Slingshot v Tri Pod 1

Discussion in 'Polaris Slingshot vs The Competition' started by TriPod1, Mar 18, 2015.

  1. TriPod1

    TriPod1 New Member

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    Hi Noah,

    The track is around 62" but will vary depending on the wheels chosen.

    Wheelbase is 100" - 102" but will depend on which donor you employ (see below re choices).

    Tire size is 195/50R15 at the front and either 195/40R17 or more likely (way more choice and better tires) 205/40R17 at the rear.

    We recommend 6" (4 x 100 pcd and preferably around +30 offset) rims up front and of course whatever came with the bike at the rear (either 5.5" or 6").

    Engine choice at this stage is Blackbird CBR1100XX, Hayabusa, ZX14 or Bandit GSF1250. The original prototype is a VTR1000F Honda V Twin but customer preference has been very much strongly towards the 4s. In reality as much as I like a twin, a four is better suited for dragging the extra weight around (smoother delivery down low and easier to get off the line). Having said that a giant HD or similar donk would do just fine but the thuddy vibes might irritate after a while...?? I'm interested in your thoughts re a biggish twin and why that is your preference. Certainly they offer more character than something as characterless as a Bandit 1250 but it is a very practical engine choice that pulls incredibly well and cleanly.

    No idea on torsional stiffness other than it is quite stiff and engineers have been happy with it re approval for the road. It is not a requirement in Aus for a torsional stiffness number to rego a three wheeler (probably for sound enough reasons, ie its sort of impossible to twist the chassis in use) but certainly is for a car. We have quite high expectations for that too. Around 4000NM per degree. I intend to measure a chassis in due course, probably fairly soon as the task moves up my todo list. It certainly feels very stiff while driving the vehicle at the track or some place the vehicle can be driven safely on the limit of adhesion (and beyond). What sort of numbers did your project generate. It certainly looks pretty heavy duty!

    Little doubt that a journalist got some facts wrong. It seems to be part of their job these days... We use bike calipers modified with spacers to fit over the 10mm thick rotors we use. We suggest our pedal box which has slightly shorter but more widely spaced pedals than your average race style pedal box. We employ bike rear master cylinders of appropriate size, un boosted of course and with a balance bar. So balance can be adjusted perfectly, and feel is enhanced without boosting. The brakes are fade free when driven at race speed at the track and achieve in excess of 1.2G stopping acceleration. In short the brakes are stunning and enormous fun to use. The main front end parts weigh -

    Upright - 2386g
    Hub - 1874g
    Rotor - 3260g
    Caliper - 1192g
    Total - 8712g (19 pounds)

    Not overly light and not nastily heavy but the set up will never break which is nice. The area for improvement is really making the mudguard bracket and mudguard itself a bit lighter.

    I appreciate your questions and thoughts.

    Regards, Andrew.
     
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  2. Noah Katz

    Noah Katz Active Member

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    Thanks for the great info!

    The weights of those unsprung components are better than I expected.

    Re engine choice, I prefer the sound (like a giant mechanical heart beating) and sensuous pulsing feel of a big low-revving twin over the frenetic sound and feel of a four, which also matches my use mode of cruising/touring, not that I don't like strafing the turns now and then.

    Big touring bikes aren't a lot heavier than the Tripod, so I don't see the issue there.

    I do like your idea of a big torquey Triumph triple; I love that ripping wail they make.

    The frame I built weighed 130 lb and torsional stiffness measured close to 5000 lb-ft (6800 N-m)/deg.
     
  3. TriPod1

    TriPod1 New Member

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    No problems re the info Noah.

    We could carve about 10% off those unsprung weight numbers fairly easily by machining off a bit of engineering overkill here and there but I haven't bothered because I doubt anyone would notice it at the tiller but lowering the other unwanted unsprung weight in the form of the guard and bracket is a goal. Driving the car without the mudguard on you notice a mild improvement in feedback from the front end through the steering wheel, but in reality even with the guard assembly on the steering is a delight so not a major priority.

    I'm with you on twins, love the sound although my preference has been towards Ducs and their jap copies such as the Superhawk rather than the larger capacity slower revving type. I'm also a fan of the Honda V fours and feel a lightweight Tri Pod with a SSSA rear end off the Interceptor and the V four engine of course would be a great sounding but smooth and tractable compromise. Maintaining the chain drive we need for our reverse and the gearing options the chain drive brings. Also the SSSA allows an 8" rim to be fitted for a better grip outcome at the rear but maybe at the expense of a tiny torque steer issue ???

    Your right of course re the weight of the big twins, but the issue I have had with those is that they are almost always belt or shaft drive, sometimes oil cooled and also quite long with a weak swing arm. Not really fitting in the back of the Tri Pod and not offering enough strength for the unusual loading in the swing arm.

    The idea of the Tri Pod is as much as anything, maximum thrills for minimal dollars so simplicity is key.

    Your frame is very stiff indeed!! I'll report back when I have some numbers on ours but obviously its not anywhere near that tight!

    Andrew.
     
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  4. Noah Katz

    Noah Katz Active Member

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    Interesting that you could feel a difference w/o the guard; how much does it weigh?

    Also, kudos for choosing sensible 15-in. wheels for the front vs. everyone else going 17 in. and up.

    An option for reverse that would eliminate the requirement for chain drive is a 12V winch motor with a roller that would swing into contact with the rear tire.

    Re swingarms, my guess is that if it's strong/stiff enough for a bike it will be for a trike.

    It doesn't see are no lateral g forces from cornering, but when heeled over the swingarm has to withstand bump forces which are unattenuated by the suspension.

    [edit] I'm going to backpedal upon further thought; tire compliance might reduce those forces to less than lateral g's when used on a trike.

    Stress analysis would settle the question.
     
  5. TriPod1

    TriPod1 New Member

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    I'm guessing our average fibreglass guard and steel bracket weighs around 4 to 5 pounds. I don't have accurate numbers like the upright assembly parts because our main aim with the guard is stiffness first and lightness second. Having the thing waving around causes weird negative feedback into the steering (steering feel) and of course the guard contacts the tire. We have tried many solutions and most work pretty well but now that we are doing the fibreglass in house again I will take a closer look at this myself rather than to some degree the fibreglasser making it up as he goes along. We could conceivably make the brackets out of some premium grade of aluminium too...

    Like I state above its stiffness that makes the most improvement in steering feel, the extra weight isn't the biggest issue but it does feel a little nicer in the front without the guards. Un sprung weight feels like its a huge issue or could be but Lotus did a study a while ago that suggested it isn't (after spending 40 - 50 years saying it was!!). I'm in the middle somewhere. Light is great but lets not let it break in half.

    The fashion for large wheels and super low profile tires is just that - fashion! Formula one cars seem to generate plenty of lateral grip with their chunky looking rubber and there is definitely a sweetspot of grip versus feel versus fun versus cost versus look versus ride quality. Same with the rear tire width. A T Rex style wide tire would not necessarily generate a whole lot more grip than our affordable narrow set up when there is only 400 pounds sitting on it. In the wet I suspect the T Rex can be a bit iffy in the rear...???

    Swingarms need to infinitely stiff laterally to avoid any torque steer. Some very subtle torque steer is probably acceptable so infinite stiffness isn't actually required in the real world but some of those Harley swingarms look awfully flimsy in the lateral plane. Harleys are limited in the lean angle they can achieve by the peg (floorboard) height so I'm guessing the engineers at HD aren't too concerned with making them real strong in that regard. Who knows?

    Torsionally most swing arms accept the serious ones off superbikes and other fairly sport things twist like a pretzel (are pretzels particularly twisty?? Dont know). Did load up the one off the Orange Prototype once (fairly beefy aluminium cast/extruded combo from Honda) and with a 200 pound guy hanging off an 8ft length of tube it twisted around 10 odd degrees (in one direction from rest position). So in theory at full corner load the rear wheel can twist from vertical a bit, and maybe generally feel a bit soggy rather than tight and crisp in the rear as it would if the wheel didn't twist and un twist between corners. I suspect a flimsy swing arm would 'feel' pretty ordinary. It may not break though, well not straight away. The handling quality of the vehicle is at the top of my priority list always.

    We will get to more of this testing in due course. The reality is that our test bed Tri Pod has done 6000 miles on horrendous dirt/gravel/sand roads and another 30000 miles on rough Australia sealed roads, always driven hard and the 'feel' in the rear is the same it has always been (same swingarm bearings too). On the basis that the Superhawk swingarm is obviously weaker than the other donor arms our vehicles use I'm comfortable that the average builder is going to get a long and safe life out of our currently specced rear ends



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  6. Noah Katz

    Noah Katz Active Member

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    4 - 5 lb is pretty heavy; good data point to know that you can feel that difference.

    > Swingarms need to infinitely stiff laterally to avoid any torque steer.

    Oh, now I know what you mean by torque steer.

    I'd think stiffness that limits deflection to roughly a mm wouldn't be perceptible given all the other compliances (esp the tire), and allowing that would save infinite weight :)
     

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