Chassis Evolution

[Ji Gantor]

This topic is for information only and the detail is not to be used without checking with manufacturers recommendations.
There may be some error in some of the data and it can be considered that this is not a technically correct document.

The heart and soul of a motocross bike may be the engine but the chassis determines how the power from the engine can be used. It also effects the steering and the absorption of impact loads due to uneven ground or jumps. The chassis also holds everything in place so all the components can work as a team.
The frame is the skeleton of a bike and goes mostly unnoticed until it flexes or cracks.

The tube frames of the early bikes were mild steel that had a tensile strength of 450mpa and weighed 7850kilos/m3.

My research is limited but in the mid 60s the Rickman brothers designed and built the first designated motocross frame out of chrome moly. I am not sure what grade it would have been but lets say it was 4130 which is the most common. 4130 Has a tensile strength of 650mpa and weighs 7850kilos/m3. I am not sure how the brothers fused the tubes together but I would assume they oxy welded the joints. Oxy welding is still used today to weld chrome moly along with Tig and Mig. Oxy and Tig heat the members up evenly while Mig is a very quick heat that cools very fast. This quick up and down heat in a small area produces brittle joints in thick walled pipes. If the tube wall thickness is less than 3mm thick then there is no need to preheat the members and Mig should work fine but some old welders still preheat the tubes before and after welding. The proper filler rod also plays a big part in joint strength. ER 80S-D2 gives the best result, ER70-S6 is still used but there is a little strength loss and 310, 312 stainless steel filler rod seems to be an old favourite.

As can be seen chrome moly is the same weight as mild steel but it is 70% stronger so the weight reduction is gained due to less material being used to do the same job. Flex or what is know as deflection in engineering speak is not really governed by strength but by depth of beam. The flex in a motocross chassis is very small because the length of the members or tubes between supports are very short. Thus flex or deflection is not the main factor when designing a chassis. Bending moment and shear would be more concerning and these forces are helped by extra strength and tube wall thickness. The next biggest problem is fatigue failure. If you have ever bent a piece of wire back and forth you will know that eventually it will break, this is fatigue failure. Remember, because the tubes are only short they don't have to flex much in both directions and say 40 000 cycles later the tube exhibits a crack. Chrome moly is more tensile than mild steel so it is more brittle thus if the frame received a direct impact, mild steel would yield or bend while chrome moly will if the force is great enough break.

Chrome moly is a great material to use for motorcycle frames and I think KTM still employ the metal in their chassis. Alloy is the new metal due to welding procedures and the loss of even more weight if used correctly.


Ji
 

[Ji Gantor]

From my limited research CZ used chrome moly for their chassis metal starting at around 1968. This produced weight savings and a stiffer frame. I can not find anything about Maico's but it would appear from the few brochures I have that they were using chrome moly for their frames in the early 70s.

These early MX bikes only had 4 inches of rear wheel travel and the chassis's must have taken a pounding over jumps designed for new models, but 30 years later those frames are still going strong. Vindication that 4130 was the right material for the job.

Ji

[Ji Gantor]

Chrome moly or CrMo is also used as axle, dampener rod and foot peg material. Titanium is the next big thing but it is very brittle. The fire arms industry used it for all sorts of applications that did not suite its true calling like handgun slides and firing pins. The firing pins shattered after only a few uses. They have been using titanium valves in four stroke engines for some time know especially in the V8 Super Car engines. To compensate for the brittleness of the valves the seats are made from beryllium a soft and deadly material that after a certain amount of use will no longer hold its shape and have to be replaced. Beryllium has high temperature stability, and low coefficient of expansion with temperature.
Titanium can not be used much at this stage if my memory serves me right as FIM only allow a limited use of the material.

Ji

[mx250]

Thanks Ji, very interesting .

[Nathan S]

The tube frames of the early bikes were mild steel that had a tensile strength of 450mpa and weighed 7850kilos/m3.

That 450MPa figure seems high. The FIA approved CDS roll-cage material (which was such a bitch to get hold of in Australia, that CAMS changed the rules to allow CDW) is only rated to 350MPa.
Schedule X (aka "dash") tubing is ~250MPa, so a "mild" steel that's 450MPa wouldn't be very mild at all???

[Ji Gantor]

The standard BHP CHS Dura Gal C350 Grade C350LO has a min Tensile strength of 430mpa here in Australia. The tube I referred to is the US standard.

The reason that the roll cage has to be only 350mpa is so it will bend instead of a catastrophic failure when impact occurs.

Ji

[Oggy Doggy]

As a slight addition to Ji's nice piece, Rickman frames are in fact made from Reynold 531 tubing which varies only slightly from 4130 chrome-moly. The tubes are butt brazed using special bronze alloy rods made specifically for the Rickmans by Sifbronze and liquid flux. Anyone who's ever done work on a Rickman, Cheney or most other 531 constructed frames will vouch that the quality and strength is beyond reprooach. You don't see too many broken 531 brazed frames.

[Ji Gantor]

Hi Oggy Doggy,
That is a great add to my topic.
Thanks mate

Ji

[VMX247]

and I'd like to add that Titanium is the best material for frying pans made so far to date  
Alison

[GMC]

Steel comes in many grades, the hard part is to find the shape of tube you want in a specific grade. (same problems in alloy)
Structural steel (large RHS & I beams) will have a higher tensile stength for structural purposes but most of the small tubes I don't think are very high.
Black flat is very low in tensile strength but bright flat (usually 1020) is much better.

4130 Cro-Mo is rated at 650 mpa.

Grade 1018-1026 is 350 mpa. I know this is brought in in tube & is used in roll cages. It's a bit cheaper than Cro-Mo & doesn't become brittle next to the welds.

From my experiance all Cro-Mo becomes brittle next to the welds (the heat affected zone) no matter what it's welded with. I always heat treat my frames to "normalise" the Cro-Mo

The Poms usually build their frames from Reynolds tube. The tensile strength is slightly higher than Cro-Mo & they usually use nickel bronze to weld it thus avoiding any brittle zones.
From what I have heard it is hard to obtain, only made in afew sizes & usually only in the UK.
Despite its higher tensile strength I haven't heard of it being used in planes, only 4130?

CZ may have claimed to use Cro-Mo as did Maico & many other brands but from my experiance of repairing frames they are just not in the same league as 4130.
They may well be Cro-Mo but of a lesser grade.

I think the current alloy frame fad is more about casting sections & CNC maching.
A modern frame may only have a dozen pieces to it.
My HL frames for instance has about 90 different pieces to be prepared before welding.

[LWC82PE]

BSA B44 GP is also made from Reynolds 531 brazed together.

someone was telling me the other day about a new bike being made (i think CCM) that has the frame made from a carbon fibre or plastic material, it was something strange and out of the norm for a bike. It was somthing that involved the different frame parts being glued togther, it might of been alloy tubes gued together with a resin or something. I wish i could remember the exact details.

[Nathan S]

Interesting point Geoff.
The yZ125E was suppsoed to have a chromoly frame, over the 'non-chromoly' of the earlier ones.
Having one D frame and one E frame in my hands at the same time, showed no sign of them being different materials - the weight difference was imperceptable and the wall thicknesses are the same.
Based on a sample of three each, YZ125D and E frames are both just as prone to corrosion as each other (ie: Nothing dramatic in either frame, but the E frames showed none of the additional corrosion resistance that Chromoly should have).

Plus... the only badly cracked YZ125 frame I've seen was an E....

[Ji Gantor]

BHP grade C350LO 430mpa CHS tubes come in 26.9mm and 21.3mm diameters.
CrMo still rusts.
Razor blades rust, chrome vanadium tools rust.
I don't think there is an easy way to tell mild steel from CrMo except via a hardness test.
Every year the frames have been getting smaller because our knowledge on how the loads are applied to the frame is getting better. Take GMC's CZ swingarm it weighs 2.3kilos. This means that Geoff has made the swingarm with a thinner wall thickness tube than stock and stiffened it up via a gusset plate, thus reducing the weight by 400 grams.
If two frames have the same shape and the same diameter tube with the same wall thickness but one is CrMo and the other is mild steel than they would weigh the same. If the frame that was CrMo had a crack in it this kinda proves that it was CrMo because it is more brittle.

Ji

[vmx42]

Hey JI,
Enjoying your chassis rave, but you need to do a bit more research on Titanium. It is certainly not known for being brittle and its applications are far more varied than just being used in V8 Supercar valves.

Most passenger and military aircraft use enormous quantities of the stuff in their airframes and engines. In fact the new Airbus A380 uses 26 tonnes of titanium in the 4 engines alone. Not really the place to put a brittle material.

Lots of production MXers use a small amount in engines, specifically their valves as you say, but Yamaha has used it recently for the rear shock spring on the YZs. Again not really an application for a brittle material.

Re the CCM: it uses a machined/fabricated aluminum frame that is bonded with aerospace epoxies and not welded. It uses a bit of carbon fibre [fuel tank and possibly airbox/subframe] but it is essentially just a production Yamaha YZ engine in the alloy frame [as described] that uses KTM suspension [including stock swingarm  and geometry] and plastic. It is an interesting bike, but not earth shattering  except for the bonded frame - the Poms always talk up their latest marque resurrection as being a 'world beater' when in fact over the last quarter century on Triumph as suceeded.

And even bonded aluminium isn't that unique. The old Suzuki DR250/350 used a bonded aluminium swingarm 20 years ago. I hope they [CCM] enjoy a bit of success, both on and off the track, but they are not pushing the envelope as they seem to be claiming.

Back to you.
VMX42

[JohnnyO]

VMX42, just on the subject of CCM i have just read where 2 CCM factory riders finished in the top 11 at the first MX1 GP. Pretty good for a small company.