Steering geometry and workable changes

mauricejota7604

mauricejota7604

Hero member
Location
Victoria Australia
Hi Andy I will be interested to see how your steering goes with the cut frame.
I fitted a set of offset Ceriani triples to my RGS race bike to see if I could get her to steer quicker but cant really say there was much of an improvement, in fact my bike had a really bad front end pattering going on in a tight left hand bend think it had anything to do with the triples.
These RGS based bikes are a big handfull on the track, very heavy to steer. 
 
mauricejota7604 said:
These RGS based bikes are a big handfull on the track, very heavy to steer.
Click to expand...

Bit of a thread hijack but are you using stock length shocks and swingarm/frame attachment points?  The easiest way to get them to turn is just by changing the ride heights.  My RGA came back to something like 26deg and ~100mm of trail with a stock geometry frame and it turns better than my Bimota. 

My recollection is that the non-parallel yokes made the forks steeper by about 1.5deg than the steering head?  Although that sounds good, what it does is make more trail which is the opposite of what you want, as trail is the primary determining factor on stability.  The RGS has a lot of trail standard (120 or 130mm!), adding 1.5deg to a ~700mm long fork will give nearly another 20mm.  If that is the case then it's little wonder it won't turn.

The pattering sounds like a separate suspension-related issue to me.
Just my $0.02... worth what you pay for it.  :D

Cheers
SteveB
 
With a little more thought, the steeper yokes will also raise the front a little, which will of course change everything again. 
I made a bunch of educated guesses at stock geometry and got 129mm trail in the original format (plausible) which changes to 155.5mm (with 27.9deg rake) if the yokes are 1.5deg steeper.  That assumes the offset is roughly the same between parallel and raked yokes, I have no idea if that is true, if it's not then things change again.  Given they're raked the offset must be slightly hard to establish exactly as there's no hard datum, and also not the same between top and bottom yoke.  Weird system.
 
Very weird system indeed. if you think about the arc it creates when swinging left-right then trail figures will vary as you countersteer and set mid-corner. Better maths-head than me if you can work out how it would vary from a parallel setup!!

Is there a clever way of measuring rake and trail figures with the bike set up on flat ground? I'll do mine by removing the front wheel and guard, taking a line down the front of the steering head to the ground contact point, calculating what to deduct to arrive at that point relative to the centreline of the steering head (starting to get tricky already!!), then doing something similar for the centreline of the triple clamps (ie offset) to the axle centre, a line from there to the ground at exactly 90 deg and that distance between the lines should be my trail figure. Rake will be calculated on trig tables, sin/cos and the like.
 
Sorry for the thread hijack and it probably wouldn't hurt to start a thread alone steering geometry but following on from the offset triples the other thing that is different about the offset Ceriani triple clamps  to the Marzocchi triple clamps is the centerline of the forks is further forward than the centerline of the triple pivot and its about 25mm so the wheelbase remains the same.
I like your idea Quentin about transposing lines to the floor so at least there are some dimensions to work to.
 
RGS has 125mm trail in standard trim. Trail on my RGS is about 115mm. The BMW K100 front end reduced it by 5mm. The rest of the reduction came from changing the stance of the bike - jacking the rear suspension up and dropping the triple clamps down the forks. 115mm is still a lot of trail by modern standards. The bike is quite stable at speed so it could be reduced more, but I reckon that would require frame mods, which I don't feel the urge to do.
 
mauricejota7604 said:
Sorry for the thread hijack and it probably wouldn't hurt to start a thread alone steering geometry but following on from the offset triples the other thing that is different about the offset Ceriani triple clamps  to the Marzocchi triple clamps is the centerline of the forks is further forward than the centerline of the triple pivot and its about 25mm so the wheelbase remains the same.
I like your idea Quentin about transposing lines to the floor so at least there are some dimensions to work to.
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OK, that's what I was talking about above, makes somewhat more sense but if wheelbase is the same then trail comes back to being near enough the same, so I don't really see how it changes much.  Maybe with a more definite Ceriani offset measurement there might show a useful difference.  The guys that swapped the other way when racing them in the 70's might be able to advise, but they used 750SFC yokes from memory, so we'd need to confirm what their offset is also.

The easiest way to determine trail is to draw it, plenty of freeware CAD options around these days.  The only thing you really need to measure on the bike is the rake, which you can do with an inclinometer app on a smart phone these days with pretty decent accuracy.  Alternatively you can drop string lines to the ground and work it out with trig.  You only then need to know offset and wheel diameter (from tyre manufacturer) and draw it up in CAD or work it out with trig if you're that way inclined.

Perhaps a mod can split this stuff off into a new topic?
 
I measured the difference between the SFC (not offset) and standard 3C (offset) Ceriani 38mm top yokes.  The SFC offset measured 60mm and the 3C was 63mm.    Pic attached.
HTH.
 

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hooksey said:
Boiler makers would be all over that system Maurice  ;) Old School
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Too true cobber.

I did a race GS1000 by tying it between benches dead upright (spirit levels).
Drew the center of the contact patch - on the floor, in chalk... - drew the point at which the center line of the steering head hit the floor - and physically measured the trail.
On 17's with long Gazi shocks it was 25.5 deg rake and 96mm trail.
Very close to current big Triumphs. Steers bloody well too.

When you steepen the rake, it's not frame stiffness at the steering head you need to worry about - it's the fork stiffness.
Any slop in the sliders is magnified. Any lack of torsional stiffness is felt as vague steering. I went to a larger diameter axle with this bike and have not had to resort to a fork brace.
 
tom3c75 said:
I measured the difference between the SFC (not offset) and standard 3C (offset) Ceriani 38mm top yokes.  The SFC offset measured 60mm and the 3C was 63mm.    Pic attached.
HTH.
Click to expand...

Thanks for measuring, that's good info.  For the Cerianis, I expect the bottom yoke offset should be about 5mm less than the top.  Is this the case?

So other things being equal, here's a set of numbers:

RGS yokes, 45mm offset, parallel : Trail 128.7mm
3C yokes, 63mm offset at the top, 1.5deg steeper fork axis: Trail 132.6mm
SFC yokes, 60mm offset, parallel : Trail 109.7mm. 

Bear in mind these are based on some assumed numbers so are not real but are indicative.  So that's why the SFC yokes were the hot tip back in the day, they do indeed reduce the trail quite usefully.
So Maurice, I would suggest parting out your SFC and nicking the yokes to put on the RGS!
 
Difficult to measure the offset of the standard 3C bottom yoke, as it's fitted to the bike. 

The SFC bottom yoke offset also measures at 60mm, as one would expect with parallel yokes.

Won't the standard (ie offset) Ceriani bottom yoke be much the same, ie. 60mm?  Since the measurement is taken at the bottom of the yoke, at the point where the stem meets the yoke, the effect of the non-perpendicular stem should not be very great at that point, no?
 
Hmmmm, tried reading all this bumph about trail and castor and the such like, not a clue frankly. So basically i went with 10mm lopped of the forks and 10mm up on the rear shocks, in my non engineering head that should make it turn faster as its steepened the steering head angle and slightly reduced the wheelbase. Yes/no???

FFFANARCHY
 
I thought the stupid Raked Yokes are only on Harleys and early Bimotas?
No Idea how to mesure out the Trail here, only by Handscatces on the Floor or something?
Anyway, the Rake and Trail is to messure without Weight of the Bike. (Wheels off the Ground)
And there is also the "Ground Trail" and the "Real Trail" ..... Big difference!
My Harris has 26? SteeringHead Degrees and 86 mm Ground Trail. Still steers like a Train!
Its cause of 18" Wheels front and rear.
The Size of the Tyres (and the Brand) also make huge differences.
 
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