Rake angle and grip
Posted: Sat Sep 07, 2013 7:27 pm
the steering head angle (also known as the 'RAKE') is still very conservative on Sidecars.
at the extreme angles of steering lock, LESS tyre is in contact as the wheel leans over. so for a right hand bend the tyre has least grip when on full lock. the less rake and steeper the steering headstock = the more grip you get at full lock.
for example (figures are examples only)
23d head angle (typical solo)
straight ahead 100%
half lock 90%
full R/H lock 60%
and then a steeper angle of say
15d head angle (speedway type)
straight ahead 100%
half lock 95%
full R/H lock 75%
the contact area is more central to the tyre allowing the suspension to work better rather than working the tyre wall on one side.
in the past, to try this steeper rake angle, folk on solos have cut a wedge out of the underside of the frame or taken a little wedge out and welded it into the gap in the top half of the frame. doing this would:
1) change the wheelbase - a few mm - not too important here but anything over say: 15mm is too much.
2) it alters the weight distribution and puts way more on the front - the weight distribution/spring rates etc = all gone to pot
3) trail (the distance the tyre sits behind the steering pivot point as it would strike the ground - this is shortened making the steering less stable.
for sidecars, with a few extra holes drilled at the base of the leading link fork to send the wheel backwards - this is no problem, and backed up if you have one by an eccentric wheel spindle we can get the trail just right again.
what's required is to weld and add metal into the front of the frame between the main frame and the headstock. this would steepen the steering head up from say 27 degrees to the (say) 23 degrees and thus keeps the front wheel in exactly the same place on the *ground.
In recent years in order to get the rider forwards the continentals have been placing their handlebars well ahead of the crown nut which causes a weird steering sensation and eventually prohibits you from doing it anymore and after this you can't put them (and therefore the rider's weight) any further ahead. a dead end for development...
however, if you make the steering headstock steeper by moving the steering head away by say 60mm to steepen the rake, then take the bars from 60mm in front of the crown nut to a position on top of it, you - the rider, start from the same riding position as before. Just go look at a trials bike with the steep rake and the bars are behind the crown nut! you then have room to move the handlebars forward (with the rider!) again if it helps for weight distribution comfort and control etc.
the rider's seated position is controlled by his comfort and angle to the bars. even on the face of a sloped seat, if the bars go forwards 60mm, the rider will sit further forwards increasing the weight over the front wheel. downward pressure on the front tyre will give a more grip.
add the two together: contact area and pressure and you have more grip on the front.
some figures to work with and why:
well, working through the angled headstock: the wheel spindle is luckily a quarter of the way between the ground and the top of the headstock on the steering line. true the wheel spindle is ahead of this line but we still use spindle height as a fulcrum.
aiming to move the headstock forwards a distance of 68mm overall, we start by rotating the top of the headstock 51mm and rotate through 4 degrees. this means that with using the spindle as a fulcrum that the point that the ground trail has lost 17mm. ask your son or daughter: they'll explain! the 'point on the ground' which is the line straight through the middle the steering stem as if drawn with a lazer.
there is also a point on the ground below the middle of the front wheel spindle. yes it is the bit the tyre sits on! but when measuring the distance this point is compared to the one through the headstock is your 'ground trail'.
the reason I'm using the steering axis lines meeting at the same height as the spindle is that when we discuss trail it is necessary.
so, so far we needed 68mm to weld on a new headstock but have gone only 51mm remember?
well to keep the same trail we should have to move the wheel back 17mm and we don't want to do that so we weld the headstock 17mm further forwards at the same new angle of 23d and THAT becomes the 68mm!
this makes it 68mm ahead of the old headstock which is roughly the size of a headstock tube!
so now the headstock angle is 23d (from 27d), it has rotated forwards 51mm, been moved forwards 17mm at the same new angle and now we have the wheel 17mm ahead of where it used to sit.
the pivot point of the front arm needs to be the same height off the ground as before as well.
the above will change the rake by just under 4 degrees. from say 27 to 23 etc. 15 degrees and less are possible but the headstock moves too far away from the rider. one step at a time though, after all you don't know whether you'll want to sit forward again.
here is a heavily changed photo, way more than 68mm at the top of the headstock:
note;
lilac line = the old headstock angle
yellow line = the new angle but look where the line strikes the ground. wheel needs to be on this for zero trail but this would shorten wheelbase
blue line = headstock shunted forwards at same angle to yellow line so the wheel has the same ground trail wheel has now moved forwards though from where it intersected the yellow/lilac/red to the light blue/red intersection.
swingarm must move back on the pivot holes until the wheel spindle is back on the yellow/lilac/red point.
wheel spindle/wheelbase finishes where it started.
white line = wheel spindle position. + and - trail starts here
red line = changes made on the plain of the wheel spindles
if you want the engine forwards, the rider forwards for grip on corners, get the steering head forwards and then you can plonk the rider further forwards to do it?
tests on solos have proven that it is the amount of trail you have that gives stability and not the rake. after all: the coffee trolley wheel works vertically (zero rake) doesn't it?
some photos added 9/10/13:
this is Steph Parr and Josh Haynes. Photo by Ann (thanks!). all photos Wakes Colne September 2013 British Round.
I've not used the wheel spindle as a fulcrum as described above this time but simply used ground trail. wherever the steering point would strike the ground then the wheel stands on it for 'zero' trail. simple!
nice photo gives a nice side on shot. IF the outfit runs zero trail he appears to have 30d headstock. I've marked the lines. Orange line through the headstock is the rake. the line down from the wheel spindle meets the orange line so it is zero trail. if the front wheel was back a bit it would be positive trail. we don't put the wheel ahead of it.... I think!
with a bit of trickery the head angle is now 15d but the wheelbase has been dramatically shortened. no good.... we have negative trail. very bad. if you stop a tea trolley and then push it back the opposite way what happens to the wheels....yeh....yug!
so we extend the headstock forwards but at the same angle as above. now we have zero trail again! lilac line is the old 30d, white line is the new 15d
the rider needs arms like an Orang-Utan at this point but everything else can go forwards now. does the rider want to sit further forwards? the only extra weight in the chassis is in the additional spine length in the frame.
regards
Taffy
at the extreme angles of steering lock, LESS tyre is in contact as the wheel leans over. so for a right hand bend the tyre has least grip when on full lock. the less rake and steeper the steering headstock = the more grip you get at full lock.
for example (figures are examples only)
23d head angle (typical solo)
straight ahead 100%
half lock 90%
full R/H lock 60%
and then a steeper angle of say
15d head angle (speedway type)
straight ahead 100%
half lock 95%
full R/H lock 75%
the contact area is more central to the tyre allowing the suspension to work better rather than working the tyre wall on one side.
in the past, to try this steeper rake angle, folk on solos have cut a wedge out of the underside of the frame or taken a little wedge out and welded it into the gap in the top half of the frame. doing this would:
1) change the wheelbase - a few mm - not too important here but anything over say: 15mm is too much.
2) it alters the weight distribution and puts way more on the front - the weight distribution/spring rates etc = all gone to pot
3) trail (the distance the tyre sits behind the steering pivot point as it would strike the ground - this is shortened making the steering less stable.
for sidecars, with a few extra holes drilled at the base of the leading link fork to send the wheel backwards - this is no problem, and backed up if you have one by an eccentric wheel spindle we can get the trail just right again.
what's required is to weld and add metal into the front of the frame between the main frame and the headstock. this would steepen the steering head up from say 27 degrees to the (say) 23 degrees and thus keeps the front wheel in exactly the same place on the *ground.
In recent years in order to get the rider forwards the continentals have been placing their handlebars well ahead of the crown nut which causes a weird steering sensation and eventually prohibits you from doing it anymore and after this you can't put them (and therefore the rider's weight) any further ahead. a dead end for development...
however, if you make the steering headstock steeper by moving the steering head away by say 60mm to steepen the rake, then take the bars from 60mm in front of the crown nut to a position on top of it, you - the rider, start from the same riding position as before. Just go look at a trials bike with the steep rake and the bars are behind the crown nut! you then have room to move the handlebars forward (with the rider!) again if it helps for weight distribution comfort and control etc.
the rider's seated position is controlled by his comfort and angle to the bars. even on the face of a sloped seat, if the bars go forwards 60mm, the rider will sit further forwards increasing the weight over the front wheel. downward pressure on the front tyre will give a more grip.
add the two together: contact area and pressure and you have more grip on the front.
some figures to work with and why:
well, working through the angled headstock: the wheel spindle is luckily a quarter of the way between the ground and the top of the headstock on the steering line. true the wheel spindle is ahead of this line but we still use spindle height as a fulcrum.
aiming to move the headstock forwards a distance of 68mm overall, we start by rotating the top of the headstock 51mm and rotate through 4 degrees. this means that with using the spindle as a fulcrum that the point that the ground trail has lost 17mm. ask your son or daughter: they'll explain! the 'point on the ground' which is the line straight through the middle the steering stem as if drawn with a lazer.
there is also a point on the ground below the middle of the front wheel spindle. yes it is the bit the tyre sits on! but when measuring the distance this point is compared to the one through the headstock is your 'ground trail'.
the reason I'm using the steering axis lines meeting at the same height as the spindle is that when we discuss trail it is necessary.
so, so far we needed 68mm to weld on a new headstock but have gone only 51mm remember?
well to keep the same trail we should have to move the wheel back 17mm and we don't want to do that so we weld the headstock 17mm further forwards at the same new angle of 23d and THAT becomes the 68mm!
this makes it 68mm ahead of the old headstock which is roughly the size of a headstock tube!
so now the headstock angle is 23d (from 27d), it has rotated forwards 51mm, been moved forwards 17mm at the same new angle and now we have the wheel 17mm ahead of where it used to sit.
the pivot point of the front arm needs to be the same height off the ground as before as well.
the above will change the rake by just under 4 degrees. from say 27 to 23 etc. 15 degrees and less are possible but the headstock moves too far away from the rider. one step at a time though, after all you don't know whether you'll want to sit forward again.
here is a heavily changed photo, way more than 68mm at the top of the headstock:
note;
lilac line = the old headstock angle
yellow line = the new angle but look where the line strikes the ground. wheel needs to be on this for zero trail but this would shorten wheelbase
blue line = headstock shunted forwards at same angle to yellow line so the wheel has the same ground trail wheel has now moved forwards though from where it intersected the yellow/lilac/red to the light blue/red intersection.
swingarm must move back on the pivot holes until the wheel spindle is back on the yellow/lilac/red point.
wheel spindle/wheelbase finishes where it started.
white line = wheel spindle position. + and - trail starts here
red line = changes made on the plain of the wheel spindles
if you want the engine forwards, the rider forwards for grip on corners, get the steering head forwards and then you can plonk the rider further forwards to do it?
tests on solos have proven that it is the amount of trail you have that gives stability and not the rake. after all: the coffee trolley wheel works vertically (zero rake) doesn't it?
some photos added 9/10/13:
this is Steph Parr and Josh Haynes. Photo by Ann (thanks!). all photos Wakes Colne September 2013 British Round.
I've not used the wheel spindle as a fulcrum as described above this time but simply used ground trail. wherever the steering point would strike the ground then the wheel stands on it for 'zero' trail. simple!
nice photo gives a nice side on shot. IF the outfit runs zero trail he appears to have 30d headstock. I've marked the lines. Orange line through the headstock is the rake. the line down from the wheel spindle meets the orange line so it is zero trail. if the front wheel was back a bit it would be positive trail. we don't put the wheel ahead of it.... I think!
with a bit of trickery the head angle is now 15d but the wheelbase has been dramatically shortened. no good.... we have negative trail. very bad. if you stop a tea trolley and then push it back the opposite way what happens to the wheels....yeh....yug!
so we extend the headstock forwards but at the same angle as above. now we have zero trail again! lilac line is the old 30d, white line is the new 15d
the rider needs arms like an Orang-Utan at this point but everything else can go forwards now. does the rider want to sit further forwards? the only extra weight in the chassis is in the additional spine length in the frame.
regards
Taffy