Frame Geometry

[M5-]

So......

I've been giving a lot of thought to the effects of frame geometry on snow bike handling of late. When I first set up a bike it was a KTM 450 EXC which is an off road bike vs a race bike. As a result it has more rake and trail than a pure race bike. After blowing that up twice, I built the CR500 which is a race bike but its an old (really ancient actually design) steelie frame and boasts a lot of rake and trail probably more than the the KTM EXC. That 500 wore me out daily but now I see it wasn't due to the power or vibrations it was the steering effort specially when we were riding slow picking through the trees. It is a biotch to ride that thing slow which means you tip over all the time and yada yada we all know how that story ends. I own three different skis and they are all basically the same, some better than others in some conditions but really no massive differences in the end its just subtle improvements here and there in different conditions.

Fast forward to this season and I'm riding the YZ/Camso setup and it is so much easier to ride its not even funny. I WAS of a mind that you would never be able to feel any differences in frame geometry strictly in the deep pow, I'm not talking about trail riding just pow, but I've had a change of heart particularly riding slow. I know there are huge differences between the TS LT kit on the 500 and the Camso kit on the YZ but there is more to it than that, its the frames.

One major issue in my mind is the relationship between the dirt bike wheel and the snow bike ski. We are asking a ski to perform what the wheel is supposed to do. I'm starting to believe you have a better shot at this with a race bike frame with less rake and trail. I was sold on the race bike tranny after switching away from the wide ratio 4 years ago but now I think the full package has to include the frame as well.

This brings me to where I'm at now and that's contemplating building an SXF/CR500 hybrid and throwing a Camso or Yeti SS on it. The YZ works great don't get me wrong but between the 4 stroke and the Camso kit I can really feel the weight difference over the 500. With a Yeti kit I could drop close to 60 lbs over the YZ.

Any KTM SXF race bike gurus out there know the difference between the different years of SXF frames. There is basically no weight penalty in using the KTM Cromo frame vs an AF conversion and the KTM swap is simple in comparison. I found an 09 SXF frame which is about the oldest I would consider. Would it be worth it to find something newer.

A little light reading.

https://www.dirtrider.com/features/protips/141_0908_tech_tips_motorcycle_rake_and_trail


M5

 

[summitboy]

My money would be on the BRC 500 motor. KTM motor in whichever ktm frame you desire. Drop in and go. It sure looks like a great piece of engineering. All fits in a new generation 250/300 frame. That would make a great bike platform with the newest engine technology !

 

[Timberview01]

Definitely an interesting point. I went directly to the Yamaha website to see what kind of difference there was between the standard YZ, and the FX. Interestingly enough, the YZ has more rake and trail than the Off-Road bike. One degree in rake and 0.2 inches of trail.

Not sure if similar comparisons on other brands will turn up the same results, but Yamaha is my personal brand of choice...

 

[chumbilly1]

I think optimally You would use a 13 or later linkage frame as it mounts your strut in the center of the swingarm vs. offset mount of the pds bikes. Yeti does make an adapter for the soft strut or the rrs shock though(something TS never could figure out). I have a 14 sxf350 frame I would be willing to part with, but it has smashed frame rails from EX practice and no title(racebike). I also have triple clamps,sx tank, plastics, no radiators though. The cr5 into sxf chassis apears to be super easy and must vibrate less than AF conversions. Let me know!

 

[portgrinder]

My money would be on the BRC 500 motor. KTM motor in whichever ktm frame you desire. Drop in and go. It sure looks like a great piece of engineering. All fits in a new generation 250/300 frame. That would make a great bike platform with the newest engine technology !

I think you are on to something with the frame for sure.

Don't wanna be the first guy with that BRC. Looks nice but I'm curious if those trans gears are going to survive.

 

[coryf89]

do you guys REALLY notice a 1 degree or .2" difference in your fork tube height or angle?

 

[captainRT]

I am working on your answer right now, ktm sx /cr500 yeti129ss, just need camso to send me my friggen parts so I can get up and running. Any ktm sx 450 frame will be miles better, I like the PDS version as you can move the track the furthest forward.

 

[chumbilly1]

I am working on your answer right now, ktm sx /cr500 yeti129ss, just need camso to send me my friggen parts so I can get up and running. Any ktm sx 450 frame will be miles better, I like the PDS version as you can move the track the furthest forward.

Agree that PDS will allow you to move kit further forward. Unfortunately it seems like it would be rather complex with the yeti mount bracket design. If you machined new mount plates that got you significantly closer to frame, then it looks like it would interfere with the bike adapter mounting holes. Im curious how you are going to get around this as I have a 129ss mounted to my PDS 300sx. If you build new mount plates let me know I would be interested in a set. Best idea I had was to eliminate the second bolt and related flange area. Those bike adapters are pretty much press fit and one bolt might be sufficiant. ????

 

[summitboy]

I think you are on to something with the frame for sure.

Don't wanna be the first guy with that BRC. Looks nice but I'm curious if those trans gears are going to survive.

I’d be very surprised if the gears didnt hold up. Everything about the BRC thus far shows the 300 can handle some upgrade. Even reeds can flow more than whats needed. I think the stock 500 output was similiar to what a new gen 300 output is. I guess time will tell. I love the fact Riley at BRC is just down the street from me lol. Maybe a guy could cryo if the gears didnt hold up ?? If a guy had this motor in a 15/16 frame it would be great imo !

 

[eric82930]

The frame geometry only effects the rake and you can change strut rods fork height and such to make the rake anything you want. The
Trail is set by the triple clamps spindle and ski saddle position.

I have 2 snow bikes with the same frames skis and spindles. The long track is coppered out and the short track fork is straight up and down by comparison. The long track steers harder only because of more ski pressure. But it has to have more ski pressure to turn the longer track.

What i notice tires me out most is the ski keel size and ski pressure caused by to long a strut too stiff front or rear springs. You probably like the camso better because the ski has no keel and the track has no rear spring.

The new ktms at least are so close on geometry between an xcw and sx all you need to do is turn the rear spring preload down half a turn or run a smaller rear tire and the rake is identical then switch the t clamp offset 2mm to match. I do this on all my xcw's.

The frame swap would be cool to get the skinny modern ergonomics but is over kill if you just want to change rake.

 

[M5-]

I don't have the number right now, I found it once but anyways the old CR500 has a ton of rake I'll find the number but its like 3 or 4 degrees more that the YZ. I don't think there is any strut adjustment that's going to fix that, the thing was designed in the 1980's. Its a full on chopper end of story. Its really hard to make a slow tight radius turn in the trees on the 500. It climbs like a mountain goat though I will give it that.

I've ridden my TS LT for 4 seasons going on 5 now and before that I had an ST kit. More ski pressure isn't the answer for the TS/CR500 bike. The Yeti ski allows the kit to track better and stay on course but the steering effort is much increased which equals tired rider. The old TS ski doesn't have enough bite for the CR5 frame and washes badly. The Camso ski doesn't have a ton of bite probably less than the old TS ski but the combination of rake and shorter frame makes the whole unit more compact which yields a tighter natural turn radius.

I'm a skier. You don't have one pair of skis for all conditions. If I'm skiing groomers which I try not to at all cost I don't want my reverse camber back country fatties I want a race ski with a GS side cut. If its really boring I want a super G. Pure slalom skis are fun but the have so much side cut that they are slow but holy crap they turn.

Snow bike skis are crude at best. None of the manufactures have played with side cut which seems like an obvious thing to me. Different conditions different side cut. So what I am getting at here is as snow bikers we need to address the turn radius, so frames, distance from kit to bike and rail spacing/width. Shocks and springs and blah blah blah, yah for sure but its more basic than just that.

I think CMX is on the right course getting the track as close to the bike as possible. The whole unit needs to be more compact. As far as PDS vs linkage if it was possible I could mod the frame as necessary to center the strut. I have a PDS roller in the shop I will have to check it out.

Are the 350 and 450 frame cradles the same. I know the 250 doesn't work all that well.

As far as the BRC goes, way cool I don't want to be the first kid on the block with one and I don't like being married to a single company/parts supplier.

M5

 

[chumbilly1]

Not sure if the 350/450 cradles are the same. Doubt it. But the motor still has to be bigger than a 2t. One could always cut the linkage tabs off as well to get the kit closer to the bike.

 

[M5-]

I'm not looking to start with scrap so how smashed is your frame. Can you post some pics. If I build this its going to be a nice bike. I may well be interested in the tank and triples, I already have the forks.

M5

 

[chumbilly1]

The frame rails are flattened from impact with rocks while endurocross training. I bought bike this way, and race it competetively in the A class. Doesnt compromise integrity at all just resale. Only reason I am considering replacing frame is that I found a 15 frame, which is powder coated orange, and has clean local title. My frame is grey, and I think the orange looks sharper.

 

[chumbilly1]

As for the BRC, I was glad to here that the motor uses a honda cr5 rod and piston. This will make some wear items easier to source. Of course if one had a catastrophic failure you could be back to square one.

 

[M5-]

I honestly haven't given the motor part of it much thought but my inclination right off the bat would be to build a CR5 motor using somebodies billet cases and my tranny etc. If the stock CR5 case halves were more available and stronger it would be a no brainer. I doubt I would bother with the Panthera e-start but I would likely do their stator setup.

M5

 

[M5-]

OK so here is a very non scientific comparison of my 2 bikes. With the bikes sitting flat on the shop floor I measured the distance between the center of the front fork axle bolt and the center of the swing arm/motor mount bolt. You would get the same numbers if the bike were on its wheels.

2010 - 2013 YZ450 35 inches c to c
1992 CR500 steelie 36.5 inches c to c

If you find yourself with a few moments throw a tape on your bike and post up the results.

Thx

M5

 

[CATSLEDMAN1]

busy busy busy

Mostly KTM in my riding group.

SX fork rake is steep and the TS kits were basically built around this concept.

I ride a 500xcw, to make my ski have the same trail manners as the SX,I have had to mod my spindle so I could run my ski bolt 2.75 inches behind the fork axle. The SX works good at 2". If you run the TS spindle in their position on any of the KTM or Yam. you will have a second class handling ski.........always busy. Checking stock TS spindle on 2 stroke EXC's ski bolt will be directly under the axle..........whoooo, not good, BUSY BUSY, again some where at 2.5 back big improvment. I have not tried an ARO setup on the 300exc yet.

Now running the ARO ski and spindle , somewhat the same issue. Swapping back and forth between Dan's SX and my XCW, not the same. I have yet to move my spindle but that is in the works. We have been experimenting with the single blade center runners, when we get a better handle on that in all kinds of conditions, then I will rotate my spindle back to approximate the position the SX runs. I am sure there is room for improvement.

 

[chumbilly1]

2013 450sxf 36 5/8" .. How about from center of axle to center of driveshaft? 45" on mine with yeti. I could move to 44" with a little work.

 

[chumbilly1]

Ill get pics of frame. Bike is in trailer for winter, and I will need to remove skidplate.