rear wheel. That is a lot of variables to deal with.
Frame flex is one of the great dark arts. It seems that only in the last few years has much real progress been made for dirt bikes. The real development has been on street bikes.
Back in the seventies, road racing was evolving fast. When the first real race tires were developed, the slicks, everyone expected great improvements in lap times from the new tires. But many decent race bikes were instantly transformed into evil handing monsters.
It took a while for the engineers to figure out what was happening. The frames on many of the bikes had a tremendous amount of flex. With the existing tires, most of the energy that was developed from flex would harmlessly work its way to the ground via the tires as they slid around. But slippery rides were blamed on poor tires, not knowing what other factors might be involved.
When the slicks came along with their improved grip, this energy now had no release. It was like a coiled spring and with no outlet at the ground, it headed the other direction, towards the rider. The result was a violent chassis reaction, as the bike attempted to spit the rider off.
Over the next 20 years, chassis’ got stiffer and stiffer, particularly with the advent of aluminum designs. They went so far as to develop new problems, such as chatter. Chatter is when the wheel hops along the ground at extreme lean angles. This was attributed to a LACK of flex. To this day race bike engineers are still challenged by the same problems, developing the ideal amount of flex.
In motocross, the standard cure all was to stiffen frames. Back in the days of steel, every factory bike had lots of extra welding and parts added. As to how scientific this was, that is anyone’s guess. I had a 1998 factory Suzuki RM250 with a gusseted frame. I never liked that bike.
In 1997 Honda went to aluminum. The original version was so stiff that McGrath up and left Honda because he considered it unrideable. It took a number of years for Honda to engineer the correct amount of flex back into the CR frames.
It was almost a decade before the rest of Japanese completely converted from steel. By then, there were enough advances in steel frames to consider it as a viable path forward. But the Japanese were already down the path like lemmings. As Honda goes, the rest of the world goes.
Interestingly, over in the Honda off road department, there was probably more real world testing of flex than anywhere else. Saddled with the long suffering and eventually successful XR600, the team had to find the best ways to make it work. I recall Johnny Campbell telling me how they would uncrate 20 new XR’s at the start of the season and completely dismantle them. The frames were sent off to the welder for a complete overhaul.
Those old XR’s had an interesting mix of flex characteristics. They were great flat trackers and would slide around with ease. When the aluminum frame XR650 came along, many long time Honda riders found themselves pitched to the ground trying to ride the new bike like they did the old one. This bike required a change in riding and techniques.
In the 2005 Baja 1000 Johnny hit a rock and destroyed a wheel and axle early in the race. If I recall, he didn’t even crash. As seen in the Dust To Glory movie, the team helicopter landed and replaced the wheel, but with the axle still damaged, he had to make his way to the next pit for replacement.
After the race, I saw the damaged wheel, it was the plain OEM part. Stock XR wheels were particularly flimsy looking. I asked why they didn’t run stronger aftermarket wheels. The reply was that they had tested many things and Johnny had to have the flex of the stock wheels to get the feel he needed for the bike. Presumably the chassis was stiff enough that they needed flex in other areas to compensate.
As for KTM, it is obvious that the newest steel frame designs are good enough for world class competition. Dungey and Roczen seem to be doing fine with them. But that was not always the case. KTM almost completely ignored frame design for well over a decade.
If you were to look at two stroke frames from 1990 and 2003, you would see that they are nearly identical. It wasn’t until 2005 that they made a serious attempt to improve strength. I have wondered about this for some time, because in some ways, my 1992 bike handles better on the track than some of the later bikes I have owned. I think one small part that contributes to this is the cylinder head stay.
On contemporary bikes, the head stay is a small bracket from the top of the head to the frame. On 2004 era models, these brackets would break on a regular basis. On the old bikes this was a much larger cast aluminum bracket that bolted lower, on the engine cases. I think it actually makes a big contribution to the stiffness of the otherwise spindly frame. It creates more of a triangulation and bolts to a stronger part of the motor.
The 2005 frame came with its own issues. The 2004 bike was a particularly good riding enduro motorcycle. It was the culmination of one era of design. The new 2005 model did not work nearly as well. KTM had not yet figured out the correct suspension settings for the stiffer frame. For the next two years they kept many aftermarket suspension companies busy. Does anyone remember when the hot set up at the time was to take the third bolt out of the lower triple clamp?
I recall one conversation with a KTM tester about how they had been unhappy with the suspension work from WP. Then they went out and tested various frame designs, all with just one set of suspension. As they swapped the fork and shock between frames, they felt like completely different parts each time. This showed just how critical the inter-working of both frame and suspension are.
Since that time, KTM has remained on a fast track for frame design. But I feel that suspension is still playing catch up just a bit. Splitting models between PDS and linkage has both helped and hindered some of that. It is like a Chinese menu, there are lots of good choices, but you may not be able to get exactly what you want. But at least there are choices.
Ryan Dungey’s race wins sell motorcycles. But does that mean that you want to ride his bike? I say probably not. The sx and xc-f designs are hard core race bikes. KTM was long known for trying to make their enduro bikes into motocrossers, now they have a true dedicated mx line.
You will notice that Kurt Caselli and Ivan Ramirez will be riding their old twin cam 450sx-f at the 2013 Baja 500 next week. This bike is not even in production anymore. Why are they not on the new Dungey inspired 450xc-f? After three years of development on the old frame, the riders are very comfortable with it. It is a comfort level they have not developed with the new stiffer framed bike yet.
As for me, I like the ride of the current xc-w chassis. It is more forgiving and easier to ride. On the 350 versions, the cast clamps are probably fine. On the heavier 450 and 500 models, I think I would change to stiffer machined clamps. I found that the 500 would wander around at times when racing.
So where does that leave us? There are no real conclusions; this is right or that is wrong. Is there a perfect balance of flex and performance? Not necessarily, it is like asking if there is a perfect suspension set up. What works for one rider may not work for the next. A stiffer motorcycle is a two sided coin. It may work better at the extreme end of performance, but it will suffer in the middle ground. It might be faster, but take more skill and effort to ride.