Framebuilders - how much trail do you use?

[gtadmin]

Hello guys,

I am building a frame with the following dimensions:
Wheelbase: 1350mm (53.15")
Ground Trail: 80mm (3.15")
Head angle between 55.8 and 56.2 degrees from horizontal (hard to measure accurately)
Front wheel diameter: 500mm (20")

The styling is somewhere between cruiser and MWB recumbent, inspired mostly by the offerings contained in "Street Machine Choppers" issue #5 (OZ mag). Also, I am aiming for 30 cruising, 45kph top speed (max 28mph). The 20" front wheel has embedded electro-magnetic braking (OK AJ, a "frock" FH205), and the rear wheel is a 26" with 8" diskbrake. It is anticipated that there will be 35/65 weight distribution. The frame is >50% complete with the major, outstanding items being the front forks and headtube/backbone triangulation.

So far, so good.

However, I was reading an old article from "Physics Today April 1970" written by David EH Jones, where he was experimenting with the effects of trail (amongst other things). One of the conclusions reached was that an excessive amount of trail, although extremely stable, became difficult to steer. No absolute measurement was given though, only 0.2 of wheel diameter, therefore 20" x 0.2 = 4" (100mm).

A website dedicated to recumbent design http://bikesmithdesign.com/design/12steps.html however states that only 6 to 12mm (1/4" to 1/2") should be used!

A third reference is "Motorcycle Handling Chassis Design Manual" by Tony Foale, where between 50 and 100mm is recommended. Note these are heavier and faster machines though.

The result of obviously too much reading is a state of confusion! I'm now thinking that I should reduce my trail to 40mm (thereby increasing the wheelbase to 1390mm). Changing the head angle would be major rework given I've already "goosenecked" it.

So the question is, what trail do you use and/or recommend, and why? For completeness and understanding, you probably would include wheelbase, head angle, front wheel diameter and weight distribution. Road manners and "feel" although subjective are welcome.

Cheers in advance,
GT

[AussieJester]

The more rake the more trail...to a point, i like to have between 2-3 inches of trail, for a upright
frame with lesser head angle, likea mtb i guess 1-2inches would suffice...How ever much or less you put will give you different handling obviously
more rake and lots of trail will give you a steady ride but not super nimble in the handling, on the opposite end reduced
head angle and lil trail will produce very precise 'nervous' type handling...

Best of luck

KiM

[gtadmin]

Yep, got that OK (and thanks). I have your dimensions and impressions of your ride from your build. In short, 45deg 1670mm wheelbase 75mm trail, tracks strongly.
I would put up a sketch, but I can't draw for shit.

Cheers mate,
GT

[John in CR]

AJ,
Sorry buddy, but you got that backwards. Increasing the rake decreases the trail. Since this gentleman wants to proceed based on a fixed head set angle, the rake and fork length are what he has to work with.


gtadmin,
With the weight of a hub motor in the front wheel, wheel flop is going to be a big issue that is likely to force you to use a shorter trail. You need to talk to someone using a front hub on a long wheelbase recumbent to get more pertinent info for your case. I have about 100mm of trail on my 60" wheelbase bike, and it's dead stable at over 90kph, but I've got a lot of wheel flop. It's only an issue if I'm sitting on the bike stopped where the wheel can flop over to the side quite rapidly. I can see though that the weight of a hub motor could make that effect extreme enough that you could feel it want to flop even while moving (at very low speed), which would be a bad thing. I've only changed the trail via changing the head angle, so I can't comment on how changing the trail by varying the rake is going to affect handling. I assume that the effect different.

[gtadmin]

John, you are technically correct with rake (which is the forward horizontal offset of the wheel axle from the extended centreline of the head tube), although I know AJ meant head angle as measured from the vertical (commonly called rake angle, or just plain rake), and with that interpretation he is also correct. The greater the angle from vertical, the more trail increases all other things being equal. Head angle itself produces almost zero effect on handling when travelling in a straight line at greater than walking speed, and it is mainly used for convenience of manufacture and aesthetics. Trail has a much greater effect on stability, as does weight supported by the front wheel. The more weight, the more stable.

Flopping at standstill will not be a serious issue, it's not like I'm going to fall off - after all the seat is about 23" from the ground! Like sitting in your kitchen chair. What is your bike like at lower speeds where you aren't wobbling because of weight? (Edit: oops, forgot you don't actually pedal.) Does your trail make it hard to steer?

Thanks for your feedback.

Cheers,
GT

[Evoforce]

I have been at a few sites that deal with wheel flop. One was for bicycles of the cargo hauling catagory and the other is for motorcycles that drive sidecars. Look into these areas to maybe give you some clue as to how to design and tackle this issue for your situation.

[gtadmin]

Thanks Evo, any links? Not to the sidecar, but the cargo.

When I talk about trail above, I am referring to real trail, which is measured from the point of contact between the tyre and the ground, at 90deg to the extended centreline through the head tube. The more head angle (from vertical) the less the real trail is for a given amount of ground trail. This is the distance when multiplied by the weight on the front wheel produces the torque that straightens your handle bars, often referred to as self-centring. So in general the more trail or the more weight, the greater the centring torque.

Real trail for AJ's bike is 75 * cos 45 = 53.0mm. For mine, it's 80 * cos 34 = 66.3mm. And because my wheelbase is 81% of AJ's, I think I can reduce my ground trail considerably and still be stable. 53 * .81 / cos 34 = 51.6mm should be the equalivent to AJ's for my wheelbase. My bike however will be lighter.

Cheers,
GT

Edit: typos

[gtadmin]

OK, for defintions I found this http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry. But who said wiki was correct ...

After reading about flop, it's effect is reduced on my bike because of the low height of the head tube and low centre of gravity. I think I talked myself into going to a shorter ground trail!

Cheers,
GT

[John in CR]

Real trail, ground trail, real rake, rake angle. No wonder we all get so confused about this topic, which is otherwise not too difficult. Are the following not the standard? Can we please stick to the standard and avoid being a source of confusion to others?
head tube angle = angle from horizontal (I like that you stated from horizontal with such
a slack head angle to avoid confusion.)
trail = the distance between where the tire touches the ground and a centerline drawn trough the headset to the ground
rake = the distance the wheel axle is offset from the centerline drawn through the headset

Regarding the wheel flop, it's not so much you falling off, but suddenly the bike flipping out right under you or next to you if you nonchalantly hold the handlebar while off the bike. It happens very quickly with our low center of gravity bikes, and landing on the throttle or it doing it while you are holding the throttle could be a real problem. Also, if you're like AJ and bike looks are more important than everything else, too much flop will make it look a bit "wrong" when parked too. The weight of the hub motor seems like it would increase these effects, though I'm not 100% clear on that effect.

John

[docnjoj]

Now you guys have me worried about the rake and trail of Joyces new trike with a front hub motor. Anybody hazard a guess as to the behavior with a 9C stuck on the front wheel. I think the rake is 60 degrees? My guess is that the trail is about 3" but I could be wrong.

Joyces trike.jpg (63.98 KiB) Viewed 864 times

otherDoc

[John in CR]

OK, for defintions I found this http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry. But who said wiki was correct ...

After reading about flop, it's effect is reduced on my bike because of the low height of the head tube and low centre of gravity. I think I talked myself into going to a shorter ground trail!

Cheers,
GT

GT,

Now you lost me. If flop is reduced with the factors of your bike, why wouldn't you want to go for the stability of a longer trail? I just put some different bike pieces together not knowing shit from shinola. It got a bit twitchy when I tucked going uphill into a big headwind, so I poked around about bike geometry, hacked the headset off, and welded it back on at a lesser head angle. It ended up a bit more trail than I intended, since I wasn't exacting like you, but I've been pleased as punch.

Oh and to answer you previous question, my bike is steady like a rock slow or fast. I've got a bit of tiller effect but once used to it I like it more because it makes one handed steering and one hand braking feel more stable.

John

[John in CR]

Now you guys have me worried about the rake and trail of Joyces new trike with a front hub motor. Anybody hazard a guess as to the behavior with a 9C stuck on the front wheel. I think the rake is 60 degrees? My guess is that the trail is about 2" but I could be wrong.

otherDoc

Rake and trail on a non-leaning delta trike...I can't wrap my brain around those effects that except it won't have an issue with flop. I like that trike though.

[docnjoj]

So this "flop" thing involves tilt from the vertical axis? Wouldn't that happen even on a delta trike due to the rake/trail difference when she turns? It would just be minimized by the not leaning. This sounds like I translated it from Chinese Does it make any sense?
otherDoc

[docnjoj]

I hope she likes it, John. It fit the needs to be higher than my trike and she is used to the configuration. If it is controllable at least it wont tip over so easily and it has really powerful brakes. When it arrives we will do a before and after test to see if the motor makes the handling different.
otherDoc

[gtadmin]

The standard is different for bicycles and motorcycles, which is why we get confusion. Wiki defines head angle for bicycles and rake for motorcycles being the same thing! The definitions I am using are taken from Tony Foale's book.

OK, a couple of posts while writing this. The recommendations from the recumbent site and the Physics Today article had me worried that my geometry would be too stable, and therefore quite difficult to turn. And I gain extra clearance between chain ring and front wheel. I had reduced the wheelbase to an absolute minimum because I need to get it through a turnstile!

Cheers,
GT

[gtadmin]

Now you guys have me worried about the rake and trail of Joyces new trike with a front hub motor. Anybody hazard a guess as to the behavior with a 9C stuck on the front wheel. I think the rake is 60 degrees? My guess is that the trail is about 3" but I could be wrong.
otherDoc

doc, hard to tell from the piccie, but it looks like you have minimal trail. I think front wheel drive tends to stabilise the bike or in your case trike. Like when towing a caravan and getting speed wobbles, you hit the accelerator to straighten it out.

Concerning flop, I don't think you will have a problem, unless she makes a habit of going round corners on two wheels. But ... I'm no expert either.

Cheers,
GT

[docnjoj]

Thanks gtadmin for the assessment. We will see how soon she gets on 2 wheels. Her accident may slow her down for a while. but I doubt it. At least on slippery surfaces she will maybe keep the shiny side up!
otherDoc

[gtadmin]

John, after re-reading the cause of flop, increased trail and weight increases the tendency to flop. My misunderstanding and I apologise. Some flop is necessary though to give "feel" when cornering. Oh, and I know you will love the fact that "real" trail is also called "mechanical" trail

This means that doc, you will get some flop with the trike, but ... you shouldn't notice the effect. When is it due to arrive?

Cheers guys, it's now Thursday and I'm off
GT

[AussieJester]

[quote="John in CR"]AJ,
Sorry buddy, but you got that backwards. [/quote

Nope i got it right..

KiM

[docnjoj]

Trike is due in the bike shop on Friday. Her Bike-e has a similar setup with a front motor and about the same rake and trail as the new trike so the differences should be the "training wheels" and the underseat steering. It is HD (heavy duty) and even thought she doesn't weigh that much it should be hard to break. They claim a load up to 400 lbs is acceptable and she weighs 165 so.................. Sun wouldn't send it to me in a box as their policy is to only have bike shops assemble and test their stuff but my LBS is a good one and they are accepting some "support" from me regarding LocTite and other "hints".
otherDoc

[FMB42]

gtadmin writes:
"OK, a couple of posts while writing this. The recommendations from the recumbent site and the Physics Today article had me worried that my geometry would be too stable, and therefore quite difficult to turn. And I gain extra clearance between chain ring and front wheel. I had reduced the wheelbase to an absolute minimum because I need to get it through a turnstile!"

You can also, of course, consider using a larger or smaller diameter wheel as a means of increasing or, in your case, decreasing relative stability.


BTW, I've had Tony Foale's "Motorcycle Handling and Chassis Design" book since the early '80s. This book offers a wealth of information on this topic (I found the "reverse rake leading link designs to be very interesting to say the least).

[gtadmin]

Trike is due in the bike shop on Friday. Her Bike-e has a similar setup with a front motor and about the same rake and trail as the new trike so the differences should be the "training wheels" and the underseat steering. It is HD (heavy duty) and even thought she doesn't weigh that much it should be hard to break. They claim a load up to 400 lbs is acceptable and she weighs 165 so.................. Sun wouldn't send it to me in a box as their policy is to only have bike shops assemble and test their stuff but my LBS is a good one and they are accepting some "support" from me regarding LocTite and other "hints".
otherDoc

HD is good, but of far more importance will be less trips to ER. Hope she is recovering well.

It will be interesting to get your feedback on how it handles a speed in a straight line.

Cheers,
GT

[gtadmin]

gtadmin writes:
"OK, a couple of posts while writing this. The recommendations from the recumbent site and the Physics Today article had me worried that my geometry would be too stable, and therefore quite difficult to turn. And I gain extra clearance between chain ring and front wheel. I had reduced the wheelbase to an absolute minimum because I need to get it through a turnstile!"

You can also, of course, consider using a larger or smaller diameter wheel as a means of increasing or, in your case, decreasing relative stability.


BTW, I've had Tony Foale's "Motorcycle Handling and Chassis Design" book since the early '80s. This book offers a wealth of information on this topic (I found the "reverse rake leading link designs to be very interesting to say the least).

Hi FMB42
Diameter of wheel is locked. Altering trail at the moment is easily achievable right up to I start welding it!

Yes, an excellent reference.

Cheers,
GT

[John in CR]

Doc,

Definitely give us a review. I haven't seen a mid suspension like that, so please include feedback specifically regarding how it handles the bumps.

[gtadmin]

For you John ....