For the new rear axle, as it is also suc­cinct­ly called at the uni­ver­si­ty, it was not nec­es­sary to “rein­vent the wheel”, but at least the hub­less dri­ve wheel with a huge wheel bear­ing is a design inno­va­tion. In which the dri­ve axle is shift­ed off-cen­tre and con­nect­ed to an out­er rim via a spur gear. 

The nice thing about start­ing a com­plete­ly new design is that you have all the design free­dom at your dis­pos­al. How­ev­er, it is also a great chal­lenge, because it is both com­pli­cat­ed and time-con­sum­ing to take into account all the require­ments and prop­er­ties of the Traction‑X rim to be designed. After all, man­u­fac­tur­ing, assem­bly and han­dling in the customer’s hands should be tak­en into account right from the start, as should the tech­ni­cal require­ments of the indi­vid­ual com­po­nents, such as the max­i­mum slid­ing speeds or the forces that can be endured. Not to men­tion the avail­abil­i­ty of afford­able com­po­nents that meet these require­ments with­out mak­ing the busi­ness case impos­si­ble, i.e. mak­ing the prod­uct unsellably expen­sive. It is all the more pleas­ant to be able to fall back on good pre­lim­i­nary work on the demon­stra­tor and patent draw­ings, despite all the con­struc­tive freedom. 

Thus, in the course of the last eight months and in the com­bi­na­tion of two indi­vid­ual project works and a bach­e­lor the­sis, a com­plete and func­tion­al CAD mod­el of patent vari­ant 1 (Dri­ve-Train Swing-Wheel) was cre­at­ed, which can be seen in Fig. 1. In addi­tion, sev­er­al finite ele­ment analy­ses deter­mined that the Traction‑X rim would with­stand the antic­i­pat­ed loads with a high degree of cer­tain­ty. At the begin­ning, it quick­ly became clear that the scope and com­plex­i­ty of the require­ments or tasks were too much for just one Bachelor’s the­sis (mine) and one indi­vid­ual project the­sis (Jonas Bur­winkel). There­fore, we sep­a­rat­ed the areas of require­ments analy­sis and pre­lim­i­nary design into a sec­ond indi­vid­ual project work, which I car­ried out. At the same time, I was able to use the time to famil­iarise myself with the oper­a­tion of Fusion360. This was fol­lowed in my bach­e­lor the­sis by the detailed design of the entire rim with com­po­nents that are actu­al­ly avail­able on the mar­ket and the mesh­ing to the finite ele­ment mod­el, start­ing from the CAD mod­el of my Traction‑X design. In order to obtain real­is­tic results for the fol­low­ing cal­cu­la­tion of the sta­t­ic stress­es, the cal­cu­la­tion para­me­ters used had to be deter­mined in the best pos­si­ble way. Fig­ure 2 shows the graph­i­cal eval­u­a­tion of one of the cal­cu­lat­ed load cas­es. It can be seen that the stress­es in the cur­rent design are low (high safe­ty) and thus mate­r­i­al or weight of the rim can be reduced. 

Par­al­lel to my work, my fel­low stu­dent at the TH Köln, Jonas Bur­winkel, took care of the design and cal­cu­la­tion of the gear shaft in his project work. 

At the begin­ning of the project, it was impor­tant to under­stand the com­plex and not entire­ly triv­ial sta­t­ics of the rim in order to be able to cal­cu­late the loads of dri­ving on the indi­vid­ual com­po­nents. How­ev­er, since the dri­ving oper­a­tion of a scoot­er is any­thing but sta­t­ic, we came up with the fol­low­ing sub­sti­tute mod­el: for the high­est start­ing loads, we imag­ined an immi­nent traf­fic light start, a “cav­a­lier start” (a some­what dusty expres­sion). The dri­ver there­fore gives “full throt­tle” or com­mands the elec­tric motor to full start­ing torque by turn­ing the knob on the right, but remains on the front brake until the traf­fic light turns green. This load case was men­tal­ly super­im­posed with fur­ther dynam­ic increas­es due to dri­ving through pot­holes. Since our sys­tem behaves very well and lin­ear­ly, this is per­mis­si­ble. How­ev­er, the free-body dia­grams of the com­plex trans­mis­sion, which do not appear in this way or in a sim­i­lar way in any sta­t­ics text­book in the world, were a problem.

With the oper­at­ing loads and the require­ments for the ser­vice life of our Traction‑X dri­ve, it was then pos­si­ble, in con­sul­ta­tion with a well-known bear­ing man­u­fac­tur­er, to select suit­able bear­ings for the gear­box and to design the gear­ing and the dri­ve shaft. The large wheel bear­ing turned out to be much more com­plex. Know­ing that in the sec­ond patent vari­ant this would be replaced by a race on three rollers, the pre­lim­i­nary choice fell on a so-called thin-ring bear­ing. Although this ful­fils the tech­ni­cal require­ments, it would be far too expen­sive for use on Traction‑X. Here, the dif­fer­ence between a mass mar­ket for afford­able scoot­ers and the mar­ket for con­struc­tion machin­ery or mil­i­tary vehi­cles, where such bear­ings are also used but where costs play a minor role or no role at all, becomes clear. How­ev­er, this expen­sive bear­ing has already been replaced in the cur­rent design ver­sion by a so-called wire-race bear­ing, which is much cheaper. 

In order to be able to trans­mit high pow­er, the rim should be pro­vid­ed with oil bath lubri­ca­tion. This requires very large radi­al shaft seal­ing rings that seal the cir­cum­fer­en­tial rim bed against the stand­ing inner rims. These seal­ing rings are addi­tion­al­ly pro­tect­ed from the out­side with caps. Since we do not want to intro­duce hub caps for two-wheel­ers, of course, we have agreed on the term “wheel ring”. These clamp in the grooves that are also used to attach the glued bal­anc­ing adhe­sive weights. All in all, this idea met with a very pos­i­tive response, also because the good design­er should famil­iarise him­self with the habits of his cus­tomers, some of whom clean their vehi­cles with a high-pres­sure clean­er out of con­ve­nience, but do not want to risk dam­ag­ing their rear wheel drive. 

To increase the clean­li­ness of the oil, met­al debris is col­lect­ed by a mag­net on the filler plug. To make the oil change par­tic­u­lar­ly user-friend­ly, the oil drain plug is eas­i­ly acces­si­ble on the out­side of the rim bed. These design details were also met with praise, as they sim­pli­fy main­te­nance and increase ser­vice life. 

Moti­vat­ed by the solu­tion of each chal­lenge and by the exchange with the team mem­bers dur­ing the bi-week­ly meet­ings, it is a plea­sure to work on Traction‑X. The good team­work is remark­able, espe­cial­ly since all com­mu­ni­ca­tion is pure­ly dig­i­tal. I would like to take this oppor­tu­ni­ty to thank all the peo­ple involved in the Traction‑X project. 

I am look­ing for­ward to accom­pa­ny­ing the fur­ther devel­op­ment of the project and to see­ing the results of the next works. The project leader added that he is also indebt­ed and that he is look­ing for­ward to the time after Coro­na because he real­ly wants to catch up on the oppor­tu­ni­ty to pat some shoul­ders (with gloves on) and shake hands.


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