Airhead salt racer rear suspension geometry

Here’s a quick rundown of the process I went through in sorting the rear suspension geometry of the salt racer.

First task was to calculate the shock specifications. I started manually calculating these with the help of a couple texts including Tony Foale’s tome on the subject: Motorcycling Handling and Chassis Design. Even though the salt racer’s geometry is relatively simple compared to modern sports bikes with all their tricky linkages, I soon gave up and bought Tony’s program: Motorcycling Analysis which is basically the book in program form – this made things very much easier with the luxury of being able to easily experiment.  Highly recommended. www.tonyfoale.com

I’m not going to describe the process in detail but after inputting the geometry of the airhead frame + oilhead swingarm, I arrived at the following specifications for the shock (which ended up being similar to the stock R1100RT shock):

Free length: 340 (fully extended)
Travel: 50mm
Spring rate: 190N/mm
Lower clevis to suit the R1100 swing arm.

I opted for Hyperpro and soon after, a very sexy looking piece of Dutch bling landed on the doorstep complete with damping adjust both ways and ride hide adjust.

The program was used to predict the behavior of the system and tweek certain parameters so that ideal suspension characteristics were obtained – in this case we were shooting for a total sag (bike + Rider) of between 25-30mm with a spring preload of no more than 15mm. Most the program parameters as defined in the program’s set up page below are locked in:  The oilhead swing arm dimensions cannot be changed, nor can the spring specifications. The only parameters that can be changed are the ‘X coord’ and Y coord (which locate the top fixed end of the shock) and the spring preload. At this point we are mainly interested in the X coord and preload. The X coord is the horizontal mounting position of the shock in relation to the swing arm pivot.  I took an educated guess at the back wheel load at 112kg with me as rider.

A few button presses revealed that a X coord of 35mm combined with a spring preload of 13mm gave a total sag of 28mm. You can see the 35mm of X coord in the scaled graphic:  the top end of the shock is just behind the swing arm pivot. A spring preload of 13mm is a little on the high side for this particular spring. I could have reduced the necessary spring preload to get the target sag by pushing the top of the shock futher behind the swingarm pivot e.g. X coord of 40 or 50mm but there were other reasons why this was not so practical including complicating the mounting arrangement to the frame. So a X coord of 35mm was settled upon.   Now the Y coord could be tweaked, but this was done on the bike with shock attached to swingarm – more of this later.

Here’s the setup page of Tony’s program showing all the vital suspension stats of the salt racer. BTW, this is only the rear suspension part of the program. You could go on to model the entire suspension behavior of a bike if you so wished.

Tony Foale motorcycle setup page

 

The following is one of the many graphs that Tony’s program spits out. It shows shock displacement V’s wheel movement. The point to note is that from a completely unloaded state, the sag under full load (rider plus bike) is 28mm (vertical black line). Remember, nobody has sat on a bike yet, we are using the program to predict what the suspension will do in order to best locate the fixing point for the top end of the shock to the frame.

 

tony_foale_B

 

Another interesting graph is the effective spring rate measured at the wheel or ‘wheel rate’ Notice that even though the spring has a linear rate, the effective wheel rate is progressive ranging from approx 12 – 18 N/mm. This is the reason that people shouldn’t blindly ‘upgrade’ their linear springs to progressive rate springs. The suspension geometry may already be designed to obtain a progressive effective rate at the wheel with a linear spring. This is particularly the case for modern bikes with complex suspension linkages.

 

tony_foale_C

 

So,  having pegged the X coord at 35mm aft of the swing arm pivot, next job was to work out the vertical co-ordinate from the ground of the shock mounting position to the frame (Y coord). With a shaft drive, what we want to happen is this: when the bike is fully loaded, the gearbox output shaft axis, drive shaft axis and center of the rear wheel are all on the same straight line.  This ensures that any mechanical inefficiencies from universal joints operating at angles are dialed out – particularly for the salt racer where I’m chasing every microhorse. In reality, of course, this straight line position will become the median about which the suspension moves.

The image below shows the bike in an unloaded state (bike supported by jack under oil pan). The top edge of the bar with the blue tape marks the position where the drive train is perfectly aligned. With the top of the shock unattached, the bike was jacked up so that the center of the wheel was approx 25-28 mm below the top top edge of the bar. With top of shock 35mm aft of the swing arm pivot (Y coord) we now had the mounting position of the shock to the frame. I then mocked up a temporary mounting arrangement to lock the shock into this position. So, theoretically, with rider aboard the suspension should sag by the predicted 25-28mm at which point the drive train becomes aligned…

 

RearSusp_Mockup3

 

Now to test our predictions! With shock  firmly fixed to the frame I hopped on board. The measured total sag was 21mm. Not bad! I didn’t expect it to be spot-on because the bike was not complete and therefore under it’s finished weight. But it’s in a workable ball-park, there is enough adjustment in the system – adjustable torque arm and ride height adjust on the shock –  to tweak from this point so the next step is to remove the temporary bracketry and properly build in the top of the shock.  To be continued….

 

RearSusp_Mockup2

 

 

RearSusp_Mockup1

The business end

The frankenswingarm is finally looking like it should. Freshly powdercoated K100 17″ wheel; powdercoated R1100 RT swingarm; R1100S bevel drive; aftermarket carbon hugger for R1100S; Custom made adjustable torque arm. All grafted to the arse of an unsuspecting airhead.

 

frankenswingarm

Airhead

calculating CR – a simpler way

Here’s a method that involves little math compared with the conventional method as described here

The following method involves bolting the head and cylinder together as an assembly on the bench using all-thread. Then carefully placing the piston in the cylinder at exactly where it would be if it were at top dead center (TDC) in an assembled motor. The  volume of the combustion chamber (Vtdc) can then be measured directly with the burrette. It’s then simply a matter of dividing this volume into the entire volume above the piston at bottom dead center (BDC) to determine the compression ratio as illustrated in the diagram below.

 

Compression_Ratio_1

Step 1. Determine piston position at TDC: We do this using the scribed line on the feeler gauge that was used  in method 1 to determine the deck height ‘dh’ and repeated her:

To measure dh, clamp the cylinder to the block using tubes on the studs, turn the motor via the back wheel or an 8mm allen key on the alternator rotor bolt to until the piston is at TDC. Slide a squared off 0.1mm feeler gauge between the cylinder wall and piston so that it rests on the 1st compression ring. Use a very pointy scribe to mark the top of the cylinder – some prussian blue would be handy here. Make sure you slide the gauge in line with the horizontal axis of the gudgeon pin (ie at 3 and 9 o’clock looking directly at the piston) as this is where piston rock is at a minimum. If excising care, you can measure dh with this method to within +/- 0.1mm.

 

Bolt cylinder to case using tubes

Bolt cylinder to case using tubes

Scribe line with piston at TDC

Scribe line with piston at TDC

TransparentPic

 

Step 2. Off to the bench: Move cylinder off the bike and onto the bench then carefully place the piston at TDC using the scribed line on the feeler gauge. Grease the rings a little first in order to make things water tight. Before bolting everything together, use a vernier caliper to take a checking measurement from the bottom of cylinder skirt to the bottom of the cylinder to give me a means of checking that the piston hadn’t moved position after bolting everything together and buggerising around.

Step 3. The bench top assembly: Bolt cylinder, gasket and head together using 10mm all-thread then use checking measurement at Step 2 to check that piston hadn’t moved during the bolt up.

Using scribed line to set piston to TDC

Using scribed line to set piston to TDC

Taking measurement to skirt bottom

Taking measurement to skirt bottom

Bolting it all together

Bolting it all together

 

Step 4.  Measuring the combustion chamber volume (Vtdc) with a burrette: If you examine a head you’ll, see that when the plug hole is vertical, a part of the outer periphery of the chamber is higher than the bottom of the plug hole and around a corner a bit like under the lip of a toilet bowl. So if you fill when the plug hole is vertical, a sizable air pocket will form at this higher peripheral part.

The way to get around this is to first fill to the bottom of the plug hole with the cylinder vertical on the bench. This allows the peripheral volume to completely fill. Then, tilt the assembly so that the plug hole is vertical and continue filling until the fluid level just reaches the bottom of the plug hole.

BTW: The burette is the cheaper acrylic type (100ml) and cost me $27.00. Graduations are 0.2ml. (The glass ones are over $100). Use water or metho with food colour or some other means of tinting.

Final filling with plug hole vertical

Final filling with plug hole vertical

ditto

ditto

TransparentPic

 

So the final number for my particular R100 BMW airhead? Combustion chamber volume Vtdc = 66.7 cc’s (or ml).

Step 5. Calculating CR: All that remains is a fairly simple calculation. In the case of my particular R100 airhead: Stroke = 70.6mm and bore = 94.25mm (first oversize)

method2_calcs

Not terribly high for a large valve early 80’s R100 that supposed to be 9.5… but that’s another story…

Adapter flange twix airhead & oilhead

An adapter flange is needed between the output flange of the airhead gearbox and the drive shaft of the oilhead swing arm.

The adapter has a round spigot that is press fit into the female spline of the uni-joint on the R1100 drive shaft. Then the adapter will be welded to the uni-joint. The adapter is turned and milled from a piece of 75mm K1045 round bar.

flange_finished

flange_onBike

 

flange_ready

ready for welding

flange_weld1

flange_weld2

Moto Guzzi Lemans Mk111 Agostini

Fixing busted head fins

I got a couple heads from a 1989 R100GS for the salt racer. These are the ones to have since they have the so-called ‘D ports’ and have larger intake spigots since they take 40mm Bings. Got them cheap because one of them was busted up: 4 broken fins, a cracked 6mm hole that takes the rocker cover stud and a stripped spark plug thread. Other then that, they were  OK!

Aside from the broken fins, you can see where I welded up the rocker cover hole ready to face and re-drill:

Fin_weld3_M

The process involves TIG welding a bead on top of  the broken fin edge, then another bead on top, then another, then another….until a thin wall is built. Then into the mill to have the sides shaved to the correct thickness then another couple beads and repeat.

Fin_weld5_M

Fin_weld4_M

And after quite a few hours of welding, milling and filing:

Fin_weld6_M

Fin_weld1_m

 

 

Dyno coastdown test

 

salt racer rear end

DIY dyno comipete!

dyno almost complete

TThe dyno is almost complete. The Datamite 111 dyno system by Performance Trends is hooked up and loaded into the laptop and I have taken the first trial runs. All that remains is to calibrate the dyno and hook up the brake caliper. Oh, and the small matter of building a room around it!

DIY_dyno_complete1

DIY_dyno_complete2

airhead salt racer gets stiffed!

TThere’s a certain satisfaction in hacking tons of detritus and redundant bits off a frame – a bare canvas on which to plonk your own detritus and redundant bits!

Effectionally known as the ‘rubber cow’ the airhead frame is well known for it’s elastic nature.  BMW, in their wisdom, took the lengendary Norton Featherbed concept but rather than continuing each loop over the top and under the tank to the head stem, the BMW loops converge at a single spine that starts at the rear end of the tank and continues to the head stem.

This is where a good deal of the elasticity lurks…. and waits to be exited by the various pendulum effects from the front end and rear swing arm sections amongst other things.  I love a simplistic look but this can be hard to achieve. I could have gone the way that so many do and add tie rods that run from the head stem down across each side of the motor to the swing arm pivot area, but to my eye, this really stuffs up the simple lines of the bike.

I stood and stared and orbited around the frame for hours before settling on a stiffening plan and opted to horizontally stiffen the spine area and tie the top loops through to the head stem in a way that replicates the original Featherbed idea. Some gusseting of the head stem area was also added. Measuring and drawing in CAD for laser cutting makes things a whole lot easier.

airhead slat racer frame stiffening.

 

BMW airhead frame stiffenin

The oil cooler is a hand- me -down from my shed mate Ross’s Triumph Bonne salt racer build

 

laser_cut_frame_stiffening

laser cut stiffening plates