R1100S - GS length torque arm (365 mm).

HerrFlick · Post by **HerrFlick** » Sat Mar 21, 2015 5:05 am

Had always thought about trying one of these but they've become as rare as hen's teeth or as expensive as rocking horse poopoo.

Returning to the saddle after two years (and fresh eyes) I found what had always made me feel slightly nervous about the front end: understeer.

Obvious in tight slow roundabouts; front wheel wants to flop over too far and push out from underneath. Needed firm push on the inside bar and pull on outside bar to hold a steady line. Then when gently accelerating out a tendency to run wide. I had become used to it previously but was never 100% confident abt the front end. Experimented with tyre pressures but no joy. Michelin Road pilots each end.

I'd already wound the rear ride height to max (helped a little) so next was the shorter brake stay arm.

Decided to make my own until I could either find a GS unit or have my "S" unit shortened.

A couple of suitably sized sections of alloy strip and viola.

Engine ends don't have the benefit of isolating rubber bushes so are a tight fit on the thru bolt. Not permanent, but will suffice on smooth roads for a week or two.

BUT BUT ... no understeer. YAY. Rolls into tight corners with a neutral feel and same on exit.

Turns out the Boxer Cup bikes also have 365mm stay arms.

Perhaps this is why "S" bikes have the reputation for being so sensitive to tyre pressures.

Cheers.

eyore · Post by **eyore** » Sat Mar 21, 2015 7:34 am

Nice job, When you say a couple of lengths of alloy section, what do you mean exactly? Is it channel back to back or what. Also how much has it raised the seat height?

HerrFlick · Post by **HerrFlick** » Sat Mar 21, 2015 11:18 am

eyore wrote:Nice job, When you say a couple of lengths of alloy section, what do you mean exactly? Is it channel back to back or what. Also how much has it raised the seat height?

Thanks Eyore.

Two flat strips of 32 x 3 mm aluminium. Only temporary until I find a GS arm.

I didn't measure the difference but I seem to remember others saying it was about 1-1/2 inches. Certainly noticeable rise in seat height.

I have the rear rear shock ride height at max also, so I could wind that down to lower the seat height but then I'd be getting back near the original front end rake/trail. A little less rake from the factory might have been better.

Boxadog 2000 · Post by **Boxadog 2000** » Sat Mar 21, 2015 1:53 pm

That is an accident waiting to happen.
The final drive exerts force forward.
They will buckle and you will be at best in casualty.
Join the two strips together and you might be safe

Corvus · Post by **Corvus** » Sat Mar 21, 2015 3:30 pm

Boxadog 2000 wrote:That is an accident waiting to happen.......

Agreed!

My opinion is take em off until you find either the pukka item or come up with a sound engineering solution.

Not worth the risk.

HerrFlick · Post by **HerrFlick** » Sat Mar 21, 2015 3:46 pm

Boxadog 2000 wrote:That is an accident waiting to happen.
The final drive exerts force forward.
They will buckle and you will be at best in casualty.
Join the two strips together and you might be safe

By Jove you're exactly right. Thankyou. (Still with some chain/sprocket mindset).

Will join them, through bolt with a separating spacer, to keep strips perfectly straight. (And Mr Euler happy).

Cheers

Boxadog 2000 · Post by **Boxadog 2000** » Sat Mar 21, 2015 7:44 pm

At Verily least
My opinion is bin it
With all due respect you do not have a clue with regards to dynamics
This not a personal affront

HerrFlick · Post by **HerrFlick** » Sun Mar 22, 2015 3:14 am

Boxadog 2000 wrote:At Verily least
My opinion is bin it
With all due respect you do not have a clue with regards to dynamics
This not a personal affront

"Not a personal affront?" Harrumph koff koff bluster splutter ... we'll see about that: slide rules at 30 paces my good fellow!

Each Al strip is 32 mm x 3 mm, 365 mm between centres, ends pinned.

Critical buckling load: 8,920 lbs per strip.
Total buckling load: 17,940 lbs.

De-rate by a factor of 10 (normal safety factor plus shock load): 1,790 lbs.

Pinning the strips together half way will triple this load capacity to 7,100 lbs

Working loads: I weigh 120 kg. Bike 240 kg. Total: 360 kg. Actually kg wt.
Ratio of wheel radius to brake-stay-mounting-point to axle = 2.14

Were I to drop the clutch and achieve 1g acceleration, horizontal inertial force at the rear axle will be: 360kg. (Actually 36kN, but since load calculation formulas use gravitational units of force there's not point changing to kN).

Multiplying by 2.14 = 780 kg.

This is the compression load applied to the brake stay strut (each end pinned).

Or in Lubs: 1717 lbs.

Given the de-rating factor I've applied, and given that I've made no allowance for the peak-stress reducing effect of the large rubber isolating bush at the rear end, (which can be around 5), I'm quite comfortable with this setup.

As mentioned earlier, with the struts pinned at the centre, the load capacity will rise to 7,100 lbs.

Analysis of the BMW arm is not so straight forward. It consists of two separate U-sections, but with a stiffening rib rolled into the U.

Considering these as simple U-sections, each has a maximum compressive load of 1942 kg, or approx 4000 lb. 8000 lbs total. (Johnson Formula. Low slenderness ratios. http://inside.mines.edu/~jsteele/411/Le ... ckling.pdf).

Using the same formula for the alloy struts gives 680 kg each, or 1300 lbs, for a total of 2600 lbs.

Pinning the struts at the centre raises the load capacity to 865 kg per strut or 1904 lbs. Total: 3808 lbs.

So I'm still comfortable with what I've fitted, but as I said in my first post, it's very short term temporary.

Why is the BMW item so strong? A few possible reasons: came from the off road bikes therefore susceptible to bending impacts. Has to cope with vibration-induced metal fatigue/cracking.

Cheers

Corvus · Post by **Corvus** » Sun Mar 22, 2015 8:41 am

HerrFlick wrote:
Boxadog 2000 wrote:At Verily least
My opinion is bin it
With all due respect you do not have a clue with regards to dynamics
This not a personal affront
"Not a personal affront?" Harrumph koff koff bluster splutter ... we'll see about that: slide rules at 30 paces my good fellow!

.......

Ha ha. That is genuinely funny.

I'm not convinced your appraisal of the loads and dynamics is right? The exact function of the strut in question is an interesting one for sure and I'm looking forward to your interpretation of it.

Not digested what you've put forward yet, but one thing which seems obviously missing to me is that you don't seem to have built in any factor for how much you and the bike combined weight will be effectively multiplied going over large bumps (as just one example). All this happening while under hard acceleration (thus mass transfer effect), with a passenger and luggage.

I can't see anything in there considering transmission reactions? True, they're related intrinsically to acceleration and deceleration, but that's not really what you've put forward?

Another thing to be wary of when considering buckling is that the values given will assume that the load is applied perfectly in line and perfectly stable. The fact that the anchor point consists of a single bolt through a rubber bush will not facilitate this perfect scenario. As soon as the load goes slightly "off centre" the buckling value will diminish alarmingly.

There's every possibility that riding around with what's on the photos will be ok, depending on how you intend to use or abuse the machine, but I wouldn't risk it personally. I'd just use the setup to assess your preferences for static geometry, taking into account propshaft angles and then have a proper machined beam made up.

All the above is just my opinion.

Cheers.

HerrFlick · Post by **HerrFlick** » Sun Mar 22, 2015 9:17 am

Corvus wrote:
HerrFlick wrote:
Boxadog 2000 wrote:At Verily least
My opinion is bin it
With all due respect you do not have a clue with regards to dynamics
This not a personal affront
"Not a personal affront?" Harrumph koff koff bluster splutter ... we'll see about that: slide rules at 30 paces my good fellow!

.......
Ha ha. That is genuinely funny.

I'm not convinced your appraisal of the loads and dynamics is right? The exact function of the strut in question is an interesting one for sure and I'm looking forward to your interpretation of it.

Cheers.

Hi Corvus.

As I've said in earlier posts, this is a short term temporary arrangement until I get a genuine GS arm. It was also to test, before I outlaid on a GS arm, if the geometry change would fix the understeer problem, which it did. Yay.

I am mindful of both axial loading eccentricity and the effects of road shock loads, so the bike is being used in town (smooth streets) just to tootle around doing errands etc.

In the light of this discussion it's interesting to examine the dimensions of after-market torque arms, and of those used on other heavy Beemers.

The curved, top mounted arm of the R1200 would be an interesting one to analyse, in that it is put under tension during acceleration and compression during braking

Cheers.

John C.

Corvus · Post by **Corvus** » Sun Mar 22, 2015 9:40 am

I'm a reasonably brave guy. Definitely adventurous thinking when it comes to challenging engineering convention. But there's no way I'd go out on that bike with 3mm ally strips between me and a wrecked transmission, to say nothing of being spat into a hedge.

You can wave all the maths you like in front of me and I still wouldn't trust it (although, curiously, if you waved a thousand quid at me I might. Go figure!).

Take the bits of baco foil off please!

Blackal · Post by **Blackal** » Sun Mar 22, 2015 10:07 am

I'd be worried about shock compressive loads on that arrangement. A small lateral deflection, and the integrity has gone ........

But - if you've got this sort of background............... Crack on!

Al

Merecat · Post by **Merecat** » Sun Mar 22, 2015 10:10 am

Quote from my chief engineer in a previous life:

Why use 3/8" plate when we have 1 1/4" in stock?

Stupid.. Yes, but the logic is sound. Always Over engineer for safety.

SP250 · Post by **SP250** » Sun Mar 22, 2015 10:38 am

I just bought a new GS one about a year after I had the bike and it speeded up the turn in and raised the rear ride height so that I wasn't decking out the cylinder heads as much. It didn't feel any less stable in a straight line and still didn't need a steering damper, was just as planted and steady in the corners whilst laid over too.
As a qualified mechanical engineer, I would only trust those bits of strip ally for a geometry check on the bike bench, no matter how many times I checked the calcs.
To ride it, I would wait till I had the correct BMW or aftermarket brace, I must be getting old as I'd rather be safe than a statistic.

HerrFlick · Post by **HerrFlick** » Sun Mar 22, 2015 4:00 pm

Great to see all the different opinions and concerns.

"As I've said in earlier posts, this is a short term temporary arrangement until I get a genuine GS arm. ...... ........... ........ ....... so the bike is being used in town (smooth streets) just to tootle around doing errands etc."

For those number-doubters, I presume you've driven over bridges like the Clifton Bridge, Forth Road Bridge, Humber Bridge, or that you've flown in those huge airliners with the bendy wings. Then there are those skyscraper lifts. Errrk.

Cheers.